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  • 1. Bartlett, Rachel E.
    et al.
    Bollasina, Massimo A.
    Booth, Ben B. B.
    Dunstone, Nick J.
    Marenco, Franco
    Messori, Gabriele
    Stockholm University, Faculty of Science, Department of Meteorology .
    Bernie, Dan J.
    Do differences in future sulfate emission pathways matter for near-term climate? A case study for the Asian monsoon2018In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 50, no 5-6, p. 1863-1880Article in journal (Refereed)
    Abstract [en]

    Anthropogenic aerosols could dominate over greenhouse gases in driving near-term hydroclimate change, especially in regions with high present-day aerosol loading such as Asia. Uncertainties in near-future aerosol emissions represent a potentially large, yet unexplored, source of ambiguity in climate projections for the coming decades. We investigated the near-term sensitivity of the Asian summer monsoon to aerosols by means of transient modelling experiments using HadGEM2-ES under two existing climate change mitigation scenarios selected to have similar greenhouse gas forcing, but to span a wide range of plausible global sulfur dioxide emissions. Increased sulfate aerosols, predominantly from East Asian sources, lead to large regional dimming through aerosol-radiation and aerosol-cloud interactions. This results in surface cooling and anomalous anticyclonic flow over land, while abating the western Pacific subtropical high. The East Asian monsoon circulation weakens and precipitation stagnates over Indochina, resembling the observed southern-flood-northern-drought pattern over China. Large-scale circulation adjustments drive suppression of the South Asian monsoon and a westward extension of the Maritime Continent convective region. Remote impacts across the Northern Hemisphere are also generated, including a northwestward shift of West African monsoon rainfall induced by the westward displacement of the Indian Ocean Walker cell, and temperature anomalies in northern midlatitudes linked to propagation of Rossby waves from East Asia. These results indicate that aerosol emissions are a key source of uncertainty in near-term projection of regional and global climate; a careful examination of the uncertainties associated with aerosol pathways in future climate assessments must be highly prioritised.

  • 2.
    Bender, Frida A.-M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Frey, Lena
    Stockholm University, Faculty of Science, Department of Meteorology .
    McCoy, Daniel T.
    Grosvenor, Daniel P.
    Mohrmann, Johannes K.
    Assessment of aerosol-cloud-radiation correlations in satellite observations, climate models and reanalysis2019In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 52, no 7-8, p. 4371-4392Article in journal (Refereed)
    Abstract [en]

    Representing large-scale co-variability between variables related to aerosols, clouds and radiation is one of many aspects of agreement with observations desirable for a climate model. In this study such relations are investigated in terms of temporal correlations on monthly mean scale, to identify points of agreement and disagreement with observations. Ten regions with different meteorological characteristics and aerosol signatures are studied and correlation matrices for the selected regions offer an overview of model ability to represent present day climate variability. Global climate models with different levels of detail and sophistication in their representation of aerosols and clouds are compared with satellite observations and reanalysis assimilating meteorological fields as well as aerosol optical depth from observations. One example of how the correlation comparison can guide model evaluation and development is the often studied relation between cloud droplet number and water content. Reanalysis, with no parameterized aerosol–cloud coupling, shows weaker correlations than observations, indicating that microphysical couplings between cloud droplet number and water content are not negligible for the co-variations emerging on larger scale. These observed correlations are, however, not in agreement with those expected from dominance of the underlying microphysical aerosol–cloud couplings. For instance, negative correlations in subtropical stratocumulus regions show that suppression of precipitation and subsequent increase in water content due to aerosol is not a dominating process on this scale. Only in one of the studied models are cloud dynamics able to overcome the parameterized dependence of rain formation on droplet number concentration, and negative correlations in the stratocumulus regions are reproduced.

  • 3.
    Bender, Frida
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica
    Stockholm University, Faculty of Science, Department of Meteorology .
    Rodhe, Henning
    Stockholm University, Faculty of Science, Department of Meteorology .
    Response to the eruption of Mount Pinatubo in relation to climate sensitivity in the CMIP3 models2010In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 35, no 5, p. 875-886Article in journal (Refereed)
    Abstract [en]

    The radiative flux perturbations and subsequent temperature responses in relation to the eruption of Mount Pinatubo in 1991 are studied in the ten general circulation models incorporated in the Coupled Model Intercomparison Project, phase 3 (CMIP3), that include a parameterization of volcanic aerosol. Models and observations show decreases in global mean temperature of up to 0.5 K, in response to radiative perturbations of up to 10 W m−2, averaged over the tropics. The time scale representing the delay between radiative perturbation and temperature response is determined by the slow ocean response, and is estimated to be centered around 4 months in the models. Although the magniude of the temperature response to a volcanic eruption has previously been used as an indicator of equilibrium climate sensitivity in models, we find these two quantities to be only weakly correlated. This may partly be due to the fact that the size of the volcano-induced radiative perturbation varies among the models. It is found that the magnitude of the modelled radiative perturbation increases with decreasing climate sensitivity, with the exception of one outlying model. Therefore, we scale the temperature perturbation by the radiative perturbation in each model, and use the ratio between the integrated temperature perturbation and the integrated radiative perturbation as a measure of sensitivity to volcanic forcing. This ratio is found to be well correlated with the model climate sensitivity, more sensitive models having a larger ratio. Further, if this correspondence between “volcanic sensitivity” and sensitivity to CO2 forcing is a feature not only among the models, but also of the real climate system, the alleged linear relation can be used to estimate the real climate sensitivity. The observational value of the ratio signifying volcanic sensitivity is hereby estimated to correspond to an equilibrium climate sensitivity, i.e. equilibrium temperature increase due to a doubling of the CO2 concentration, between 1.7 and 4.1 K. Several sources of uncertainty reside in the method applied, and it is pointed out that additional model output, related to ocean heat storage and radiative forcing, could refine the analysis, as could reduced uncertainty in the observational record, of temperature as well as forcing.

  • 4. Bollasina, Massimo A.
    et al.
    Messori, Gabriele
    Stockholm University, Faculty of Science, Department of Meteorology . Met Office Hadley Centre, UK.
    On the link between the subseasonal evolution of the North Atlantic Oscillation and East Asian climate2018In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 51, no 9-10, p. 3537-3557Article in journal (Refereed)
    Abstract [en]

    We analyse the impact of the North Atlantic Oscillation (NAO) on the climate of East Asia at subseasonal time scales during both winter and summer. These teleconections have mainly been investigated at seasonal and longer time scales, while higher-frequency links are largely unexplored. The NAO is defined using extended empirical orthogonal functions on pentad-mean observations, which allows to elucidate the oscillation’s spatial and temporal evolution and clearly separate the development and decay phases. The downstream dynamical imprint and associated temperature and precipitation anomalies are quantified by means of a linear regression analysis. It is shown that the NAO generates a significant climate response over East Asia during both the dry and wet seasons, whose spatial pattern is highly dependent on the phase of the NAO’s life cycle. Temperature and precipitation anomalies develop concurrently with the NAO mature phase, and reach maximum amplitude 5–10 days later. These are shown to be systematically related to mid and high-latitude teleconnections across the Eurasian continent via eastward-propagating quasi-stationary Rossby waves instigated over the Atlantic and terminating in the northeastern Pacific. These findings underscore the importance of rapidly evolving dynamical processes in governing the NAO’s downstream impacts and teleconnections with East Asia.

  • 5.
    Brodeau, Laurent
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Koenigk, Torben
    Extinction of the northern oceanic deep convection in an ensemble of climate model simulations of the 20th and 21st centuries2016In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 46, no 9, p. 2863-2882Article in journal (Refereed)
    Abstract [en]

    We study the variability and the evolution of oceanic deep convection in the northern North Atlantic and the Nordic Seas from 1850 to 2100 using an ensemble of 12 climate model simulations with EC-Earth. During the historical period, the model shows a realistic localization of the main sites of deep convection, with the Labrador Sea accounting for most of the deep convective mixing in the northern hemisphere. Labrador convection is partly driven by the NAO (correlation of 0.6) and controls part of the variability of the AMOC at the decadal time scale (correlation of 0.6 when convection leads by 3-4 years). Deep convective activity in the Labrador Sea starts to decline and to become shallower in the beginning of the twentieth century.  The decline is primarily caused by a decrease of the sensible heat loss to the atmosphere in winter resulting from increasingly warm atmospheric conditions. It occurs stepwise and is mainly the consequence of two severe drops in deep convective activity during the 1920s and the 1990s.  These two events can both be linked to the low-frequency variability of the NAO. A warming of the sub-surface, resulting from reduced convective mixing, combines with an increasing influx of freshwater from the Nordic Seas to rapidly strengthen the surface stratification and prevent any possible resurgence of deep convection in the Labrador Sea after the 2020s. Deep convection in the Greenland Sea starts to decline in the 2020s, until complete extinction in 2100. As a response to the extinction of deep convection in the Labrador and Greenland Seas, the AMOC undergoes a linear decline at a rate of about -0.3 Sv per decade during the twenty-first century.

  • 6.
    Caron, Louis-Philippe
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Jones, Colin G.
    Understanding and simulating the link between African easterly waves and Atlantic tropical cyclones using a regional climate model: the role of domain size and lateral boundary conditions2012In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 39, no 1-2, p. 113-135Article in journal (Refereed)
    Abstract [en]

    Using a suite of lateral boundary conditions, we investigate the impact of domain size and boundary conditions on the Atlantic tropical cyclone and african easterly Wave activity simulated by a regional climate model. Irrespective of boundary conditions, simulations closest to observed climatology are obtained using a domain covering both the entire tropical Atlantic and northern African region. There is a clear degradation when the high-resolution model domain is diminished to cover only part of the African continent or only the tropical Atlantic. This is found to be the result of biases in the boundary data, which for the smaller domains, have a large impact on TC activity. In this series of simulations, the large-scale Atlantic atmospheric environment appears to be the primary control on simulated TC activity. Weaker wave activity is usually accompanied by a shift in cyclogenesis location, from the MDR to the subtropics. All ERA40-driven integrations manage to capture the observed interannual variability and to reproduce most of the upward trend in tropical cyclone activity observed during that period. When driven by low-resolution global climate model (GCM) integrations, the regional climate model captures interannual variability (albeit with lower correlation coefficients) only if tropical cyclones form in sufficient numbers in the main development region. However, all GCM-driven integrations fail to capture the upward trend in Atlantic tropical cyclone activity. In most integrations, variations in Atlantic tropical cyclone activity appear uncorrelated with variations in African easterly wave activity.

  • 7.
    Caron, Louis-Philippe
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Jones, Colin G.
    Doblas-Reyes, Francisco
    Multi-year prediction skill of Atlantic hurricane activity in CMIP5 decadal hindcasts2014In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 42, no 9-10, p. 2675-2690Article in journal (Refereed)
    Abstract [en]

    Using a statistical relationship between simulated sea surface temperature and Atlantic hurricane activity, we estimate the skill of a CMIP5 multi-model ensemble at predicting multi-annual level of Atlantic hurricane activity. The series of yearly-initialized hindcasts show positive skill compared to simpler forecasts such as persistence and climatology as well as non-initialized forecasts and return anomaly correlation coefficients of similar to 0.6 and similar to 0.8 for five and nine year forecasts, respectively. Some skill is shown to remain in the later years and making use of those later years to create a lagged-ensemble yields, for individual models, results that approach that obtained by the multi-model ensemble. Some of the skill is shown to come from persisting rather than predicting the climate shift that occur in 1994-1995. After accounting for that shift, the anomaly correlation coefficient for five-year forecasts is estimated to drop to 0.4, but remains statistically significant up to lead years 3-7. Most of the skill is shown to come from the ability of the forecast systems at capturing change in Atlantic sea surface temperature, although the failure of most systems at reproducing the observed slow down in warming over the tropics in recent years leads to an underestimation of hurricane activity in the later period.

  • 8.
    Caron, Louis-Philippe
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Jones, Colin G.
    Vaillancourt, Paul A.
    Winger, Katja
    On the relationship between cloud-radiation interaction, atmospheric stability and Atlantic tropical cyclones in a variable-resolution climate model2013In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 40, no 5-6, p. 1257-1269Article in journal (Refereed)
    Abstract [en]

    We compare two 28-year simulations performed with two versions of the Global Environmental Multiscale model run in variable-resolution mode. The two versions differ only by small differences in their radiation scheme. The most significant modification introduced is a reduction in the ice effective radius, which is observed to increase absorption of upwelling infrared radiation and increase temperature in the upper troposphere. The resulting change in vertical lapse rate is then observed to drive a resolution-dependent response of convection, which in turn modifies the zonal circulation and induces significant changes in simulated Atlantic tropical cyclone activity. The resulting change in vertical lapse rate and its implication in the context of anthropogenic climate change are discussed.

  • 9.
    Colleoni, Florence
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Joseph Fourier University, France.
    Liakka, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Krinner, Gerhard
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Masina, Simona
    Peyaud, Vincent
    The sensitivity of the Late Saalian (140 ka) and LGM (21 ka) Eurasian ice sheets to sea surface conditions2011In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 37, no 3-4, p. 531-553Article in journal (Refereed)
    Abstract [en]

    This work focuses on the Late Saalian (140 ka) Eurasian ice sheets’ surface mass balance (SMB) sensitivity to changes in sea surface temperatures (SST). An Atmospheric General Circulation Model (AGCM), forced with two preexisting Last Glacial Maximum (LGM, 21 ka) SST reconstructions, is used to compute climate at 140 and 21 ka (reference glaciation). Contrary to the LGM, the ablation almost stopped at 140 ka due to the climatic cooling effect from the large ice sheet topography. Late Saalian SST are simulated using an AGCM coupled with a mixed layer ocean. Compared to the LGM, these 140 ka SST show an inter-hemispheric asymmetry caused by the larger ice-albedo feedback, cooling climate. The resulting Late Saalian ice sheet SMB is smaller due to the extensive simulated sea ice reducing the precipitation. In conclusion, SST are important for the stability and growth of the Late Saalian Eurasian ice sheet.

  • 10. Cook, Edward R.
    et al.
    Krusic, Paul J.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Anchukaitis, Kevin J.
    Buckley, Brendan M.
    Nakatsuka, Takeshi
    Sano, Masaki
    Tree-ring reconstructed summer temperature anomalies for temperate East Asia since 800 CE2013In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 41, no 11-12, p. 2957-2972Article in journal (Refereed)
    Abstract [en]

    We develop a summer temperature reconstruction for temperate East Asia based on a network of annual tree-ring chronologies covering the period 800-1989 C.E. The East Asia reconstruction is the regional average of 585 individual grid point summer temperature reconstructions produced using an ensemble version of point-by-point regression. Statistical calibration and validation tests indicate that the regional average possesses sufficient overall skill to allow it to be used to study the causes of temperature variability and change over the region. The reconstruction suggests a moderately warm early medieval epoch (ca. 850-1050 C.E.), followed by generally cooler 'Little Ice Age' conditions (ca. 1350-1880 C.E.) and 20th century warming up to the present time. Since 1990, average temperature has exceeded past warm epochs of comparable duration, but it is not statistically unprecedented. Superposed epoch analysis reveals a volcanic forcing signal in the East Asia summer temperature reconstruction, resulting in pulses of cooler summer conditions that may persist for several years. Substantial uncertainties remain, however, particularly at lower frequencies, thus requiring caution and scientific prudence in the interpretation of this record.

  • 11. de Vries, Hylke
    et al.
    Scher, Sebastian
    Stockholm University, Faculty of Science, Department of Meteorology .
    Haarsma, Rein
    Drijfhout, Sybren
    van Delden, Aarnout
    How Gulf-Stream SST-fronts influence Atlantic winter storms: Results from a downscaling experiment with HARMONIE to the role of modified latent heat fluxes and low-level baroclinicity2019In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 52, no 9-10, p. 5899-5909Article in journal (Refereed)
    Abstract [en]

    The strong horizontal gradients in sea surface temperature (SST) of the Atlantic Gulf Stream exert a detectable influence on extratropical cyclones propagating across the region. This is shown in a sensitivity experiment where 24 winter storms taken from ERA-Interim are simulated with HARMONIE at 10-km resolution. Each storm is simulated twice. First, using observed SST (REF). In the second simulation a smoothed SST is offered (SMTH), while lateral and upper-level boundary conditions are unmodified. Each storm pair propagates approximately along the same track, however their intensities (as measured by maximal near-surface wind speed or 850-hPa relative vorticity) differ up to +/- 25%. A 30-member ensemble created for one of the storms shows that on a single-storm level the response is systematic rather than random. To explain the broad response in storm strength, we show that the SST-adjustment modifies two environmental parameters: surface latent heat flux (LHF) and low-level baroclinicity (B). LHF influences storms by modifying diabatic heating and boundary-layer processes such as vertical mixing. The position of each storm's track relative to the SST-front is important. South of the SST-front the smoothing leads to lower SST, reduced LHF and storms with generally weaker maximum near-surface winds. North of the SST-front the increased LHF tend to enhance the winds, but the accompanying changes in baroclinicity are not necessarily favourable. Together these mechanisms explain up to 80% of the variability in the near-surface maximal wind speed change. Because the mechanisms are less effective in explaining more dynamics-oriented indicators like 850 hPa relative vorticity, we hypothesise that part of the wind-speed change is related to adjustment of the boundary-layer processes in response to the LHF and B changes.

  • 12.
    Donges, Jonathan F.
    et al.
    Stockholm University, Faculty of Science, Stockholm Resilience Centre. Potsdam Institute for Climate Impact Research, Germany.
    Petrova, Irina
    Loew, Alexander
    Marwan, Norbert
    Kurths, Jürgen
    How complex climate networks complement eigen techniques for the statistical analysis of climatological data2015In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 45, no 9-10, p. 2407-2424Article in journal (Refereed)
    Abstract [en]

    Eigen techniques such as empirical orthogonal function (EOF) or coupled pattern (CP)/maximum covariance analysis have been frequently used for detecting patterns in multivariate climatological data sets. Recently, statistical methods originating from the theory of complex networks have been employed for the very same purpose of spatio-temporal analysis. This climate network (CN) analysis is usually based on the same set of similarity matrices as is used in classical EOF or CP analysis, e.g., the correlation matrix of a single climatological field or the cross-correlation matrix between two distinct climatological fields. In this study, formal relationships as well as conceptual differences between both eigen and network approaches are derived and illustrated using global precipitation, evaporation and surface air temperature data sets. These results allow us to pinpoint that CN analysis can complement classical eigen techniques and provides additional information on the higher-order structure of statistical interrelationships in climatological data. Hence, CNs are a valuable supplement to the statistical toolbox of the climatologist, particularly for making sense out of very large data sets such as those generated by satellite observations and climate model intercomparison exercises.

  • 13. Esper, Jan
    et al.
    Klippel, Lara
    Krusic, Paul J.
    Stockholm University, Faculty of Science, Department of Physical Geography. University of Cambridge, UK; Navarino Environmental Observatory, Greece.
    Konter, Oliver
    Raible, Christoph C.
    Xoplaki, Elena
    Luterbacher, Jürg
    Büntgen, Ulf
    Eastern Mediterranean summer temperatures since 730 CE from Mt. Smolikas tree-ring densities2019In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894Article in journal (Refereed)
    Abstract [en]

    The Mediterranean has been identified as particularly vulnerable to climate change, yet a high-resolution temperature reconstruction extending back into the Medieval Warm Period is still lacking. Here we present such a record from a high-elevation site on Mt. Smolikas in northern Greece, where some of Europe's oldest trees provide evidence of warm season temperature variability back to 730 CE. The reconstruction is derived from 192 annually resolved, latewood density series from ancient living and relict Pinus heldreichii trees calibrating at r(1911-2015) = 0.73 against regional July-September (JAS) temperatures. Although the recent 1985-2014 period was the warmest 30-year interval (JAS Twrt.1961-1990 = + 0.71 degrees C) since the eleventh century, temperatures during the ninth to tenth centuries were even warmer, including the warmest reconstructed 30-year period from 876-905 (+ 0.78 degrees C). These differences between warm periods are statistically insignificant though. Several distinct cold episodes punctuate the Little Ice Age, albeit the coldest 30-year period is centered during high medieval times from 997-1026 (- 1.63 degrees C). Comparison with reconstructions from the Alps and Scandinavia shows that a similar cold episode occurred in central Europe but was absent at northern latitudes. The reconstructions also reveal different millennial-scale temperature trends (NEur = - 0.73 degrees C/1000 years, CEur = - 0.13 degrees C, SEur = + 0.23 degrees C) potentially triggered by latitudinal changes in summer insolation due to orbital forcing. These features, the opposing millennial-scale temperature trends and the medieval multi-decadal cooling recorded in Central Europe and the Mediterranean, are not well captured in state-of-the-art climate model simulations.

  • 14. Feldhoff, Jan H.
    et al.
    Lange, Stefan
    Volkholz, Jan
    Donges, Jonathan F.
    Stockholm University, Faculty of Science, Stockholm Resilience Centre. Potsdam Institute for Climate Impact Research, Germany.
    Kurths, Juergen
    Gerstengarbe, Friedrich-Wilhelm
    Complex networks for climate model evaluation with application to statistical versus dynamical modeling of South American climate2015In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 44, no 06-maj, p. 1567-1581Article in journal (Refereed)
    Abstract [en]

    In this study we introduce two new node-weighted difference measures on complex networks as a tool for climate model evaluation. The approach facilitates the quantification of a model's ability to reproduce the spatial covariability structure of climatological time series. We apply our methodology to compare the performance of a statistical and a dynamical regional climate model simulating the South American climate, as represented by the variables 2 m temperature, precipitation, sea level pressure, and geopotential height field at 500 hPa. For each variable, networks are constructed from the model outputs and evaluated against a reference network, derived from the ERA-Interim reanalysis, which also drives the models. We compare two network characteristics, the (linear) adjacency structure and the (nonlinear) clustering structure, and relate our findings to conventional methods of model evaluation. To set a benchmark, we construct different types of random networks and compare them alongside the climate model networks. Our main findings are: (1) The linear network structure is better reproduced by the statistical model statistical analogue resampling scheme (STARS) in summer and winter for all variables except the geopotential height field, where the dynamical model CCLM prevails. (2) For the nonlinear comparison, the seasonal differences are more pronounced and CCLM performs almost as well as STARS in summer (except for sea level pressure), while STARS performs better in winter for all variables.

  • 15.
    Grand Graversen, Rune
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Wang, Minghuai
    Polar amplification in a coupled climate model with locked albedo2009In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 33, no 5, p. 629-643Article in journal (Refereed)
    Abstract [en]

    In recent years, a substantial reduction of the sea ice in the Arctic has been observed. At the same time, the near-surface air in this region is warming at a rate almost twice as large as the global average—this phenomenon is known as the Arctic amplification. The role of the ice-albedo feedback for the Arctic amplification is still a matter of debate. Here the effect of the surface-albedo feedback (SAF) was studied using a coupled climate model CCSM3 from the National Center for Atmospheric Research. Experiments, where the SAF was suppressed by locking the surface albedo in the entire coupled model system, were conducted. The results reveal polar temperature amplification when this model, with suppressed albedo, is forced by a doubling of the atmospheric CO2 content. Comparisons with variable albedo experiments show that SAF amplifies the surface-temperature response in the Arctic area by about 33%, whereas the corresponding value for the global-mean surface temperature is about 15%. Even though SAF is an important process underlying excessive warming at high latitudes, the Arctic amplification is only 15% larger in the variable than in the locked-albedo experiments. It is found that an increase of water vapour and total cloud cover lead to a greenhouse effect, which is larger in the Arctic than at lower latitudes. This is expected to explain a part of the Arctic surface–air-temperature amplification.

  • 16.
    Graversen, Rune
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Drijfhout, S.
    Hazeleger, W.
    van de Wal, R.
    Bintanja, R.
    Helsen, M.
    Greenland’s contribution to global sea-level rise by the end of the 21st century2011In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 37, p. 1427-1442Article in journal (Refereed)
    Abstract [en]

    The Greenland ice sheet holds enough water to raise the global sea level with *7 m. Over the last few decades, observations manifest a substantial increase of the mass loss of this ice sheet. Both enhanced melting and increase of the dynamical discharge, associated with calving at the outlet-glacier fronts, are contributing to the mass imbalance. Using a dynamical and thermodynamical ice-sheet model, and taking into account speed up of outlet glaciers, we estimate Greenland’s contribution to the 21stcentury global sea-level rise and the uncertainty of this estimate. Boundary fields of temperature and precipitation extracted from coupled climate-model projections used for the IPCC Fourth Assessment Report, are applied to the icesheet model. We implement a simple parameterization for increased flow of outlet glaciers, which decreases the bias of the modeled present-day surface height. It also allows for taking into account the observed recent increase in dynamical discharge, and it can be used for future projections associated with outlet-glacier speed up. Greenland contributes 0–17 cm to global sea-level rise by the end of the 21st century. This range includes the uncertainties in climate-model projections, the uncertainty associated with scenarios of greenhouse-gas emissions, as well as the uncertainties in future outlet-glacier discharge. In addition, the range takes into account the uncertainty of the ice-sheet model and its boundary fields.

  • 17. Graversen, Rune G.
    et al.
    Mauritsen, Thorsten
    Drijfhout, Sybren
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Mårtensson, Sebastian
    Stockholm University, Faculty of Science, Department of Meteorology .
    Warm winds from the Pacific caused extensive Arctic sea-ice melt in summer 20072011In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 36, no 11-12, p. 2103-2112Article in journal (Refereed)
    Abstract [en]

    During summer 2007 the Arctic sea-ice shrank to the lowest extent ever observed. The role of the atmospheric energy transport in this extreme melt event is explored using the state-of-the-art ERA-Interim reanalysis data. We find that in summer 2007 there was an anomalous atmospheric flow of warm and humid air into the region that suffered severe melt. This anomaly was larger than during any other year in the data (1989-2008). Convergence of the atmospheric energy transport over this area led to positive anomalies of the downward longwave radiation and turbulent fluxes. In the region that experienced unusual ice melt, the net anomaly of the surface fluxes provided enough extra energy to melt roughly one meter of ice during the melting season. When the ocean successively became ice-free, the surface-albedo decreased causing additional absorption of shortwave radiation, despite the fact that the downwelling solar radiation was smaller than average. We argue that the positive anomalies of net downward longwave radiation and turbulent fluxes played a key role in initiating the 2007 extreme ice melt, whereas the shortwave-radiation changes acted as an amplifying feedback mechanism in response to the melt.

  • 18.
    Grudd, H
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Torneträsk tree-ring width and density AD 500 – 2004: A test of climatic sensitivity and a new 1500-year reconstruction of north Fennoscandian summers2008In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 31, p. 843-857Article in journal (Refereed)
    Abstract [en]

    This paper presents updated tree-ring width(TRW) and maximum density (MXD) from Tornetra¨sk innorthern Sweden, now covering the period AD 500–2004. Byincluding data from relatively young trees for the most recentperiod, a previously noted decline in recent MXD is eliminated.Non-climatological growth trends in the data areremoved using Regional Curve Standardization (RCS), thusproducingTRWandMXDchronologies with preserved lowfrequencyvariability. The chronologies are calibrated usinglocal and regional instrumental climate records. A bootstrappedresponse function analysis using regional climatedata shows that tree growth is forced by April–August temperaturesand that the regression weights for MXD are muchstronger than for TRW. The robustness of the reconstructionequation is verified by independent temperature data andshows that 63–64% of the instrumental inter-annual variationis captured by the tree-ring data. This is a significantimprovement compared to previously published reconstructionsbased on tree-ring data from Tornetra¨sk. Adivergence phenomenon around AD 1800, expressed as anincrease in TRW that is not paralleled by temperature andMXD, is most likely an effect of major changes in the densityof the pine population at this northern tree-line site. The biasintroduced by this TRW phenomenon is assessed by producinga summer temperature reconstruction based onMXDexclusively. The new data show generally higher temperatureestimates than previous reconstructions based onTornetra¨sk tree-ring data. The late-twentieth century, however,is not exceptionally warm in the new record: Ondecadal-to-centennial timescales, periods around AD 750,1000, 1400, and 1750 were equally warm, or warmer. The200-year long warm period centered on AD 1000 was significantlywarmer than the late-twentieth century (p\0.05)and is supported by other local and regional paleoclimatedata. The new tree-ring evidence from Tornetra¨sk suggeststhat this ‘‘Medieval WarmPeriod’’ in northern Fennoscandiawas much warmer than previously recognized.

  • 19.
    Gunnarson, Björn
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Linderholm, Hans W.
    Moberg, Anders
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Improving a tree-ring reconstruction from west-central Scandinavia: 900 years of warm-season temperatures2011In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 36, no 1-2, p. 97-108Article in journal (Refereed)
    Abstract [en]

    Dendroclimatological sampling of Scots pine (Pinus sylvestris L.) has been made in the province of Jamtland, in the west-central Scandinavian mountains, since the 1970s. The tree-ring width (TRW) chronology spans several thousand years and has been used to reconstruct June August temperatures back to 1632 BC. A maximum latewood density (MXD) dataset, covering the period AD 1107-1827 (with gap 1292-1315) was presented in the 1980s by Fritz Schweingruber. Here we combine these historical MXD data with recently collected MXD data covering AD 1292-2006 into a single reconstruction of April September temperatures for the period AD 1107 2006. Regional curve standardization (RCS) provides more low-frequency variability than non-RCS and stronger correlation with local seasonal temperatures (51% variance explained). The MXD chronology shows a stronger relationship with temperatures than the TRW data, but the two chronologies show similar multi-decadal variations back to AD 1500. According to the MXD chronology, the period since AD 1930 and around AD 1150-1200 were the warmest during the last 900 years. Due to large uncertainties in the early part of the combined MXD chronology, it is not possible to conclude which period was the warmest. More sampling of trees growing near the tree-line is needed to further improve the MXD chronology.

  • 20.
    Han, Zixuan
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography. Lanzhou University, China.
    Su, Tao
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Wen, Qin
    Feng, Guolin
    Thermodynamic and dynamic effects of increased moisture sources over the Tropical Indian Ocean in recent decades2019In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 53, no 11, p. 7081-7096Article in journal (Refereed)
    Abstract [en]

    In the present work, the mechanisms for the changes in moisture sources (evaporation minus precipitation; EmP) during boreal summer (May-September) are explored over the tropical Indian Ocean during 1979-2016. We apply a moisture budget analysis to quantify the thermodynamic and dynamic effects. Our results show that the EmP in the tropical central-eastern and southwestern Indian Oceans experienced significant increasing trends during boreal summer. The increased EmP in the tropical central-eastern Indian Ocean is due to the enhanced dynamic divergence (account for approximately 51%), while a stronger dynamic advection contributes more moisture supply to the southwestern Indian Ocean (account for approximately 34%). We find that during recent decades, the enhanced east-west thermal gradient in the Pacific strengthens the Walker Circulation, which leads to a westward shift in convection over the Indian Ocean warm pool, resulting in weakened convection and ascent over the tropical central-eastern Indian Ocean. The weakened convection leads to an anomalous low-level atmospheric divergent circulation, which intensifies the dynamic divergence contributing to the enhanced EmP over the tropical central-eastern Indian Ocean. Additionally, the warming climate during recent decades also increases the land-sea thermal contrast in the vicinity of the Indian Ocean, which enhances the southeastern wind in the low-level troposphere and leads to an enhanced EmP over the southwestern Indian Ocean.

  • 21.
    Hannachi, Abdelwaheb
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Barnes, Elizabeth A.
    Woollings, Tim
    Behaviour of the winter North Atlantic eddy-driven jet stream in the CMIP3 integrations2013In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 41, no 3-4, p. 995-1007Article in journal (Refereed)
    Abstract [en]

    A systematic analysis of the winter North Atlantic eddy-driven jet stream latitude and wind speed from 52 model integrations, taken from the coupled model intercomparison project phase 3, is carried out and compared to results obtained from the ERA-40 reanalyses. We consider here a control simulation, twentieth century simulation, and two time periods (2046-2065 and 2081-2100) from a twenty-first century, high-emission A2 forced simulation. The jet wind speed seasonality is found to be similar between the twentieth century simulations and the ERA-40 reanalyses and also between the control and forced simulations although nearly half of the models overestimate the amplitude of the seasonal cycle. A systematic equatorward bias of the models jet latitude seasonality, by up to 7A degrees, is observed, and models additionally overestimate the seasonal cycle of jet latitude about the mean, with the majority of the models showing equatorward and poleward biases during the cold and warm seasons respectively. A main finding of this work is that no GCM under any forcing scenario considered here is able to simulate the trimodal behaviour of the observed jet latitude distribution. The models suffer from serious problems in the structure of jet variability, rather than just quantitiative errors in the statistical moments.

  • 22. Harrison, S. P.
    et al.
    Bartlein, P. J.
    Brewer, S.
    Prentice, I. C.
    Boyd, Meighan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Hessler, I.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Izumi, K.
    Willis, K.
    Climate model benchmarking with glacial and mid-Holocene climates2014In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 43, no 3-4, p. 671-688Article in journal (Refereed)
    Abstract [en]

    Past climates provide a test of models' ability to predict climate change. We present a comprehensive evaluation of state-of-the-art models against Last Glacial Maximum and mid-Holocene climates, using reconstructions of land and ocean climates and simulations from the Palaeoclimate Modelling and Coupled Modelling Intercomparison Projects. Newer models do not perform better than earlier versions despite higher resolution and complexity. Differences in climate sensitivity only weakly account for differences in model performance. In the glacial, models consistently underestimate land cooling (especially in winter) and overestimate ocean surface cooling (especially in the tropics). In the mid-Holocene, models generally underestimate the precipitation increase in the northern monsoon regions, and overestimate summer warming in central Eurasia. Models generally capture large-scale gradients of climate change but have more limited ability to reproduce spatial patterns. Despite these common biases, some models perform better than others.

  • 23.
    Hind, Alistair
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Moberg, Anders
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Past millennial solar forcing magnitude: A statistical hemispheric-scale climate model versus proxy data comparison2013In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 41, no 9-10, p. 2527-2537Article in journal (Refereed)
    Abstract [en]

    A set of global climatemodel simulations for the lastthousand years developed by the Max Planck Institute is comparedwith paleoclimate proxy data and instrumental data,focusing on surface temperatures for land areas between 30 and75N. The proxy data are obtained from six previously publishedNorthern Hemispheric-scale temperature reconstructions, here recalibratedfor consistency, which are compared with the simulationsutilizing a newly developed statistical framework forranking several competing simulations by means of their statisticaldistance against past climate variations. The climate modelsimulations are driven by either ‘‘low’’ or ‘‘high’’ solar forcingamplitudes (0.1 and 0.25 % smaller total solar irradiance in the Maunder Minimum period compared to the present) in addition toseveral other known climate forcings of importance. Our resultsindicate that the high solar forcing amplitude results in a poorermatch with the hemispheric-scale temperature reconstructionsand lends stronger statistical support for the low-amplitude solarforcing. However, results are likely conditional upon the sensitivityof the climate model used and strongly dependent on thechoice of temperature reconstruction, hence a greater consensus isneeded regarding the reconstruction of past temperatures as thiscurrently provides a great source of uncertainty.

  • 24. Hochman, Assaf
    et al.
    Alpert, Pinhas
    Kunin, Pavel
    Rostkier-Edelstein, Dorita
    Harpaz, Tzvi
    Saaroni, Hadas
    Messori, Gabriele
    Stockholm University, Faculty of Science, Department of Meteorology . Uppsala University, Sweden.
    The dynamics of cyclones in the twentyfirst century: the Eastern Mediterranean as an example2019In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894Article in journal (Refereed)
    Abstract [en]

    The Mediterranean region is projected to be significantly affected by climate change through warming and drying. The Eastern Mediterranean (EM) is particularly vulnerable since the bulk of the precipitation in the region is associated with a specific circulation pattern, known as Cyprus Low (CL). Here, we study the influence of increased greenhouse gases on the average properties and dynamics of CLs, using a regional semi-objective synoptic classification. The classification is applied to NCEP/NCAR reanalysis data for the present day (1986-2005) as well as to eight CMIP5 models for the present day and for the end of the century (2081-2100; RCP8.5). This is complemented by a dynamical systems analysis, which is used to investigate changes in the dynamics and intrinsic predictability of the CLs. Finally, a statistical downscaling algorithm, based on past analogues, is applied to eighteen rain stations over Israel, and is used to project precipitation changes associated with CLs. Significant changes in CL properties are found under climate change. The models project an increase in CL meridional pressure gradient (0.5-1.5 hPa/1000 km), which results primarily from a strong increase in the pressure over the southern part of the study region. Our results further point to a decrease in CL frequency (- 35%, as already noted in an earlier study) and persistence (- 8%). Furthermore, the daily precipitation associated with CL occurrences over Israel for 2081-2100 is projected to significantly reduce (- 26%). The projected drying over the EM can be partitioned between a decrease in CL frequency ( 137 mm year(-1)) and a reduction in CL-driven daily precipitation ( 67 mm year(-1)). The models further indicate that CLs will be less predictable in the future.

  • 25.
    Iqbal, Waheed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Leung, Wai-Nang
    Hannachi, Abdel
    Stockholm University, Faculty of Science, Department of Meteorology .
    Analysis of the variability of the North Atlantic eddy-driven jet stream in CMIP52018In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 51, no 1-2, p. 235-247Article in journal (Refereed)
    Abstract [en]

    The North Atlantic eddy-driven jet is a dominant feature of extratropical climate and its variability is associated with the large-scale changes in the surface climate of midlatitudes. Variability of this jet is analysed in a set of General Circulation Models (GCMs) from the Coupled Model Inter-comparison Project phase-5 (CMIP5) over the North Atlantic region. The CMIP5 simulations for the 20th century climate (Historical) are compared with the ERA40 reanalysis data. The jet latitude index, wind speed and jet persistence are analysed in order to evaluate 11 CMIP5 GCMs and to compare them with those from CMIP3 integrations. The phase of mean seasonal cycle of jet latitude and wind speed from historical runs of CMIP5 GCMs are comparable to ERA40. The wind speed mean seasonal cycle by CMIP5 GCMs is overestimated in winter months. A positive (negative) jet latitude anomaly in historical simulations relative to ERA40 is observed in summer (winter). The ensemble mean of jet latitude biases in historical simulations of CMIP3 and CMIP5 with respect to ERA40 are and respectively. Thus indicating improvements in CMIP5 in comparison to the CMIP3 GCMs. The comparison of historical and future simulations of CMIP5 under RCP4.5 and RCP8.5 for the period 2076-2099, shows positive anomalies in the jet latitude implying a poleward shifted jet. The results from the analysed models offer no specific improvements in simulating the trimodality of the eddy-driven jet.

  • 26. Jensen, Mari F.
    et al.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nisancioglu, Kerim H.
    The interaction between sea ice and salinity-dominated ocean circulation: implications for halocline stability and rapid changes of sea ice cover2016In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 47, no 9-10, p. 3301-3317Article in journal (Refereed)
    Abstract [en]

    Changes in the sea ice cover of the Nordic Seas have been proposed to play a key role for the dramatic temperature excursions associated with the Dansgaard-Oeschger events during the last glacial. In this study, we develop a simple conceptual model to examine how interactions between sea ice and oceanic heat and freshwater transports affect the stability of an upper-ocean halocline in a semi-enclosed basin. The model represents a sea ice covered and salinity stratified Nordic Seas, and consists of a sea ice component and a two-layer ocean. The sea ice thickness depends on the atmospheric energy fluxes as well as the ocean heat flux. We introduce a thickness-dependent sea ice export. Whether sea ice stabilizes or destabilizes against a freshwater perturbation is shown to depend on the representation of the diapycnal flow. In a system where the diapycnal flow increases with density differences, the sea ice acts as a positive feedback on a freshwater perturbation. If the diapycnal flow decreases with density differences, the sea ice acts as a negative feedback. However, both representations lead to a circulation that breaks down when the freshwater input at the surface is small. As a consequence, we get rapid changes in sea ice. In addition to low freshwater forcing, increasing deep-ocean temperatures promote instability and the disappearance of sea ice. Generally, the unstable state is reached before the vertical density difference disappears, and the temperature of the deep ocean do not need to increase as much as previously thought to provoke abrupt changes in sea ice.

  • 27.
    Kapsch, Marie-Luise
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Max-Planck Institute for Meteorology, Germany.
    Skific, Natasa
    Graversen, Rune G.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Francis, Jennifer A.
    Summers with low Arctic sea ice linked to persistence of spring atmospheric circulation patterns2019In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 52, no 3-4, p. 2497-2512Article in journal (Refereed)
    Abstract [en]

    The declining trend of Arctic September sea ice constitutes a significant change in the Arctic climate system. Large year-to-year variations are superimposed on this sea-ice trend, with the largest variability observed in the eastern Arctic Ocean. Knowledge of the processes important for this variability may lead to an improved understanding of seasonal and long-term changes. Previous studies suggest that transport of heat and moisture into the Arctic during spring enhances downward surface longwave radiation, thereby controlling the annual melt onset, setting the stage for the September ice minimum. In agreement with these studies, we find that years with a low September sea-ice concentration (SIC) are characterized by more persistent periods in spring with enhanced energy flux to the surface in forms of net longwave radiation plus turbulent fluxes, compared to years with a high SIC. Two main atmospheric circulation patterns related to these episodes are identified: one resembles the so-called Arctic dipole anomaly that promotes transport of heat and moisture from the North Pacific, whereas the other is characterized by negative geopotential height anomalies over the Arctic, favoring cyclonic flow from Siberia and the Kara Sea into the eastern Arctic Ocean. However, differences between years with low and high September SIC appear not to be due to different spring circulation patterns; instead it is the persistence and intensity of processes associated with these patterns that distinguish the two groups of anomalous years: Years with low September SIC feature episodes that are consistently stronger and more persistent than years with high SIC.

  • 28.
    Karlsson, Johannes
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    The simulation of Arctic clouds and their influence on the winter surface temperature in present-day climate in the CMIP3 multi-model dataset2011In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 36, no 3-4, p. 623-635Article in journal (Refereed)
    Abstract [en]

    We investigate the influence of clouds on the surface energy budget and surface temperature in the sea-ice covered parts of the ocean north of the Arctic circle in present-day climate in nine global climate models participating in the Coupled Model Intercomparison Project phase 3, CMIP3. Monthly mean simulated surface skin temperature, radiative fluxes and cloud parameters are evaluated using retrievals from the extended AVHHR Polar Pathfinder (APP-x) product. We analyzed the annual cycle but the main focus is on the winter, in which large parts of the region experience polar night. We find a smaller across-model spread as well as better agreement with observations during summer than during winter in the simulated climatological annual cycles of total cloudiness and surface skin temperature. The across-model spread in liquid and ice water paths is substantial during the whole year. These results qualitatively agree with earlier studies on the present-day Arctic climate in GCMs. The climatological ensemble model mean annual cycle of surface cloud forcing shows good agreement with observations in summer. However, during winter the insulating effect of clouds tends to be underestimated in models. During winter, most of the models as well as the observations show higher monthly mean total cloud fractions, associated with larger positive surface cloud forcing. Most models also show good correlation between the surface cloud forcing and the vertically integrated ice and liquid cloud condensate. The wintertime ensemble model mean total cloud fraction (69%) shows excellent agreement with observations. The across-model spread in the winter mean cloudiness is substantial (36-94%) however and several models significantly underestimate the cloud liquid water content. If the two models not showing any relationship between cloudiness and surface cloud forcing are disregarded, a tentative across-model relation exists, in such a way that models that simulate large winter mean cloudiness also show larger surface cloud forcing. Even though the across-model spread in wintertime surface cloud forcing is large, no clear relation to the surface temperature is found. This indicates that other processes, not explicitly cloud related, are important for the simulated across-model spread in surface temperature.

  • 29. Koenigk, Torben
    et al.
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Arctic climate and its interaction with lower latitudes under different levels of anthropogenic warming in a global coupled climate model2017In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 49, no 1-2, p. 471-492Article in journal (Refereed)
    Abstract [en]

    Three quasi-equilibrium simulations using constant greenhouse gas forcing corresponding to years 2000, 2015 and 2030 have been performed with the global coupled model EC-Earth in order to analyze the Arctic climate and interactions with lower latitudes under different levels of anthropogenic warming. The model simulations indicate an accelerated warming and ice extent reduction in the Arctic between the year-2030 and year-2015 simulations compared to the change between the year-2015 and year-2000 simulations. Both Arctic warming and sea ice reduction are closely linked to the increase of ocean heat transport into the Arctic, particularly through the Barents Sea Opening. Decadal variations of Arctic sea ice extent and ice volume are of the same order of magnitude as the observed ice extent reductions in the last 30 years and are dominated by the variability of the ocean heat transports through the Barents Sea Opening and the Bering Strait. Despite a general warming of mid and high northern latitudes, a substantial cooling is found in the subpolar gyre of the North Atlantic under year-2015 and year-2030 conditions. This cooling is related to a strong reduction in the AMOC, itself due to reduced deep water formation in the Labrador Sea. The observed trend towards a more negative phase of the North Atlantic Oscillation (NAO) and the observed linkage between autumn Arctic ice variations and NAO are reproduced in our model simulations for selected 30-year periods but are not robust over longer time periods. This indicates that the observed linkages between ice and NAO might not be robust in reality either, and that the observational time period is still too short to reliably separate the trend from the natural variability.

  • 30. Koenigk, Torben
    et al.
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ocean heat transport into the Arctic in the twentieth and twenty-first century in EC-Earth2013In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 42, no 11-12, p. 3101-3120Article in journal (Refereed)
    Abstract [en]

    The ocean heat transport into the Arctic and the heat budget of the Barents Sea are analyzed in an ensemble of historical and future climate simulations performed with the global coupled climate model EC-Earth. The zonally integrated northward heat flux in the ocean at 70°N is strongly enhanced and compensates for a reduction of its atmospheric counterpart in the twenty first century. Although an increase in the northward heat transport occurs through all of Fram Strait, Canadian Archipelago, Bering Strait and Barents Sea Opening, it is the latter which dominates the increase in ocean heat transport into the Arctic. Increased temperature of the northward transported Atlantic water masses are the main reason for the enhancement of the ocean heat transport. The natural variability in the heat transport into the Barents Sea is caused to the same extent by variations in temperature and volume transport. Large ocean heat transports lead to reduced ice and higher atmospheric temperature in the Barents Sea area and are related to the positive phase of the North Atlantic Oscillation. The net ocean heat transport into the Barents Sea grows until about year 2050. Thereafter, both heat and volume fluxes out of the Barents Sea through the section between Franz Josef Land and Novaya Zemlya are strongly enhanced and compensate for all further increase in the inflow through the Barents Sea Opening. Most of the heat transported by the ocean into the Barents Sea is passed to the atmosphere and contributes to warming of the atmosphere and Arctic temperature amplification. Latent and sensible heat fluxes are enhanced. Net surface long-wave and solar radiation are enhanced upward and downward, respectively and are almost compensating each other. We find that the changes in the surface heat fluxes are mainly caused by the vanishing sea ice in the twenty first century. The increasing ocean heat transport leads to enhanced bottom ice melt and to an extension of the area with bottom ice melt further northward. However, no indication for a substantial impact of the increased heat transport on ice melt in the Central Arctic is found. Most of the heat that is not passed to the atmosphere in the Barents Sea is stored in the Arctic intermediate layer of Atlantic water, which is increasingly pronounced in the twenty first century.

  • 31. Koenigk, Torben
    et al.
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Graversen, RuneGrand
    Stockholm University, Faculty of Science, Department of Meteorology .
    Karlsson, Johannes
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Willén, Ulrika
    Wyser, Klaus
    Arctic climate change in 21st century CMIP5 simulations with EC-Earth2012In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 40, no 11-12Article in journal (Refereed)
    Abstract [en]

    The Arctic climate change is analyzed in anensemble of future projection simulations performed withthe global coupled climate model EC-Earth2.3. EC-Earthsimulates the twentieth century Arctic climate relativelywell but the Arctic is about 2 K too cold and the sea icethickness and extent are overestimated. In the twenty-firstcentury, the results show a continuation and strengtheningof the Arctic trends observed over the recent decades,which leads to a dramatically changed Arctic climate,especially in the high emission scenario RCP8.5. Theannually averaged Arctic mean near-surface temperatureincreases by 12 K in RCP8.5, with largest warming in theBarents Sea region. The warming is most pronounced inwinter and autumn and in the lower atmosphere. The Arcticwinter temperature inversion is reduced in all scenarios anddisappears in RCP8.5. The Arctic becomes ice free inSeptember in all RCP8.5 simulations after a rapid reductionevent without recovery around year 2060. Taking intoaccount the overestimation of ice in the twentieth century,our model results indicate a likely ice-free Arctic inSeptember around 2040. Sea ice reductions are most pronouncedin the Barents Sea in all RCPs, which lead to themost dramatic changes in this region. Here, surface heatfluxes are strongly enhanced and the cloudiness is substantiallydecreased. The meridional heat flux into theArctic is reduced in the atmosphere but increases in theocean. This oceanic increase is dominated by an enhancedheat flux into the Barents Sea, which strongly contributes tothe large sea ice reduction and surface-air warming in thisregion. Increased precipitation and river runoff lead to morefreshwater input into the Arctic Ocean. However, most ofthe additional freshwater is stored in the Arctic Ocean whilethe total Arctic freshwater export only slightly increases.

  • 32.
    Koenigk, Torben
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden; Swedish e-Science Research Centre, Sweden.
    Caian, Mihaela
    Nikulin, Grigory
    Schimanke, Semjon
    Regional Arctic sea ice variations as predictor for winter climate conditions2016In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 46, no 1-2, p. 317-337Article in journal (Refereed)
    Abstract [en]

    Seasonal prediction skill of winter mid and high northern latitudes climate from sea ice variations in eight different Arctic regions is analyzed using detrended ERA-interim data and satellite sea ice data for the period 1980-2013. We find significant correlations between ice areas in both September and November and winter sea level pressure, air temperature and precipitation. The prediction skill is improved when using November sea ice conditions as predictor compared to September. This is particularly true for predicting winter NAO-like patterns and blocking situations in the Euro-Atlantic area. We find that sea ice variations in Barents Sea seem to be most important for the sign of the following winter NAO-negative after low ice-but amplitude and extension of the patterns are modulated by Greenland and Labrador Seas ice areas. November ice variability in the Greenland Sea provides the best prediction skill for central and western European temperature and ice variations in the Laptev/East Siberian Seas have the largest impact on the blocking number in the Euro-Atlantic region. Over North America, prediction skill is largest using September ice areas from the Pacific Arctic sector as predictor. Composite analyses of high and low regional autumn ice conditions reveal that the atmospheric response is not entirely linear suggesting changing predictive skill dependent on sign and amplitude of the anomaly. The results confirm the importance of realistic sea ice initial conditions for seasonal forecasts. However, correlations do seldom exceed 0.6 indicating that Arctic sea ice variations can only explain a part of winter climate variations in northern mid and high latitudes.

  • 33.
    Koenigk, Torben
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Gao, Y.
    Gastineau, G.
    Keenlyside, N.
    Nakamura, T.
    Ogawa, F.
    Orsolini, Y.
    Semenov, V.
    Suo, L.
    Tian, T.
    Wang, T.
    Wettstein, J. J.
    Yang, S.
    Impact of Arctic sea ice variations on winter temperature anomalies in northern hemispheric land areas2019In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 52, no 5-6, p. 3111-3137Article in journal (Refereed)
    Abstract [en]

    Coordinated numerical ensemble experiments with six different state-of-the-art atmosphere models have been used in order to evaluate the respective impact of the observed Arctic sea ice and sea surface temperature (SST) variations on air temperature variations in mid and high latitude land areas. Two sets of experiments have been designed; in the first set (EXP1), observed daily sea ice concentration and SST variations are used as lower boundary forcing over 1982-2014 while in the second set (EXP2) the SST variations are replaced by the daily SST climatology. The observed winter 2m air temperature (T2m) variations are relatively well reproduced in a number of mid and high latitude land areas in EXP1, with best agreement in southwestern North America and northern Europe. Sea ice variations are important for the interannual T2m variations in northern Europe but have limited impact on all other mid and high latitude land regions. In particular, sea ice variations do not contribute to the observed opposite variations in the Arctic and mid latitude in our model experiments. The spread across ensemble members is large and many ensemble members are required to reproduce the observed T2m variations over northern Europe in our models. The amplitude of T2m anomalies in the coldest observed winters over northern Europe is not reproduced by our multi-model ensemble means. However, the sea ice conditions in these respective winters and mainly the thermodynamic response to the ice anomalies lead to an enhanced likelihood for occurrence of colder than normal winters and extremely cold winters. Still, the main reason for the observed extreme cold winters is internal atmospheric dynamics. The coldest simulated northern European winters in EXP1 and EXP2 between 1982 and 2014 show the same large scale T2m and atmospheric circulation anomaly patterns as the observed coldest winters, indicating that the models are well able to reproduce the processes, which cause these cold anomalies. The results are robust across all six models used in this study.

  • 34.
    Latif, Muhammad
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . COMSATS Institute of Information Technology, Pakistan.
    Syed, F. S.
    Hannachi, Abdelwaheb
    Stockholm University, Faculty of Science, Department of Meteorology .
    Rainfall trends in the South Asian summer monsoon and its related large-scale dynamics with focus over Pakistan2017In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 48, no 11, p. 3565-3581Article in journal (Refereed)
    Abstract [en]

    The study of regional rainfall trends over South Asia is critically important for food security and economy, as both these factors largely depend on the availability of water. In this study, South Asian summer monsoon rainfall trends on seasonal and monthly (June-September) time scales have been investigated using three observational data sets. Our analysis identify a dipole-type structure in rainfall trends over the region north of the Indo-Pak subcontinent, with significant increasing trends over the core monsoon region of Pakistan and significant decreasing trends over the central-north India and adjacent areas. The dipole is also evident in monthly rainfall trend analyses, which is more prominent in July and August. We show, in particular, that the strengthening of northward moisture transport over the Arabian Sea is a likely reason for the significant positive trend of rainfall in the core monsoon region of Pakistan. In contrast, over the central-north India region, the rainfall trends are significantly decreasing due to the weakening of northward moisture transport over the Bay of Bengal. The leading empirical orthogonal functions clearly show the strengthening (weakening) patterns of vertically integrated moisture transport over the Arabian Sea (Bay of Bengal) in seasonal and monthly interannual time scales. The regression analysis between the principal components and rainfall confirm the dipole pattern over the region. Our results also suggest that the extra-tropical phenomena could influence the mean monsoon rainfall trends over Pakistan by enhancing the cross-equatorial flow of moisture into the Arabian Sea.

  • 35.
    Lewinschal, Anna
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Körnich, Heiner
    The role of precipitation in aerosol-induced changes in northern hemisphere wintertime stationary waves2013In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 41, no 3-4, p. 647-661Article in journal (Refereed)
    Abstract [en]

    The coupled climate model EC-Earth2 is used to investigate the impact of direct radiative effects of aerosols on stationary waves in the northern hemisphere wintertime circulation. The direct effect of aerosols is simulated by introducing prescribed mixing ratios of different aerosol compounds representing pre-industrial and present-day conditions, no indirect effects are included. In the EC-Earth2 results, the surface temperature response is uncorrelated with the highly asymmetric aerosol radiative forcing pattern. Instead, the anomalous extratropical temperature field bears a strong resemblance to the aerosol-induced changes in the stationary-wave pattern. It is demonstrated that the main features of the wave pattern of EC-Earth2 can be replicated by a linear, baroclinic model forced with latent heat changes corresponding to the anomalous convective precipitation generated by EC-Earth2. The tropical latent heat release is an effective means of generating stationary wave trains that propagate into the extratropics. Hence, the results of the present study indicate that aerosol-induced convective precipitation anomalies govern the extratropical wave-field changes, and that the far-field temperature response dominates over local effects of aerosol radiative forcing.

  • 36.
    Liakka, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Löfverström, Marcus
    Stockholm University, Faculty of Science, Department of Meteorology .
    Interactions between stationary waves and ice sheets: linear versus nonlinear atmospheric response2012In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 38, no 5-6, p. 1249-1262Article in journal (Refereed)
    Abstract [en]

    This study examines the mutual interaction between topographically-forced atmospheric stationary waves and continental-scale ice sheets using a thermomechanical ice-sheet model coupled to a linear as well as a fully-nonlinear dry atmospheric primitive equation model. The focus is on how the stationary-wave induced ablation feeds back on the ice sheet. Simulations are conducted in which an embryonal ice mass, on an idealised “North American” continent, evolves to an equilibrium ice sheet. Under the coupling to the linear atmospheric model, the equilibrium ice sheet is primarily controlled by the ratio between the wavelength of the stationary waves and the zonal continental extent. When this ratio is near two, the ice sheet has its center of mass shifted far eastward and its shape is broadly reminiscent of the Laurentide ice sheet at LGM. For wavelengths comparable to the continental extent, however, the ice margin extends far equatorward on the central continent but is displaced poleward near the eastern coast. Remarkably, the coupling to the nonlinear atmospheric model yields equilibrium ice sheets that are virtually identical to the ones obtained in uncoupled simulations, i.e. a symmetric ice sheet with a zonal southern margin. Thus, the degree of linearity of the atmospheric response should control to what extent topographically-forced stationary waves can reorganise the structure of ice sheets. If the stationary-wave response is linear, the present results suggest that spatial reconstructions of past ice sheets can provide some information on the zonal-mean atmospheric circulation that prevailed.

  • 37. Linderholm, Hans W.
    et al.
    Björklund, Jesper
    Seftigen, Kristina
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Fuentes, Mauricio
    Fennoscandia revisited: a spatially improved tree-ring reconstruction of summer temperatures for the last 900 years2015In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 45, no 3-4, p. 933-947Article in journal (Refereed)
    Abstract [en]

    Despite the spatially homogenous summer temperature pattern in Fennoscandia, there are large spreads among the many existing reconstructions, resulting in an uncertainty in the timing and amplitude of past changes. Also, there has been a general bias towards northernmost Fennoscandia. In an attempt to provide a more spatially coherent view of summer (June-August, JJA) temperature variability within the last millennium, we utilized seven density and three blue intensity Scots pine (Pinus sylvestris L.) chronologies collected from the altitudinal (Scandinavian Mountains) and latitudinal (northernmost part) treeline. To attain a JJA temperature signal as strong as possible, as well as preserving multicentury-scale variability, we used a new tree-ring parameter, where the earlywood information is removed from the maximum density and blue intensity, and a modified signal-free standardization method. Two skilful reconstructions for the period 11002006 CE were made, one regional reconstruction based on an average of the chronologies, and one field (gridded) reconstruction. The new reconstructions were shown to have much improved spatial representations compared to those based on data from only northern sites, thus making it more valid for the whole region. An examination of some of the forcings of JJA mean temperatures in the region shows an association with sea-surface temperature over the eastern North Atlantic, but also the subpolar and subtropical gyres. Moreover, using Superposed Epoch Analysis, a significant cooling in the year following a volcanic eruption was noted, and for the largest explosive eruptions, the effect could remain for up to 4 years. This new improved reconstruction provides a mean to reinforce our understanding of forcings on summer temperatures in the North European sector.

  • 38.
    Lindvall, Jenny
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    The diurnal temperature range in the CMIP5 models2015In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 44, no 1-2, p. 405-421Article in journal (Refereed)
    Abstract [en]

    This paper analyzes the diurnal temperature range (DTR) over land in simulations of the recent past and in future projections by 20 models participating in the Coupled Model Intercomparison Project phase 5 (CMIP5). The annually averaged DTR is evaluated for the present-day climate using two gridded datasets (HadGHCND and CRU). The DTR varies substantially between different CMIP5 models, particularly in the subtropics, and is generally underestimated. In future projections of the high emission scenario RCP8.5, the models disagree on both the sign and the magnitude of the change in DTR. Still, a majority of the models project a globally averaged reduction in the DTR, with an increase over Europe, a decrease over the Sahara desert and a substantial decrease in DTR at high latitudes in winter. The general DTR reduction is partly linked to the enhancement of the downwelling clear sky longwave radiation due to greenhouse gases. At high latitudes in winter, the decrease in DTR seems to be enforced by an increase in cloudiness, but in most other regions counteracted by decreases in cloud fraction. Changes in the hydrological cycle and in the clear sky shortwave radiation also impact the DTR. The DTR integrates many processes and neither the model differences in the DTR nor in the change in DTR can be attributed to a single parameter. Which variables that impact the model discrepancies vary both regionally and seasonally. However, clouds seem to matter in most regions and seasons and the evaporative fraction is important in summer.

  • 39.
    Lindvall, Jenny
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden.
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden.
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden.
    The impact of changes in parameterizations of surface drag and vertical diffusion on the large-scale circulation in the Community Atmosphere Model (CAM5)2017In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 48, no 11, p. 3741-3758Article in journal (Refereed)
    Abstract [en]

    Simulations with the Community Atmosphere Model version 5 (CAM5) are used to analyze the sensitivity of the large-scale circulation to changes in parameterizations of orographic surface drag and vertical diffusion. Many GCMs and NWP models use enhanced turbulent mixing in stable conditions to improve simulations, while CAM5 cuts off all turbulence at high stabilities and instead employs a strong orographic surface stress parameterization, known as turbulent mountain stress (TMS). TMS completely dominates the surface stress over land and reduces the near-surface wind speeds compared to simulations without TMS. It is found that TMS is generally beneficial for the large-scale circulation as it improves zonal wind speeds, Arctic sea level pressure and zonal anomalies of the 500-hPa stream function, compared to ERA-Interim. It also alleviates atmospheric blocking frequency biases in the Northern Hemisphere. Using a scheme that instead allows for a modest increase of turbulent diffusion at higher stabilities only in the planetary boundary layer (PBL) appears to in some aspects have a similar, although much smaller, beneficial effect as TMS. Enhanced mixing throughout the atmospheric column, however, degrades the CAM5 simulation. Evaluating the simulations in comparison with detailed measurements at two locations reveals that TMS is detrimental for the PBL at the flat grassland ARM Southern Great Plains site, giving too strong wind turning and too deep PBLs. At the Sodankyla forest site, the effect of TMS is smaller due to the larger local vegetation roughness. At both sites, all simulations substantially overestimate the boundary layer ageostrophic flow.

  • 40.
    Mauritsen, Thorsten
    et al.
    Max Planck Inst Meteorol, Hamburg, Germany.
    Graversen, Rune Grand
    Stockholm University, Faculty of Science, Department of Meteorology .
    Klocke, Daniel
    European Ctr Medium Range Weather Forecasts, Reading, Berks, England.
    Langen, Peter L.
    DMI, Copenhagen, Denmark.
    Stevens, Bjorn
    DMI, Copenhagen, Denmark.
    Tomassini, Lorenzo
    DMI, Copenhagen, Denmark.
    Climate feedback efficiency and synergy2013In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 41, no 9-10, p. 2539-2554Article in journal (Refereed)
    Abstract [en]

    Earth's climate sensitivity to radiative forcing induced by a doubling of the atmospheric CO2 is determined by feedback mechanisms, including changes in atmospheric water vapor, clouds and surface albedo, that act to either amplify or dampen the response. The climate system is frequently interpreted in terms of a simple energy balance model, in which it is assumed that individual feedback mechanisms are additive and act independently. Here we test these assumptions by systematically controlling, or locking, the radiative feedbacks in a state-of-the-art climate model. The method is shown to yield a near-perfect decomposition of change into partial temperature contributions pertaining to forcing and each of the feedbacks. In the studied model water vapor feedback stands for about half the temperature change, CO2-forcing about one third, while cloud and surface albedo feedback contributions are relatively small. We find a close correspondence between forcing, feedback and partial surface temperature response for the water vapor and surface albedo feedbacks, while the cloud feedback is inefficient in inducing surface temperature change. Analysis suggests that cloud-induced warming in the upper tropical troposphere, consistent with rising convective cloud anvils in a warming climate enhances the negative lapse-rate feedback, thereby offsetting some of the warming that would otherwise be attributable to this positive cloud feedback. By subsequently combining feedback mechanisms we find a positive synergy acting between the water vapor feedback and the cloud feedback; that is, the combined cloud and water vapor feedback is greater than the sum of its parts. Negative synergies surround the surface albedo feedback, as associated cloud and water vapor changes dampen the anticipated climate change induced by retreating snow and ice. Our results highlight the importance of treating the coupling between clouds, water vapor and temperature in a deepening troposphere.

  • 41. Meier, H. E. M.
    et al.
    Hordoir, R.
    Andersson, H. C.
    Dieterich, C.
    Eilola, K.
    Gustafsson, Bo G.
    Stockholm University, Stockholm Resilience Centre, Baltic Nest Institute.
    Hoglund, A.
    Schimanke, S.
    modeling the combined impact of changing climate and changing nutrient loads on the baltic sea environment in an ensemble of transient simulations for 1961 20992012In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 39, no 9-10, p. 2421-2441Article in journal (Refereed)
    Abstract [en]

    The combined future impacts of climate change and industrial and agricultural practices in the Baltic Sea catchment on the Baltic Sea ecosystem were assessed. For this purpose 16 transient simulations for 1961-2099 using a coupled physical-biogeochemical model of the Baltic Sea were performed. Four climate scenarios were combined with four nutrient load scenarios ranging from a pessimistic business-as-usual to a more optimistic case following the Baltic Sea Action Plan (BSAP). Annual and seasonal mean changes of climate parameters and ecological quality indicators describing the environmental status of the Baltic Sea like bottom oxygen, nutrient and phytoplankton concentrations and Secchi depths were studied. Assuming present-day nutrient concentrations in the rivers, nutrient loads from land increase during the twenty first century in all investigated scenario simulations due to increased volume flows caused by increased net precipitation in the Baltic catchment area. In addition, remineralization rates increase due to increased water temperatures causing enhanced nutrient flows from the sediments. Cause-and-effect studies suggest that both processes may play an important role for the biogeochemistry of eutrophicated seas in future climate partly counteracting nutrient load reduction efforts like the BSAP.

  • 42.
    Messori, Gabriele
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Davini, Paolo
    Alvarez-Castro, M. Carmen
    Pausata, Francesco S. R.
    Stockholm University, Faculty of Science, Department of Meteorology . Université du Québec à Montréal, Canada.
    Yiou, Pascal
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    On the low-frequency variability of wintertime Euro-Atlantic planetary wave-breaking2019In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 52, no 3-4, p. 2431-2450Article in journal (Refereed)
    Abstract [en]

    Planetary wave-breaking can lead to large-scale atmospheric circulation anomalies and favour high-impact weather occurrences. For example, the simultaneous occurrence of anti-cyclonic wave-breaking to the south of the North Atlantic jet and cyclonic wave-breaking to the north, here termed double wave-breaking, has been linked to heightened frequencies of explosive cyclones in the Atlantic basin and destructive windstorms over Western and Continental Europe. The present study analyses the long-term temporal variability of wintertime cyclonic and anti-cyclonic wave-breaking, and the resulting double wave-breaking, in the North Atlantic. We use reanalysis data, proxy reconstructions of the North Atlantic Oscillation (NAO) and a 1000-year coupled global climate model equilibrium simulation under constant pre-industrial forcing. The wave-breaking wavelet spectra highlight a significant ultra-centennial variability in double wave-breaking frequency, which is largely mirrored in the variability of the NAO. However, we note that the NAO wavelet spectra in the different datasets display significant discrepancies. The low-frequency wave-breaking variability is reflected in long-term anomalies of the large-scale atmospheric circulation in the Euro-Atlantic sector. The 100-year periods with the most and least double wave-breaking occurrences display significant and opposite anomalies in both upper and lower-level wind, as well as in the frequency of extreme temperature events and in the magnitude of wind destructiveness over Europe. The latter broadly resembles the wind destructiveness anomalies associated with individual double wave-breaking instances in reanalysis data. The existence of low-frequency variability in an atmospheric pattern related to high-impact weather events has important implications for the study and interpretation of climate change projections and of possible future NAO changes.

  • 43.
    Mortin, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Graversen, Rune G.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Evaluation of pan-Arctic melt-freeze onset in CMIP5 climate models and reanalyses using surface observations2014In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 42, no 7-8, p. 2239-2257Article in journal (Refereed)
    Abstract [en]

    The seasonal melt-freeze transitions are fun- damental features of the Arctic climate system. The representation of the pan-Arctic melt and freeze onset (north of 60°N) is assessed in two reanalyses and eleven CMIP5 global circulation models (GCMs). The seasonal melt-freeze transitions are retrieved from surface air temperature (SAT) across the land and sea-ice domains and evaluated against surface observations. While monthly averages of SAT are reasonably well represented in models, large model-observation and model–model disparities of timing of melt and freeze onset are evident. The evaluation against surface observations reveals that the ERA-Interim reanalysis performs the best, closely followed by some of the climate models. GCMs and reanalyses capture the seasonal melt-freeze transitions better in the central Arctic than in the marginal seas and across the land areas. The GCMs project that during the 21st century, the summer length—the period between melt and freeze onset—will increase over land by about 1 month at all latitudes, and over sea ice by 1 and 3 months at low and high latitudes, respectively. This larger summer-length increase over sea ice at pro- gressively higher latitudes is related to a retreat of summer sea ice during the 21st century, since open water freezes roughly 40 days later than ice-covered ocean. As a consequence, by the year 2100, the freeze onset is projected to be initiated within roughly 10 days across the whole Arctic Ocean, whereas this transition varies by about 80 days today.

  • 44. Rinke, Annette
    et al.
    Dethloff, Klaus
    Cassano, John
    Christensen, Jens
    Curry, Judy
    Du, P
    Girard, Eric
    Haugen, J-E
    Jacob, Daniela
    Jones, Colin
    Kaltzow, M
    Laprise, Rene
    Serreze, Mark
    Shaw, M
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Wyser, Klaus
    Zagar, Mark
    Stockholm University, Faculty of Science, Department of Meteorology .
    Evaluation of an ensemble of Arctic regional climate models: Spatiotemporal fields during the SHEBA year2006In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 26, p. 459-472Article in journal (Refereed)
  • 45. Rohrschneider, Tim
    et al.
    Stevens, Bjorn
    Mauritsen, Thorsten
    Stockholm University, Faculty of Science, Department of Meteorology .
    On simple representations of the climate response to external radiative forcing2019In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 53, no 5-6, p. 3131-3145Article in journal (Refereed)
    Abstract [en]

    Global warming in response to external radiative forcing is determined by the feedback of the climate system. Recent studies have suggested that simple mathematical models incorporating a radiative response which is related to upper- and deep-ocean disequilibrium (ocean heat uptake efficacy), inhomogeneous patterns of surface warming and radiative feedbacks (pattern effect), or an explicit dependence of the strength of radiative feedbacks on surface temperature change (feedback temperature dependence) may explain the climate response in atmosphere-ocean coupled general circulation models (AOGCMs) or can be useful for interpreting the instrumental record. We analyze a two-layer model with an ocean heat transport efficacy, a two-region model with region specific heat capacities and radiative responses; a one-layer model with a temperature dependent feedback; and a model which combines elements of the two-layer/region models and the state-dependent feedback parameter. We show that, from the perspective of the globally averaged surface temperature and radiative imbalance, the two-region and two-layer models are equivalent. State-dependence of the feedback parameter introduces a nonlinearity in the system which makes the adjustment timescales forcing-dependent. Neither the linear two-region/layer models, nor the state-dependent feedback model adequately describes the behavior of complex climate models. The model which combines elements of both can adequately describe the response of more comprehensive models but may require more experimental input than is available from single forcing realizations.

  • 46. Rydval, Miloš
    et al.
    Loader, Neil J.
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Druckenbrod, Daniel L.
    Linderholm, Hans W.
    Moreton, Steven G.
    Wood, Cheryl V.
    Wilson, Rob
    Reconstructing 800 years of summer temperatures in Scotland from tree rings2017In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 49, no 9-10, p. 2951-2974Article in journal (Refereed)
    Abstract [en]

    This study presents a summer temperature reconstruction using Scots pine tree-ring chronologies for Scotland allowing the placement of current regional temperature changes in a longer-term context. 'Living-tree' chronologies were extended using 'subfossil' samples extracted from nearshore lake sediments resulting in a composite chronology > 800 years in length. The North Cairngorms (NCAIRN) reconstruction was developed from a set of composite blue intensity high-pass and ring-width low-pass filtered chronologies with a range of detrending and disturbance correction procedures. Calibration against July-August mean temperature explains 56.4% of the instrumental data variance over 1866-2009 and is well verified. Spatial correlations reveal strong coherence with temperatures over the British Isles, parts of western Europe, southern Scandinavia and northern parts of the Iberian Peninsula. NCAIRN suggests that the recent summer-time warming in Scotland is likely not unique when compared to multi-decadal warm periods observed in the 1300s, 1500s, and 1730s, although trends before the mid-sixteenth century should be interpreted with some caution due to greater uncertainty. Prominent cold periods were identified from the sixteenth century until the early 1800s-agreeing with the so-called Little Ice Age observed in other tree-ring reconstructions from Europe-with the 1690s identified as the coldest decade in the record. The reconstruction shows a significant cooling response 1 year following volcanic eruptions although this result is sensitive to the datasets used to identify such events. In fact, the extreme cold (and warm) years observed in NCAIRN appear more related to internal forcing of the summer North Atlantic Oscillation.

  • 47. Schleussner, C. -F.
    et al.
    Divine, D. V.
    Donges, Jonathan F.
    Stockholm University, Faculty of Science, Stockholm Resilience Centre. Potsdam Institute for Climate Impact Research, Germany.
    Miettinen, A.
    Donner, R. V.
    Indications for a North Atlantic ocean circulation regime shift at the onset of the Little Ice Age2015In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 45, no 11-12, p. 3623-3633Article in journal (Refereed)
    Abstract [en]

    A prominent characteristic of the reconstructed Northern Hemisphere temperature signal over the last millennium is the transition from the Medieval Climate Anomaly to the Little Ice Age (LIA). Here we report indications for a non-linear regime shift in the North Atlantic ocean circulation at the onset of the LIA. Specifically, we apply a novel statistical test based on horizontal visibility graphs to two ocean sediment August sea-surface temperature records from the Norwegian Sea and the central subpolar basin and find robust indications of time-irreversibility in both records during the LIA onset. Despite a basin-wide cooling trend, we report an anomalous warming in the central subpolar basin during the LIA that is reproduced in ensemble simulations with the climate model of intermediate complexity CLIMBER-3 as a result of a non-linear regime shift in the subpolar North Atlantic ocean circulation. The identified volcanically triggered non-linear transition in the model simulations provides a plausible explanation for the signatures of time-irreversibility found in the ocean sediment records. Our findings indicate a potential multi-stability of the North Atlantic ocean circulation and its importance for regional climate change on centennial time scales.

  • 48.
    Sedlar, Joseph
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Mauritsen, T.
    Shupe, M.D.
    Brooks, I.M.
    Persson, P.O.G.
    Birch, C.E.
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sirevaag, A.
    Nicolaus, M.
    A transitioning Arctic surface energy budget: the impacts of solar zenith angle, surface albedo and cloud radiative forcing2011In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 37, no 7-8, p. 1643-1660Article in journal (Refereed)
    Abstract [en]

    Snow surface and sea-ice energy budgets were measured near 87.5A degrees N during the Arctic Summer Cloud Ocean Study (ASCOS), from August to early September 2008. Surface temperature indicated four distinct temperature regimes, characterized by varying cloud, thermodynamic and solar properties. An initial warm, melt-season regime was interrupted by a 3-day cold regime where temperatures dropped from near zero to -7A degrees C. Subsequently mean energy budget residuals remained small and near zero for 1 week until once again temperatures dropped rapidly and the energy budget residuals became negative. Energy budget transitions were dominated by the net radiative fluxes, largely controlled by the cloudiness. Variable heat, moisture and cloud distributions were associated with changing air-masses. Surface cloud radiative forcing, the net radiative effect of clouds on the surface relative to clear skies, is estimated. Shortwave cloud forcing ranged between -50 W m(-2) and zero and varied significantly with surface albedo, solar zenith angle and cloud liquid water. Longwave cloud forcing was larger and generally ranged between 65 and 85 W m(-2), except when the cloud fraction was tenuous or contained little liquid water; thus the net effect of the clouds was to warm the surface. Both cold periods occurred under tenuous, or altogether absent, low-level clouds containing little liquid water, effectively reducing the cloud greenhouse effect. Freeze-up progression was enhanced by a combination of increasing solar zenith angles and surface albedo, while inhibited by a large, positive surface cloud forcing until a new air-mass with considerably less cloudiness advected over the experiment area.

  • 49. Sterl, Andreas
    et al.
    Bintanja, Richard
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Gleeson, Emily
    Koenigk, Torben
    Schmith, Torben
    Semmler, Tido
    Severijns, Camiel
    Wyser, Klaus
    Yang, Shuting
    A look at the ocean in the EC-Earth climate model2012In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 39, no 11, p. 2631-2657Article in journal (Refereed)
    Abstract [en]

    EC-Earth is a newly developed global climate system model. Its core components are the Integrated Forecast System (IFS) of the European Centre for Medium Range Weather Forecasts (ECMWF) as the atmosphere component and the Nucleus for European Modelling of the Ocean (NEMO) developed by Institute Pierre Simon Laplace (IPSL) as the ocean component. Both components are used with a horizontal resolution of roughly one degree. In this paper we describe the performance of NEMO in the coupled system by comparing model output with ocean observations. We concentrate on the surface ocean and mass transports. It appears that in general the model has a cold and fresh bias, but a much too warm Southern Ocean. While sea ice concentration and extent have realistic values, the ice tends to be too thick along the Siberian coast. Transports through important straits have realistic values, but generally are at the lower end of the range of observational estimates. Exceptions are very narrow straits (Gibraltar, Bering) which are too wide due to the limited resolution. Consequently the modelled transports through them are too high. The strength of the Atlantic meridional overturning circulation is also at the lower end of observational estimates. The interannual variability of key variables and correlations between them are realistic in size and pattern. This is especially true for the variability of surface temperature in the tropical Pacific (El Nio). Overall the ocean component of EC-Earth performs well and helps making EC-Earth a reliable climate model.

  • 50.
    Syed, Faisal Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kornich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    On the fog variability over south Asia2012In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 39, no 12, p. 2993-3005Article in journal (Refereed)
    Abstract [en]

    An increasing trend in fog frequencies over south Asia during winter in the last few decades has resulted in large economical losses and has caused substantial difficulties in the daily lives of people. In order to better understand the fog phenomenon, we investigated the climatology, inter-annual variability and trends in the fog occurrence from 1976 to 2010 using observational data from 82 stations, well distributed over India and Pakistan. Fog blankets large area from Pakistan to Bangladesh across north India from west to east running almost parallel to south of the Himalayas. An EOF analysis revealed that the fog variability over the whole region is coupled and therefore must be governed by some large scale phenomenon on the inter-annual time scale. Significant positive trends were found in the fog frequency but this increase is not gradual, as with the humidity, but comprises of two distinct regimes shifts, in 1990 and 1998, with respect to both mean and variance. The fog is also detected in ERA-Interim 3 hourly, surface and model level forecast data when using the concept of cross-over temperature combined with boundary layer stability. This fog index is able to reproduce the regime shift around 1998 and shows that the method can be applied to analyze fog over south Asia. The inter-annual variability seems to be associated with the wave train originating from the North Atlantic in the upper troposphere that when causing higher pressure over the region results in an increased boundary layer stability and surface-near relative humidity. The trend and shifts in the fog occurrence seems to be associated with the gradual increasing trend in relative humidity from 1990 onwards.

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