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  • 1.
    Chen, Hans W.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Penn State University .
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Meteorology . Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute.
    Chen, Deliang
    A robust mode of climate variability in the Arctic: The Barents Oscillation2013In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 11, 2856-2861 p.Article in journal (Refereed)
    Abstract [en]

    The Barents Oscillation (BO) is an anomalous wintertime atmospheric circulation pattern in the Northern Hemisphere that has been linked to the meridional flow over the Nordic Seas. There are speculations that the BO has important implications for the Arctic climate; however, it has also been suggested that the pattern is an artifact of Empirical Orthogonal Function (EOF) analysis due to an eastward shift of the Arctic Oscillation/North Atlantic Oscillation (AO/NAO). In this study, EOF analyses are performed to show that a robust pattern resembling the BO can be found during different time periods, even when the AO/NAO is relatively stationary. This BO has a high and stable temporal correlation with the geostrophic zonal wind over the Barents Sea, while the contribution from the AO/NAO is small. The surface air temperature anomalies over the Barents Sea are closely associated with this mode of climate variability.

  • 2.
    Finné, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Bar-Matthews, Miryam
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Sundqvist, Hanna S.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Liakopoulos, Ilias
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Speleothem evidence for late Holocene climate variability and floods in Southern Greece2014In: Quaternary Research, ISSN 0033-5894, E-ISSN 1096-0287, Vol. 81, no 2, 213-227 p.Article in journal (Refereed)
    Abstract [en]

    We present stable isotope data (delta O-18, delta C-13) from a detrital rich stalagmite from Kapsia Cave, the Peloponnese, Greece. The cave is rich in archeological remains and there are reasons to believe that flooding of the cave has directly affected humans using the cave. Using a combination of U-Th and C-14 dating to constrain a site-specific correction factor for (Th-232/U-238) detrital molar ratio, a linear age model was constructed. The age model shows that the stalagmite grew during the period from ca. 950 BC to ca. AD 830. The stable oxygen record from Kapsia indicates cyclical changes of close to 500 yr in precipitation amount, with rapid shifts towards wetter conditions followed by slowly developing aridity. Superimposed on this signal, wetter conditions are inferred around 850, 700, 500 and 400-100 BC, and around AD 160-300 and AD 770; and driest conditions are inferred to have occurred around 450 BC, AD 100-150 and AD 650. Detrital horizons in the stalagmite indicate that three major floods took place in the cave at 500 BC, 70 BC and AD 450. The stable carbon isotope record reflects changes in biological activity being a result of both climate and human activities. (c) 2014 University of Washington.

  • 3. Gaetani, Marco
    et al.
    Messori, Gabriele
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Flamant, Cyrille
    Pausata, Francesco S. R.
    Stockholm University, Faculty of Science, Department of Meteorology . University of Quebec in Montreal, Canada.
    Understanding the Mechanisms behind the Northward Extension of the West African Monsoon during the Mid-Holocene2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 19, 7621-7642 p.Article in journal (Refereed)
    Abstract [en]

    Understanding the West African monsoon (WAM) dynamics in the mid-Holocene (MH) is a crucial issue in climate modeling, because numerical models typically fail to reproduce the extensive precipitation suggested by proxy evidence. This discrepancy may be largely due to the assumption of both unrealistic land surface cover and atmospheric aerosol concentration. In this study, the MH environment is simulated in numerical experiments by imposing extensive vegetation over the Sahara and the consequent reduction in airborne dust concentration. A dramatic increase in precipitation is simulated across the whole of West Africa, up to the Mediterranean coast. This precipitation response is in better agreement with proxy data, in comparison with the case in which only changes in orbital forcing are considered. Results show a substantial modification of the monsoonal circulation, characterized by an intensification of large-scale deep convection through the entire Sahara, and a weakening and northward shift (similar to 6.5 degrees) of the African easterly jet. The greening of the Sahara also leads to a substantial reduction in the African easterly wave activity and associated precipitation. The reorganization of the regional atmospheric circulation is driven by the vegetation effect on radiative forcing and associated heat fluxes, with the reduction in dust concentration to enhance this response. The results for the WAM in the MH present important implications for understanding future climate scenarios in the region and in teleconnected areas, in the context of projected wetter conditions in West Africa.

  • 4. Helsen, Michiel M.
    et al.
    van de Wal, Roderik S. W.
    Reerink, Thomas J.
    Bintanja, Richard
    Madsen, Marianne S.
    Yang, Shuting
    Li, Qiang
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    On the importance of the albedo parameterization for the mass balance of the Greenland ice sheet in EC-Earth2017In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 11, no 4, 1949-1965 p.Article in journal (Refereed)
    Abstract [en]

    The albedo of the surface of ice sheets changes as a function of time due to the effects of deposition of new snow, ageing of dry snow, bare ice exposure, melting and run-off. Currently, the calculation of the albedo of ice sheets is highly parameterized within the earth system model EC-Earth by taking a constant value for areas with thick perennial snow cover. This is an important reason why the surface mass balance (SMB) of the Greenland ice sheet (GrIS) is poorly resolved in the model. The purpose of this study is to improve the SMB forcing of the GrIS by evaluating different parameter settings within a snow albedo scheme. By allowing ice-sheet albedo to vary as a function of wet and dry conditions, the spatial distribution of albedo and melt rate improves. Nevertheless, the spatial distribution of SMB in EC-Earth is not significantly improved. As a reason for this, we identify omissions in the current snow albedo scheme, such as separate treatment of snow and ice and the effect of refreezing. The resulting SMB is downscaled from the lower-resolution global climate model topography to the higher-resolution ice-sheet topography of the GrIS, such that the influence of these different SMB climatologies on the long-term evolution of the GrIS is tested by ice-sheet model simulations. From these ice-sheet simulations we conclude that an albedo scheme with a short response time of decaying albedo during wet conditions performs best with respect to long-term simulated ice-sheet volume. This results in an optimized albedo parameterization that can be used in future EC-Earth simulations with an interactive ice-sheet component.

  • 5.
    Hind, Alistair
    et al.
    Stockholm University, Faculty of Science, Department of Mathematics. Stockholm University, Faculty of Science, Department of Physical Geography.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Brattström, Gudrun
    Stockholm University, Faculty of Science, Department of Mathematics.
    Problems encountered when defining Arctic amplification as a ratio2016In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, 30469Article in journal (Refereed)
    Abstract [en]

    In climate change science the term 'Arctic amplification' has become synonymous with an estimation of the ratio of a change in Arctic temperatures compared with a broader reference change under the same period, usually in global temperatures. Here, it is shown that this definition of Arctic amplification comes with a suite of difficulties related to the statistical properties of the ratio estimator itself. Most problematic is the complexity of categorizing uncertainty in Arctic amplification when the global, or reference, change in temperature is close to 0 over a period of interest, in which case it may be impossible to set bounds on this uncertainty. An important conceptual distinction is made between the 'Ratio of Means' and 'Mean Ratio' approaches to defining a ratio estimate of Arctic amplification, as they do not only possess different uncertainty properties regarding the amplification factor, but are also demonstrated to ask different scientific questions. Uncertainty in the estimated range of the Arctic amplification factor using the latest global climate models and climate forcing scenarios is expanded upon and shown to be greater than previously demonstrated for future climate projections, particularly using forcing scenarios with lower concentrations of greenhouse gases.

  • 6. Jungclaus, Johann H.
    et al.
    Bard, Edouard
    Baroni, Mélanie
    Braconnot, Pascale
    Cao, Jian
    Chini, Louise P.
    Egorova, Tania
    Evans, Michael
    González-Rouco, J. Fidel
    Goosse, Hugues
    Hurtt, George C.
    Joos, Fortunat
    Kaplan, Jed O.
    Khodri, Myriam
    Goldewijk, Kees Klein
    Krivova, Natalie
    LeGrande, Allegra N.
    Lorenz, Stephan J.
    Luterbacher, Jürg
    Man, Wenmin
    Maycock, Amanda C.
    Meinshausen, Malte
    Moberg, Anders
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Muscheler, Raimund
    Nehrbass-Ahles, Christoph
    Otto-Bliesner, Bette I.
    Phipps, Steven J.
    Pongratz, Julia
    Rozanov, Eugene
    Schmidt, Gavin A.
    Schmidt, Hauke
    Schmutz, Werner
    Schurer, Andrew
    Shapiro, Alexander I.
    Sigl, Michael
    Smerdon, Jason E.
    Solanki, Sami K.
    Timmreck, Claudia
    Toohey, Matthew
    Usoskin, Ilya G.
    Wagner, Sebastian
    Wu, Chi-Ju
    Yeo, Kok Leng
    Zanchettin, Davide
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Zorita, Eduardo
    The PMIP4 contribution to CMIP6-Part 3: The last millennium, scientific objective, and experimental design for the PMIP4 past1000 simulations2017In: Geoscientific Model Development, ISSN 1991-959X, E-ISSN 1991-9603, Vol. 10, no 11, 4005-4033 p.Article in journal (Refereed)
    Abstract [en]

    The pre-industrial millennium is among the periods selected by the Paleoclimate Model Intercomparison Project (PMIP) for experiments contributing to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) and the fourth phase of the PMIP (PMIP4). The past1000 transient simulations serve to investigate the response to (mainly) natural forcing under background conditions not too different from today, and to discriminate between forced and internally generated variability on interannual to centennial timescales. This paper describes the motivation and the experimental set-ups for the PMIP4-CMIP6 past1000 simulations, and discusses the forcing agents orbital, solar, volcanic, and land use/land cover changes, and variations in greenhouse gas concentrations. The past1000 simulations covering the pre-industrial millennium from 850 Common Era (CE) to 1849 CE have to be complemented by historical simulations (1850 to 2014 CE) following the CMIP6 protocol. The external forcings for the past1000 experiments have been adapted to provide a seamless transition across these time periods. Protocols for the past1000 simulations have been divided into three tiers. A default forcing data set has been defined for the Tier 1 (the CMIP6 past1000) experiment. However, the PMIP community has maintained the flexibility to conduct coordinated sensitivity experiments to explore uncertainty in forcing reconstructions as well as parameter uncertainty in dedicated Tier 2 simulations. Additional experiments (Tier 3) are defined to foster collaborative model experiments focusing on the early instrumental period and to extend the temporal range and the scope of the simulations. This paper outlines current and future research foci and common analyses for collaborative work between the PMIP and the observational communities (reconstructions, instrumental data).

  • 7. Kageyama, Masa
    et al.
    Albani, Samuel
    Braconnot, Pascale
    Harrison, Sandy P.
    Hopcroft, Peter O.
    Ivanovic, Ruza F.
    Lambert, Fabrice
    Marti, Olivier
    Peltier, W. Richard
    Peterschmitt, Jean-Yves
    Roche, Didier M.
    Tarasov, Lev
    Zhang, Xu
    Brady, Esther C.
    Haywood, Alan M.
    LeGrande, Allegra N.
    Lunt, Daniel J.
    Mahowald, Natalie M.
    Mikolajewicz, Uwe
    Nisancioglu, Kerim H.
    Otto-Bliesner, Bette L.
    Renssen, Hans
    Tomas, Robert A.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Abe-Ouchi, Ayako
    Bartlein, Patrick J.
    Cao, Jian
    Li, Qiang
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Lohmann, Gerrit
    Ohgaito, Rumi
    Shi, Xiaoxu
    Volodin, Evgeny
    Yoshida, Kohei
    Zhang, Xiao
    Zheng, Weipeng
    The PMIP4 contribution to CMIP6-Part 4: Scientific objectives and experimental design of the PMIP4-CMIP6 Last Glacial Maximum experiments and PMIP4 sensitivity experiments2017In: Geoscientific Model Development, ISSN 1991-959X, E-ISSN 1991-9603, Vol. 10, no 11, 4035-4055 p.Article in journal (Refereed)
    Abstract [en]

    The Last Glacial Maximum (LGM, 21 000 years ago) is one of the suite of paleoclimate simulations included in the current phase of the Coupled Model Intercomparison Project (CMIP6). It is an interval when insolation was similar to the present, but global ice volume was at a maximum, eustatic sea level was at or close to a minimum, greenhouse gas concentrations were lower, atmospheric aerosol loadings were higher than today, and vegetation and land-surface characteristics were different from today. The LGM has been a focus for the Paleoclimate Modelling Intercomparison Project (PMIP) since its inception, and thus many of the problems that might be associated with simulating such a radically different climate are well documented. The LGM state provides an ideal case study for evaluating climate model performance because the changes in forcing and temperature between the LGM and pre-industrial are of the same order of magnitude as those projected for the end of the 21st century. Thus, the CMIP6 LGM experiment could provide additional information that can be used to constrain estimates of climate sensitivity. The design of the Tier 1 LGM experiment (lgm) includes an assessment of uncertainties in boundary conditions, in particular through the use of different reconstructions of the ice sheets and of the change in dust forcing. Additional (Tier 2) sensitivity experiments have been designed to quantify feedbacks associated with land-surface changes and aerosol loadings, and to isolate the role of individual forcings. Model analysis and evaluation will capitalize on the relative abundance of paleoenvironmental observations and quantitative climate reconstructions already available for the LGM.

  • 8.
    Moberg, Anders
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Renssen, Hans
    Sundqvist, Hanna S.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Meteorology .
    Preface "Holocene climate variability over Scandinavia – A special issue originating from a workshop organized by the Bert Bolin Centre for Climate Research"2010In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 6, 719-721 p.Article in journal (Other academic)
  • 9.
    Muschitiello, Francesco
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Sundqvist, Hanna S.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Davies, Frazer J.
    Renssen, Hans
    Arctic climate response to the termination of the African Humid Period2015In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 125, 91-97 p.Article in journal (Refereed)
    Abstract [en]

    The Earth's climate response to the rapid vegetation collapse at the termination of the African Humid Period (AHP) (5.5-5.0 kyr BP) is still lacking a comprehensive investigation. Here we discuss the sensitivity of mid-Holocene Arctic climate to changes in albedo brought by a rapid desertification of the Sahara. By comparing a network of surface temperature reconstructions with output from a coupled global climate model, we find that, through a system of land-atmosphere feedbacks, the end of the AHP reduced the atmospheric and oceanic poleward heat transport from tropical to high northern latitudes. This entails a general weakening of the mid-latitude Westerlies, which results in a shift towards cooling over the Arctic and North Atlantic regions, and a change from positive to negative Arctic Oscillation-like conditions. This mechanism would explain the sign of rapid hydro-climatic perturbations recorded in several reconstructions from high northern latitudes at 5.5-5.0 kyr BP, suggesting that these regions are sensitive to changes in Saharan land cover during the present interglacial. This is central in the debate surrounding Arctic climate amplification and future projections for subtropical precipitation changes.

  • 10. Otto-Bliesner, Bette L.
    et al.
    Braconnot, Pascale
    Harrison, Sandy P.
    Lunt, Daniel J.
    Abe-Ouchi, Ayako
    Albani, Samuel
    Bartlein, Patrick J.
    Capron, Emilie
    Carlson, Anders E.
    Dutton, Andrea
    Fischer, Hubertus
    Goelzer, Heiko
    Govin, Aline
    Haywood, Alan
    Joos, Fortunat
    LeGrande, Allegra N.
    Lipscomb, William H.
    Lohmann, Gerrit
    Mahowald, Natalie
    Nehrbass-Ahles, Christoph
    Pausata, Francesco S. R.
    Peterschmitt, Jean-Yves
    Phipps, Steven J.
    Renssen, Hans
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    The PMIP4 contribution to CMIP6-Part 2: Two interglacials, scientific objective and experimental design for Holocene and Last Interglacial simulations2017In: Geoscientific Model Development, ISSN 1991-959X, E-ISSN 1991-9603, Vol. 10, no 11, 3979-4003 p.Article in journal (Refereed)
    Abstract [en]

    Two interglacial epochs are included in the suite of Paleoclimate Modeling Intercomparison Project (PMIP4) simulations in the Coupled Model Intercomparison Project (CMIP6). The experimental protocols for simulations of the mid-Holocene (midHolocene, 6000 years before present) and the Last Interglacial (lig127k, 127 000 years before present) are described here. These equilibrium simulations are designed to examine the impact of changes in orbital forcing at times when atmospheric greenhouse gas levels were similar to those of the preindustrial period and the continental configurations were almost identical to modern ones. These simulations test our understanding of the interplay between radiative forcing and atmospheric circulation, and the connections among large-scale and regional climate changes giving rise to phenomena such as land-sea contrast and high-latitude amplification in temperature changes, and responses of the monsoons, as compared to today. They also provide an opportunity, through carefully designed additional sensitivity experiments, to quantify the strength of atmosphere, ocean, cryosphere, and land-surface feedbacks. Sensitivity experiments are proposed to investigate the role of freshwater forcing in triggering abrupt climate changes within interglacial epochs. These feedback experiments naturally lead to a focus on climate evolution during interglacial periods, which will be examined through transient experiments. Analyses of the sensitivity simulations will also focus on interactions between extratropical and tropical circulation, and the relationship between changes in mean climate state and climate variability on annual to multi-decadal timescales. The comparative abundance of paleoenvironmental data and of quantitative climate reconstructions for the Holocene and Last Interglacial make these two epochs ideal candidates for systematic evaluation of model performance, and such comparisons will shed new light on the importance of external feedbacks (e.g., vegetation, dust) and the ability of state-of-the-art models to simulate climate changes realistically.

  • 11.
    Pausata, Francesco S. R.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Emanuel, Kerry A.
    Chiacchio, Marc
    Stockholm University, Faculty of Science, Department of Meteorology .
    Diro, Gulilat T.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Sushama, Laxmi
    Stager, J. Curt
    Donnelly, Jeffrey P.
    Tropical cyclone activity enhanced by Sahara greening and reduced dust emissions during the African Humid Period2017In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 114, no 24, 6221-6226 p.Article in journal (Refereed)
    Abstract [en]

    Tropical cyclones (TCs) can have devastating socioeconomic impacts. Understanding the nature and causes of their variability is of paramount importance for society. However, historical records of TCs are too short to fully characterize such changes and paleosediment archives of Holocene TC activity are temporally and geographically sparse. Thus, it is of interest to apply physical modeling to understanding TC variability under different climate conditions. Here we investigate global TC activity during a warm climate state (mid-Holocene, 6,000 yBP) characterized by increased boreal summer insolation, a vegetated Sahara, and reduced dust emissions. We analyze a set of sensitivity experiments in which not only solar insolation changes are varied but also vegetation and dust concentrations. Our results show that the greening of the Sahara and reduced dust loadings lead to more favorable conditions for tropical cyclone development compared with the orbital forcing alone. In particular, the strengthening of the West African Monsoon induced by the Sahara greening triggers a change in atmospheric circulation that affects the entire tropics. Furthermore, whereas previous studies suggest lower TC activity despite stronger summer insolation and warmer sea surface temperature in the Northern Hemisphere, accounting for the Sahara greening and reduced dust concentrations leads instead to an increase of TC activity in both hemispheres, particularly over the Caribbean basin and East Coast of North America. Our study highlights the importance of regional changes in land cover and dust concentrations in affecting the potential intensity and genesis of past TCs and suggests that both factors may have appreciable influence on TC activity in a future warmer climate.

  • 12.
    Pausata, Francesco S. R.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Messori, Gabriele
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Impacts of dust reduction on the northward expansion of the African monsoon during the Green Sahara period2016In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 434, 298-307 p.Article in journal (Refereed)
    Abstract [en]

    The West African Monsoon (WAM) is crucial for the socio-economic stability of millions of people living in the Sahel. Severe droughts have ravaged the region in the last three decades of the 20th century, highlighting the need for a better understanding of the WAM dynamics. One of the most dramatic changes in the West African Monsoon (WAM) occurred between 15000-5000 yr BP, when increased summer rainfall led to the so-called Green Sahara and to a reduction in dust emissions from the region. However, model experiments are unable to fully reproduce the intensification and geographical expansion of the WAM during this period, even when vegetation over the Sahara is considered. Here, we use a fully coupled simulation for 6000 yr BP (Mid-Holocene) in which prescribed Saharan vegetation and dust concentrations are changed in turn. A closer agreement with proxy records is obtained only when both the Saharan vegetation changes and dust decrease are taken into account. The dust reduction strengthens the vegetation-albedo feedback, extending the monsoon's northern limit approximately 500 km further than the vegetation-change case only. We therefore conclude that accounting for changes in Saharan dust loadings is essential for improving model simulations of the WAM during the Mid-Holocene.

  • 13.
    Pausata, Francesco S. R.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . University of Quebec in Montreal (UQÀM), Canada.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Muschitiello, Francesco
    Stockholm University, Faculty of Science, Department of Geological Sciences. Columbia University, USA; Uni Research Climate, Norway; Bjerknes Centre for Climate Research, Norway.
    Lu, Zhengyao
    Chafik, Léon
    Niedermeyer, Eva M.
    Stager, J. Curt
    Cobb, Kim M.
    Liu, Zhengyu
    Greening of the Sahara suppressed ENSO activity during the mid-Holocene2017In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 8, 16020Article in journal (Refereed)
    Abstract [en]

    The evolution of the El Nino-Southern Oscillation (ENSO) during the Holocene remains uncertain. In particular, a host of new paleoclimate records suggest that ENSO internal variability or other external forcings may have dwarfed the fairly modest ENSO response to precessional insolation changes simulated in climate models. Here, using fully coupled ocean-atmosphere model simulations, we show that accounting for a vegetated and less dusty Sahara during the mid-Holocene relative to preindustrial climate can reduce ENSO variability by 25%, more than twice the decrease obtained using orbital forcing alone. We identify changes in tropical Atlantic mean state and variability caused by the momentous strengthening of the West Africa Monsoon (WAM) as critical factors in amplifying ENSO's response to insolation forcing through changes in the Walker circulation. Our results thus suggest that potential changes in the WAM due to anthropogenic warming may influence ENSO variability in the future as well.

  • 14.
    Salih, Abubakr A. M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Elagib, Nadir Ahmed
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Characterization of the Sahelian-Sudan rainfall based on observations and regional climate modelsManuscript (preprint) (Other academic)
    Abstract [en]

    Sudan is part of the African Sahel region, which is known to be highly vulnerable to climate variability and change. However, the characteristics of observed and modeled rainfall in the country are rarely available in literature. The focus of this paper is two-fold: to analyze the rainfall aspects of Sahelian Sudan in terms of distribution of rain-days and amount, and to examine whether regional climate models can capture these rainfall features. Outputs from three regional models, namely REMO, RCA and RegCM4, have been evaluated against gridded observations and rain-gauge data from six arid and semi-arid weather stations spread across Sahelian Sudan over the period 1989 to 2008. Most of the observed rain-days are characterized by weak (0.1 – 1.0 mm/day) to moderate (>1.0 – 10.0 mm/day) rainfalls, with average frequencies of 18.5% and 48.0% out of the total annual rain-days, respectively. Although very strong rainfall events (>30.0 mm/day) occur rarely, they account for a large portion of the annual rainfall amount. The performance of the models varies both spatially and temporally. RegCM4 output is the closest to the observations in reproducing the annual rainfall cycle, especially for the more arid locations, but an unusual peak in June is present. All three models fail to capture the frequency of very strong rainfall events and, thus, underestimate the contribution of extreme rainfall events to the total annual number of rain-days and rainfall amount. Nevertheless, more moderate rainfall events from the models compensate this underestimated rainfall amount. REMO and RCA show a systematic tendency to overestimate the number of rain-days. Generally, the source of errors in rainfall modeling can be attributed to the convection parameterization in the models. This study suggests that rainfall occurring due to large-scale atmospheric circulation also contributes to the error. The present study uncovers some of the models’ limitations in skillfully reproducing the observed climate over dry regions.  It will help climate and hydrological modeling communities, i.e. developers, in improving the models and also users in recognizing the uncertainties in model outputs.

  • 15.
    Salih, Abubakr A. M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Pausata, Francesco S. R.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sources of Sahelian Sudan moisture: insights from a moisture-tracing atmospheric modelManuscript (preprint) (Other academic)
    Abstract [en]

    The summer rainfall across Sahelian Sudan is one of the main sources of water for agriculture, human and animal needs. However, the rainfall is characterized by large interannual variability, which has attracted extensive scientific effort to understand it. This study attempts to identify the source regions that contribute to Sahelian Sudan moisture budget during the monsoon months – July through September. We have used an atmospheric general circulation model with embedded water-tracing module (CAM3), forced by observed (1979-2013) sea-surface temperatures (SST). The result suggests that about 38.6% of the moisture comes with the atmospheric flow associated with the seasonal migration of the Intertropical convergence zone (ITCZ), which originates from Guinea Coast, Central Africa and the Western Sahel. The Mediterranean, Arabian Peninsula, and South Indian Ocean regions accounts for 10.2%, 8.1%, and 6.4%, respectively. Local evaporation and the rest of the globe supply the region with 20.3% and 13.2%, respectively. We also compared the result from this study to previous analysis that used the Lagrangian model FLEXPART. The two approaches differ when comparing individual regions, but are in better agreement when neighboring regions of similar atmospheric flow features are grouped together. Interannual variability of the rainfall over the region is highly correlated with contribution from regions that are associated with the ITCZ movement, which is linked to the Atlantic Multidecadal Oscillation (AMO). A dry regime during the time period 1979-1994 seems to be associated with the cold phase of AMO, and relatively wet period 1995-2013 is associated with the warm mode phase of AMO. 

  • 16.
    Salih, Abubakr A. M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Pausata, Francesco S. R.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sources of Sahelian-Sudan moisture: Insights from a moisture-tracing atmospheric model2016In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 121, no 13, 7819-7832 p.Article in journal (Refereed)
    Abstract [en]

    The summer rainfall across Sahelian-Sudan is one of the main sources of water for agriculture, human, and animal needs. However, the rainfall is characterized by large interannual variability, which has attracted extensive scientific efforts to understand it. This study attempts to identify the source regions that contribute to the Sahelian-Sudan moisture budget during July through September. We have used an atmospheric general circulation model with an embedded moisture-tracing module (Community Atmosphere Model version 3), forced by observed (1979-2013) sea-surface temperatures. The result suggests that about 40% of the moisture comes with the moisture flow associated with the seasonal migration of the Intertropical Convergence Zone (ITCZ) and originates from Guinea Coast, central Africa, and the Western Sahel. The Mediterranean Sea, Arabian Peninsula, and South Indian Ocean regions account for 10.2%, 8.1%, and 6.4%, respectively. Local evaporation and the rest of the globe supply the region with 20.3% and 13.2%, respectively. We also compared the result from this study to a previous analysis that used the Lagrangian model FLEXPART forced by ERA-Interim. The two approaches differ when comparing individual regions, but are in better agreement when neighboring regions of similar atmospheric flow features are grouped together. Interannual variability with the rainfall over the region is highly correlated with contributions from regions that are associated with the ITCZ movement, which is in turn linked to the Atlantic Multidecadal Oscillation. Our result is expected to provide insights for the effort on seasonal forecasting of the rainy season over Sahelian Sudan.

  • 17.
    Salih, Abubakr A. M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lagrangian tracing of Sahelian Sudan moisture sources2015In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 120, no 14, 6793-6808 p.Article in journal (Refereed)
    Abstract [en]

    The Sahelian Sudan is an arid to semiarid region that depends on the seasonal rainfall as the main source of water, but its rainfall has large interannual variability. Such dry regions usually have their main moisture sources elsewhere; thus, the rainfall variability is directly related to the moisture transport. This study seeks to identify source regions of water vapor for Sahelian Sudan during the monsoon period, from July to September. We have used the Lagrangian trajectory model FLEXPART driven by ERA-Interim reanalysis for the time period 1998 to 2008. The results show that most of the air masses that reach this region during the monsoon period have their major origins over the Arabian Peninsula, Central Africa, or are associated with the tropical easterly jet. Flow associated with Intertropical Convergence Zone contributes almost half of the total precipitated water; most of it comes from Central Africa. This suggests that moisture recycling is the major contributor, compared to Oceanic sources. The flows from the northeast (Arabian Peninsula and north Asia) and east (Horn of Africa and north Indian Ocean) contribute about one third of the precipitated water. The rest of precipitated water comes from the Mediterranean, subtropical Atlantic, and western Sahel, all with smaller contribution. Our results also indicate that different subregions of Sahelian Sudan have different moisture sources. Such result needs to be taken into account in seasonal forecasting practices.

  • 18.
    Seguinot, Julien
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Khroulev, Constantine
    Rogozhina, Irina
    Stroeven, Arjen P.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    The effect of climate forcing on numerical simulations of the Cordilleran ice sheet at the Last Glacial Maximum2014In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 8, no 3, 1087-1103 p.Article in journal (Refereed)
    Abstract [en]

    We present an ensemble of numerical simulations of the Cordilleran ice sheet during the Last Glacial Maximum performed with the Parallel Ice Sheet Model (PISM), applying temperature offsets to the present-day climatologies from five different data sets. Monthly mean surface air temperature and precipitation from WorldClim, the NCEP/NCAR reanalysis, the ERA-Interim reanalysis, the Climate Forecast System Reanalysis and the North American Regional Reanalysis are used to compute surface mass balance in a positive degree-day model. Modelled ice sheet outlines and volumes appear highly sensitive to the choice of climate forcing. For three of the four reanalysis data sets used, differences in precipitation are the major source for discrepancies between model results. We assess model performance against a geomorphological reconstruction of the ice margin at the Last Glacial Maximum, and suggest that part of the mismatch is due to unresolved orographic precipitation effects caused by the coarse resolution of reanalysis data. The best match between model output and the reconstructed ice margin is obtained using the high-resolution North American Regional Reanalysis, which we retain for simulations of the Cordilleran ice sheet in the future.

  • 19.
    Sturm, Christophe
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Meteorology .
    Noone, David
    University of Colorado.
    An introduction to stable water isotopes in climate models: benefits of forward proxy modelling for paleoclimatology2010In: Climate of the past, ISSN 1814-9332, Vol. 6, 115-129 p.Article in journal (Refereed)
    Abstract [en]

    Stable water isotopes have been measured in a wide range of climate archives, with the purpose of reconstructing regional climate variations. Yet the common assumption that the isotopic signal is a direct indicator of temperature proves to be misleading under certain circumstances, since its relationship with temperature also depends on e.g. atmospheric circulation and precipitation seasonality. The present article introduces the principles, benefits and caveats of using climate models with embedded water isotopes as a support for the interpretation of isotopic climate archives. A short overview of the limitations of empirical calibrations of isotopic proxy records is presented, with emphasis on the physical processes that infirm its underlying hypotheses. The simulation of climate and its associated isotopic signal, despite difficulties related to downscaling and intrinsic atmospheric variability, can provide a "transfer function" between the isotopic signal and the considered climate variable. The multi-proxy data can then be combined with model output to produce a physically consistent climate reconstruction and its confidence interval. A sensitivity study with the isotope-enabled global circulation model CAM3iso under idealised present-day, pre-industrial and mid-Holocene is presented to illustrate the impact of a changing climate on the isotope-temperature relationship.

  • 20.
    Sundqvist, Hanna S.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Fohlmeister, J.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Bar Matthews, M.
    Spoetl, C.
    Kornich, H.
    Evidence of a large cooling between 1690 and 1740 AD in southern Africa2013In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, 1767- p.Article in journal (Refereed)
    Abstract [en]

    A 350-year-long, well-dated delta O-18 stalagmite record from the summer rainfall region in South Africa is positively correlated with regional air surface temperatures at interannual time scales. The coldest period documented in this record occurred between 1690 and 1740, slightly lagging the Maunder Minimum (1645-1710). A temperature reconstruction, based on the correlation between regional surface temperatures and the stalagmite delta O-18 variations, indicates that parts of this period could have been as much as 1.4 degrees C colder than today. Significant cycles of 22, 11 and 4.8 years demonstrate that the solar magnetic and the El Nino-Southern Oscillation cycle could be important drivers of multidecadal to interannual climate variability in this region. The observation that the most important driver of stalagmite delta O-18 on interannual time scales from this subtropical region is regional surface temperature cautions against deterministic interpretations of delta O-18 variations in low-latitude stalagmites as mainly driven by the amount of precipitation.

  • 21.
    Sundqvist, Hanna S.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Meteorology .
    Moberg, Anders
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Northern high-latitude climate change between the mid and late Holocene: Part 1: Proxy data evidence2009In: Climate of the Past Discussions, ISSN 1814-9340, Vol. 5, no 4, 1819-1852 p.Article in journal (Other academic)
  • 22.
    Sundqvist, Hanna S.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Meteorology .
    Moberg, Anders
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brattström, Gudrun
    Stockholm University, Faculty of Science, Department of Mathematics.
    Climate change between the mid and late Holocene in the northern high latitudes: Part I: Survey of temperature and precipitation proxy data2010In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 6, 591-608 p.Article in journal (Refereed)
    Abstract [en]

    We undertake a study in two parts, where theoverall aim is to quantitatively compare results from climateproxy data with results from several climate model simulationsfrom the Paleoclimate Modelling IntercomparisonProject for the mid-Holocene period and the pre-industrial,conditions for the pan-arctic region, north of 60 N. In thisfirst paper, we survey the available published local temperatureand precipitation proxy records. We also discuss andquantifiy some uncertainties in the estimated difference inclimate between the two periods as recorded in the availabledata. The spatial distribution of available published localproxies has a marked geographical bias towards land areassurrounding the North Atlantic sector, especially Fennoscandia.The majority of the reconstructions are terrestrial, andthere is a large over-representation towards summer temperaturerecords. The available reconstructions indicate that thenorthern high latitudes were warmer in both summer, winterand the in annual mean temperature at the mid-Holocene(6000 BP±500 yrs) compared to the pre-industrial period(1500AD±500 yrs). For usage in the model-data comparisons(in Part 1), we estimate the calibration uncertainty andalso the internal variability in the proxy records, to derive acombined minimum uncertainty in the reconstructed temperaturechange between the two periods. Often, the calibrationuncertainty alone, at a certain site, exceeds the actual reconstructedclimate change at the site level. In high-density regions,however, neighbouring records can be merged into aCorrespondence to: H. S. Sundqvist(hanna.sundqvist@natgeo.su.se)composite record to increase the signal-to-noise ratio. Thechallenge of producing reliable inferred climate reconstructionsfor the Holocene cannot be underestimated, consideringthe fact that the estimated temperature and precipitationfluctuations during this period are in magnitude similar to, orlower than, the uncertainties the reconstructions. We advocatea more widespread practice of archiving proxy recordsas most of the potentially available reconstructions are notpublished in digital form.

  • 23. Xu, Guobao
    et al.
    Liu, Xiaohong
    Wu, Guoju
    Chen, Tuo
    Wang, Wenzhi
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Zhang, Youfu
    Zeng, Xiaomin
    Qin, Dahe
    Sun, Weizhen
    Zhang, Xuanwen
    Tree ring O-18's indication of a shift to a wetter climate since the 1880s in the western Tianshan Mountains of northwestern China2015In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 120, no 13, 6409-6425 p.Article in journal (Refereed)
    Abstract [en]

    Central Asian droughts have drastically and significantly affected agriculture and water resource management in these arid and semiarid areas. Based on tree ring O-18 from native, dominant Schrenk spruce (Picea schrenkiana Fisch. et Mey.), we developed a 300year (1710-2010) standard precipitation-evaporation index (SPEI) reconstruction from January to August for China's western Tianshan Mountains. The regression model explained 37.6% of the variation in the SPEI reconstruction during the calibration period from 1950 to 2010. Comparison with previous drought reconstructions confirmed the robustness of our reconstruction. The 20th century has been a relatively wet period during the past 300years. The SPEI showed quasi 2, 5, and 10year cycles. Several pluvials and droughts with covariability over large areas were revealed clearly in the reconstruction. The two longest pluvials (lasting for 12years), separated by 50years, appeared in the 1900s and the 1960s. The most severe drought occurred from 1739 to 1761 and from 1886 to 1911 was the wettest period since 1710. Compared to previous investigations of hydroclimatic changes in the western Tianshan Mountains, our reconstruction revealed more low-frequency variability and indicated that climate in the western Tianshan Mountains shifted from dry to wet in 1886. This regime shift was generally consistent with other moisture reconstructions for the northeastern Tibetan Plateau and northern Pakistan and may have resulted from a strengthened westerly circulation. The opposite hydrological trends in the western Tianshan Mountains and southeastern Tibetan Plateau reveal a substantial influence of strengthened westerlies and weakening of the Indian summer monsoon.

  • 24. Yang, Haijun
    et al.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Meteorology .
    Anatomizing the Ocean´s role in ENSO changes under global warming2008In: Journal of climate, ISSN 1520-0442, Vol. 21, no 24, 6539-6555 p.Article in journal (Refereed)
    Abstract [en]

    A revisit on observations shows that the tropical El Niño–Southern Oscillation (ENSO) variability, after removing both the long-term trend and decadal variation of the background climate, has been enhanced by as much as 50% during the past 50 yr. This is inconsistent with the changes in the equatorial atmosphere, which shows a slowdown of the zonal Walker circulation and tends to stabilize the tropical coupling system. The ocean role is highlighted in this paper. The enhanced ENSO variability is attributed to the strengthened equatorial thermocline that acts as a destabilizing factor of the tropical coupling system. To quantify the dynamic effect of the ocean on the ENSO variability under the global warming, ensemble experiments are performed using a coupled climate model [Fast Ocean Atmosphere Model (FOAM)], following the “1pctto2x” scenario defined in the Intergovernmental Panel on Climate Change (IPCC) reports. Term balance analyses on the temperature variability equation show that the anomalous upwelling of the mean vertical temperature gradient (referred as the “local term”) in the eastern equatorial Pacific is the most important destabilizing factor to the temperature variabilities. The magnitude of local term and its change are controlled by its two components: the mean vertical temperature gradient Tz and the “virtual vertical heat flux” −w′T′. The former can be viewed as the background of the latter and these two components are positively correlated. A stronger Tz is usually associated with a bigger upward heat flux −w′T′, which implies a bigger impact of thermocline depth variations on SST. The Tz is first enhanced during the transient stage of the global warming with a 1% yr−1 increase of CO2, and then reduced during the equilibrium stage with a fixed doubled CO2. This turnaround in Tz determines the turnaround of ENSO variability in the entire global warming period.

  • 25. Yang, Haijun
    et al.
    Zhao, Yingying
    Liu, Zhengyu
    Li, Qing
    He, Feng
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Heat Transport Compensation in Atmosphere and Ocean over the Past 22,000 Years2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, 16661Article in journal (Refereed)
    Abstract [en]

    The Earth's climate has experienced dramatic changes over the past 22,000 years; however, the total meridional heat transport (MHT) of the climate system remains stable. A 22,000-year-long simulation using an ocean-atmosphere coupled model shows that the changes in atmosphere and ocean MHT are significant but tend to be out of phase in most regions, mitigating the total MHT change, which helps to maintain the stability of the Earth's overall climate. A simple conceptual model is used to understand the compensation mechanism. The simple model can reproduce qualitatively the evolution and compensation features of the MHT over the past 22,000 years. We find that the global energy conservation requires the compensation changes in the atmosphere and ocean heat transports. The degree of compensation is mainly determined by the local climate feedback between surface temperature and net radiation flux at the top of the atmosphere. This study suggests that an internal mechanism may exist in the climate system, which might have played a role in constraining the global climate change over the past 22,000 years.

  • 26.
    Zhang, Qiong
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Sundqvist, Hanna
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Decadal Rainfall Dipole Oscillation over Southern Africa Modulated by Variation of Austral Summer Land-Sea Contrast along the East Coast of Africa2015In: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 72, no 5, 1827-1836 p.Article in journal (Refereed)
    Abstract [en]

    A rainfall dipole mode characterized by negative correlation between subtropical southern Africa and equatorial eastern Africa is identified in instrumental observation data in the recent 100 years. The dipole mode shows a pronounced oscillation signal at a time scale of about 18 years. This study investigates the underlying dynamical mechanisms responsible for this dipole pattern. It is found that the southern African rainfall dipole index is highly correlated to the land-sea contrast along the east coast of Africa. When the land-sea thermal contrast strengthens, the easterly flow toward the continent becomes stronger. The stronger easterly flow, via its response to east coast topography and surface heating, leads to a low pressure circulation anomaly over land south of the maximum easterly flow anomalies and thus causes more rainfall in the south. On a decadal time scale, an ENSO-like SST pattern acts to modulate this land-sea contrast and the consequent rainfall dipole. During a wet in the south and dry in the north dipole, there are warm SSTs over the central Indian Ocean and cold SSTs over the western Indian Ocean. The cold SSTs over the western Indian Ocean further enhance the land-sea contrast during austral summer. Moreover, these cold western Indian Ocean SSTs also play an important role in regulating land temperature, thereby suppressing clouds and warming the land via increased shortwave radiation over the less-cloudy land. This cloud-SST coupling acts to further strengthen the land-sea contrast.

  • 27.
    Zhang, Qiong
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    How well do reanalyses represent the southern African precipitation?2013In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 40, no 3-4, 951-962 p.Article in journal (Refereed)
    Abstract [en]

    Monthly-mean precipitation observations over southern Africa are used to evaluate the performance of eight global reanalyses: ERA-40, ERA-interim, JRA-25, MERRA, CFSR, NCEP-R1, NCEP-R2 and 20CRv2. All eight reanalyses reproduce the regionally averaged seasonal cycle fairly well; a few spatial mismatches with the observations are found in the climate mean for the rainy season. Principal component analyses show a dipole in the leading modes of all reanalyses, however with crucial differences in its spatial position. Possible reasons for the differences between the reanalyses are discussed on the basis of the ERA-interim and 20CRv2 results. A comparison between the moisture transports shows that ERA-interim manifests a very strong moisture convergence over the eastern equatorial Atlantic, resulting in the strong precipitation here. This excessive convergence may be due to the water-vapor assimilation and convection parameterization. Over the Indian Ocean, the ITCZ is shifted northward in ERA-interim compared to its position in 20CRv2. This discrepancy is most likely attributable to the meridional SST gradients in the Indian Ocean which are significantly larger in the ERA-interim than those in the 20CRv2, and the resulting atmospheric response prevents a southward shift of the ITCZ. Overall, the consistent description of the dynamical circulation of the atmosphere and the hydrological cycle appears as a crucial benchmark for reanalysis data. Based on our evaluation, the preferential reanalysis for investigating the climate variability over southern Africa is 20CRv2 that furthermore spans the longest time period, hence permitting the most precise investigations of interannual to decadal variability.

  • 28.
    Zhang, Qiong
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sundqvist, Hanna
    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.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Northern high-latitude climate change between the mid and late Holocene: Part 2: Model-data comparisons2009In: Climate of the Past Discussions, ISSN 1814-9340, Vol. 5, no 3, 1659-1696 p.Article in journal (Other academic)
  • 29.
    Zhang, Qiong
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sundqvist, Hanna
    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.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Climate change between the mid and late Holocene in northern high latitudes: Part 2: Model-data comparisons2010In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 6, 609-626 p.Article in journal (Refereed)
    Abstract [en]

    The climate response over northern high latitudesto the mid-Holocene orbital forcing has been investigated inthree types of PMIP (Paleoclimate Modelling IntercomparisonProject) simulations with different complexity of themodelled climate system. By first undertaking model-datacomparison, an objective selection method has been appliedto evaluate the capability of the climate models to reproducethe spatial response pattern seen in proxy data. The possiblefeedback mechanisms behind the climate response havebeen explored based on the selected model simulations. Subsequentmodel-model comparisons indicate the importanceof including the different physical feedbacks in the climatemodels. The comparisons between the proxy-based reconstructionsand the best fit selected simulations show that overthe northern high latitudes, summer temperature change followsclosely the insolation change and shows a commonfeature with strong warming over land and relatively weakwarming over ocean at 6 ka compared to 0 ka. Furthermore,the sea-ice-albedo positive feedback enhances this response.The reconstructions of temperature show a strongerresponse to enhanced insolation in the annual mean temperaturethan winter and summer temperature. This is verified inthe model simulations and the behaviour is attributed to thelarger contribution from the large response in autumn. Despitea smaller insolation during winter at 6 ka, a pronouncedwarming centre is found over Barents Sea in winter in thesimulations, which is also supported by the nearby northernEurasian continental and Fennoscandian reconstructions.This indicates that in the Arctic region, the response of theocean and the sea ice to the enhanced summer insolationis more important for the winter temperature than the synchronousdecrease of the insolation.

1 - 29 of 29
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