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  • 1.
    Bengtsson, Lisa
    et al.
    Sveriges meteorologiska och hydrologiska institut (SMHI), Norrköping.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Källén, Erland
    Svensson, Gunilla
    Stockholm University, Faculty of Science, Department of Meteorology .
    Large-Scale Dynamical Response to Subgrid-Scale Organization Provided by Cellular Automata2011In: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 68, no 12, p. 3132-3144Article in journal (Refereed)
    Abstract [en]

    Due to the limited resolution of numerical weather prediction (NWP) models, sub-grid scale physical processes are parameterized, and represented by grid-box means. However, some physical processes are better represented by a mean and its variance, a typical example being deep convection, with scales varying from individual updraughts to organized meso-scale systems. In this study, we investigate, in an idealized setting, whether a cellular automaton (CA) can be used in order to enhance sub-grid scale organization by forming clusters representative of the convective scales, and yield a stochastic representation of sub-grid scale variability. We study the transfer of energy from the convective to the larger atmospheric scales through nonlinear wave interactions. This is done using a shallow water (SW) model initialized with equatorial wave modes. By letting a CA act on a finer resolution than that of the SW model, it can be expected to mimic the effect of, for instance, gravity wave propagation on convective organization. Employing the CA-scheme allows to reproduce the observed behaviour of slowing down equatorial Kelvin modes in convectively active regions, while random perturbations fail to feed back on the large-scale flow. The analysis of kinetic energy spectra demonstrates that the CA sub-grid scheme introduces energy back-scatter from the smallest model scales to medium scales. However, the amount of energy back-scattered depends almost solely on the memory time scale introduced to the sub-grid scheme, whereas any variation in spatial scales generated does not influence the energy spectra markedly.

  • 2. Brandefelt, Jenny
    et al.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology.
    Northern Hemisphere stationary waves in future climate projections.2008In: Journal of Climate, Vol. 21, no 23, p. 6341–6353-Article in journal (Refereed)
    Abstract [en]

    The response of the atmospheric large-scale circulation to an enhanced greenhouse gas (GHG) forcing varies among coupled global climate model (CGCM) simulations. In this study, sixteen CGCM simulations of the response of the climate system to a 1% per year increase in the atmospheric CO2 concentration to quadrupling are analysed with focus on Northern Hemisphere winter. A common signal in fourteen out of sixteen simulations is an increased or unchanged stationary wave amplitude. A majority of the simulations may be categorised into one of three groups based on the GHG induced changes in the atmospheric stationary waves. The response of the zonal mean barotropic wind is similar within each group. 50% of the simulations belong to the first group which is categorised by a stationary wave with five waves encompassing the whole NH and a strengthening of the zonal mean barotropic wind. The second and third group, consisting of three and two simulations respectively, are characterised by a broadening and a northward shift of the zonal mean barotropic wind respectively. A linear model of barotropic vorticity is employed to study the importance of these mean flow changes to the stationary wave response. The linear calculations indicate that the GHG induced mean wind changes explain 50%, 4% and 37% of the stationary wave changes in each group respectively. Thus, for the majority of simulations the zonal mean wind changes do significantly explain the stationary wave response.

  • 3.
    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, p. 2856-2861Article 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.

  • 4. Gabriel, Axel
    et al.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lossow, Stefan
    Urban, Joachim
    Murtagh, Donal
    Zonal asymmetries in middle atmospheric ozone and water vapour derived from Odin satellite data 2001–20102011In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, p. 9865-9885Article in journal (Refereed)
    Abstract [en]

    Stationary wave patterns in middle atmospheric ozone (O3) and water vapour (H2O) are an important factor in the atmospheric circulation, but there is a strong gap in diagnosing and understanding their configuration and origin. Based on Odin satellite data from 2001 to 2010 we investigate the stationary wave patterns in O3 and H2O as indicated by the seasonal long-term means of the zonally asymmetric components O3* = O3-[O3] and H2O* = H2O-[H2O] ([O3], [H2O]: zonal means). At mid- and polar latitudes we find a pronounced wave one pattern in both constituents. In the Northern Hemisphere, the wave patterns increase during autumn, maintain their strength during winter and decay during spring, with maximum amplitudes of about 10–20 % of the zonal mean values. During winter, the wave one in O3* shows a maximum over the North Pacific/Aleutians and a minimum over the North Atlantic/Northern Europe and a double-peak structure with enhanced amplitude in the lower and in the upper stratosphere. The wave one in H2O* extends from the lower stratosphere to the upper mesosphere with a westward shift in phase with increasing height including a jump in phase at upper stratosphere altitudes. In the Southern Hemisphere, similar wave patterns occur mainly during southern spring. By comparing the observed wave patterns in O3* and H2O* with a linear solution of a steady-state transport equation for a zonally asymmetric tracer component we find that these wave patterns are primarily due to zonally asymmetric transport by geostrophically balanced winds, which are derived from observed temperature profiles. In addition temperature-dependent photochemistry contributes substantially to the spatial structure of the wave pattern in O3* . Further influences, e.g., zonal asymmetries in eddy mixing processes, are discussed.

  • 5.
    Graversen, Rune Grand
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Källén, Erland
    Stockholm University, Faculty of Science, Department of Meteorology.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology.
    Atmospheric mass-budget inconsistency in the ERA-40 reanalysis2007In: Quarterly Journal of the Royal Meteorological society, ISSN 0035-9009, Vol. 133, p. 673-680Article in journal (Refereed)
  • 6.
    Gumbel, Jörg
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Bailey, S. M.
    Lubken, F. -J
    Morris, R.
    Special issue on layered phenomena in the mesopause region Foreword2011In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 73, no 14-15, p. 2045-2048Article in journal (Refereed)
  • 7.
    Hultgren, Kristoffer
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kornich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Gumbel, Jörg
    Stockholm University, Faculty of Science, Department of Meteorology .
    Gerding, Michael
    Hoffmann, Peter
    Lossow, Stefan
    Megner, Linda
    What caused the exceptional mid-latitudinal Noctilucent Cloud event in July 2009?2011In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 73, no 14-15, p. 2125-2131Article in journal (Refereed)
    Abstract [en]

    Noctilucent Clouds (NLCs) are rarely observed at mid-latitudes. In July 2009, strong NLCs were recorded from both Paris and Nebraska, located at latitudes 48 degrees N and 41 degrees N, respectively. The main focus of this work is on the atmospheric conditions that have led to NLCs at these latitudes. We investigate to what extent these clouds may be explained by local formation or by transport from higher latitudes. The dynamical situation is analyzed in terms of wind fields created from Aura/MLS temperature data and measured by radar. We discuss possible tidal effects on the transport and examine the general planetary wave activity during these days. The winds do not seem sufficient to transport NLC particles long southward distances. Hence a local formation is rather likely. In order to investigate the possibility of local NLC formation, the CARMA microphysical model has been applied with temperature data from MLS as input. The results from the large-scale datasets are compared to NLC observations by Odin and to local NLC, temperature and wind measurements by lidar and radar. The reason for the exceptional NLC formation is most likely a combination of local temperature variations by diurnal tides, advantageously located large-scale planetary waves, and general mesospheric temperature conditions that were 5-10 K colder than in previous years. The results also point to that NLCs are very unlikely to occur at latitudes below 50 degrees N during daytime. This conclusion can be made from a tidal temperature mode with cold temperatures during nighttime and temperatures above the limit for NLC occurrence during daytime. The best time for observing mid-latitude NLCs is during the early morning hours.

  • 8.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Impact of the Vertical Sampling Scenarios on NWP and Stratospheric Wind Analysis2010Report (Other academic)
  • 9.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Predictability of the coupled troposphere-stratosphere system2010In: Seminar on Predictability in the European and Atlantic regions from days to years., Reading, UK.: ECMWF , 2010Conference paper (Other academic)
    Abstract [en]

    Tropospheric predictability is typically limited to about 20 days due to the chaotic nature of weather. The tropospheric variability contains planetary waves which can propagate vertically into the winter stratosphere, where they break and drive a residual meridional circulation. For cases of exceptionally strong planetary wave activity, this circulation can induce a polar stratospheric warming and a subsequent downward propagation of the circulation anomaly to the troposphere. This process provides an increased predictability for the troposphere. The predictability is associated with the zonal mean zonal wind around 60ºN and the Northern Annular Mode, which tends to a negative phase after a stratospheric warming event. The negative Northern Annular Mode phase yields colder temperatures in Mid- and Northern Europe. Thus, the coupled troposphere-stratosphere system improves tropospheric predictability on monthly to seasonal time-scales. This article reviews briefly the observed phenomena, the current theoretical understanding, and the role for numerical weather prediction.

  • 10.
    Körnich, Heiner
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Becker, Erich
    A simple model for the interhemispheric coupling of the middle atmosphere circulation2010In: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, Vol. 45, no 5, p. 661-668Article in journal (Refereed)
    Abstract [en]

    The interhemispheric coupling of the middle atmosphere general circulation is characterized by a global anomaly pattern of the zonal-mean temperature. This pattern reflects an anomalous stratospheric and mesospheric residual circulation, in which a weaker (stronger) stratospheric winter circulation is linked to an upward (downward) shift of its upper mesospheric branch reaching from the summer to the winter pole. This phenomenon is robust in observational data and several middle atmosphere general circulation models. In the present study, the recently proposed mechanism of the interhemispheric coupling is unequivocally proven within the framework of a zonally symmetric model that excludes any additional effects due to resolved waves and non-zonally propagating gravity waves. Two simulations are conducted that differ in the strength of the polar vortex. A weaker polar vortex results in a downward shift of the winter mesospheric gravity wave drag. This leads to changes also in the summer upper mesosphere via a feedback solely between gravity wave breaking and the zonal-mean state. The accompanying temperature anomaly reproduces the pattern of the interhemispheric coupling. (C) 2009 COSPAR.

  • 11.
    Körnich, Heiner
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Källén, Erland
    Stockholm University, Faculty of Science, Department of Meteorology .
    Climate sensitivity and variability examined in a global climate model2009In: Multiscale Modeling and Simulation in Science., Berlin: Springer Verlag , 2009, p. 299-302Chapter in book (Other academic)
  • 12.
    Körnich, Heiner
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Källén, Erland
    Stockholm University, Faculty of Science, Department of Meteorology.
    Combining the mid-latitudinal and equatorial mass/wind balance relationships in global data assimilation2008In: Tellus. Series A: Dynamic Meteorology and Oceanography, Vol. 60, no 2, p. 261-272Article in journal (Refereed)
  • 13.
    Nilsson, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology.
    A conceptual model of the surface salinity distribution in the oceanic Hadley cell2008In: Journal of Climate, Vol. 21, no 24, p. 6586-6598Article in journal (Refereed)
    Abstract [en]

    A conceptual model of the salinity distribution in the oceanic Hadley cell is presented. The model pertains to the region of tropical easterly surface winds, where the surface salinity increases poleward from a local salinity minimum near the equator to a subtropical salinity maximum. A fundamental constraint is that the meridional freshwater transports in the atmosphere and the ocean have the same magnitude but opposite directions. A key assumption is that the strength of the meridional overturning cells in the atmosphere and the ocean is proportional and set by the surface layer Ekman transport. It is further assumed that, to the lowest order of approximation, the zonal-mean Ekman transports accomplish the meridional freshwater transports, that is, eddy fluxes and gyre-induced transports are ignored. The model predicts that the salinity variation in the oceanic cell is directly proportional to the specific humidity of the near-surface air, but independent of the meridional mass transport (as long as the atmospheric and oceanic mass transports remain proportional). If the relative humidity of the near-surface air is constant, the salinity variation in the oceanic Hadley cell varies essentially with the surface temperature according to the Clausius–Clapeyron expression for the saturation vapor pressure. Further, the model is compared to observations and a global warming simulation and found to give a leading-order description of the tropical surface salinity range.

  • 14. Peters, Dieter
    et al.
    Vargin, Pawel
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology.
    A study of the zonally asymmetric tropospheric forcing of the austral vortex splitting during September 20022007In: Tellus. Series A: Dynamic Meteorology and Oceanography, ISSN 0280-6495, Vol. 59, no 3, p. 384-394Article in journal (Refereed)
  • 15.
    Salih, Abubakr A. M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Climate impact of deforestation over South Sudan in a regional climate model2013In: International Journal of Climatology, ISSN 0899-8418, E-ISSN 1097-0088, Vol. 33, no 10, p. 2362-2375Article in journal (Refereed)
    Abstract [en]

    This study examines the sensitivity of climate to changes in vegetation cover and land use in South Sudan. The focus lies on the effect of deforestation on precipitation and surface temperature especially during the rainy season. Sensitivity experiments are performed with the third version of the Abdus Salam International Centre for Theoretical Physics Regional Climate Model (RegCM3) where the present forest and vegetation cover south of 10 degrees N in Sudan are replaced by either grass or, as an extreme case, desert. The model experiments were conducted for a time period of almost 21 years, from January 1989 to August 2009, and were preceded by a control experiment to ascertain the fidelity of the model simulations. The experiments indicate that the vegetation changes affect precipitation and surface temperature in both Southern and Central Sudan significantly although the land cover changes were imposed only in the south. The precipitation during the rainy season (June through September) was reduced in the perturbed region by about 0.1-2.1 mm d(-1) for the desert scenario and by 0.1-0.9 mm d(-)1 for the grass scenario. The surface temperature increases by about 1.2 and 2.4 degrees C in the grass and desert scenario, respectively. The precipitation reduction is thus not only local but also extends to Central Sudan and neighbouring regions. The study demonstrates significant dependency for Southern and Central Sudan precipitation on the land use in Southern Sudan and indicates that the deforestation has both local and non-local regional climatic effects.

  • 16.
    Stoffelen, Ad
    et al.
    KNMI, The Netherlands.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Marseille, Gert-Jan
    KNMI, The Netherlands.
    Houchi, Karim
    KNMI, The Netherlands.
    de Kloe, Jos
    KNMI, The Netherlands.
    Assessment of Optical and Dynamical Atmospheric Heterogeneity2009Report (Other academic)
  • 17.
    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, p. 1819-1852Article in journal (Other academic)
  • 18.
    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, p. 591-608Article 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.

  • 19.
    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.

  • 20.
    Syed, Faisal Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Körnich, 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 AsiaIn: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894Article 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 must therefore be governed by some large scale phenomenon on the inter-annual time scale. Significant trends were found in the fog frequencies and this increase is not gradual, as seen in the humidity, but shows 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 detected fog index is able to reproduce the regime shift around 1998 and shows that the method can be applied to detect fog over south Asia. The inter-annual variability seems to be associated with the wave train originating from north Atlantic in the upper atmosphere that causes higher pressure over the region, resulting in 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.

  • 21.
    Syed, Faisal Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Yoo, J. H.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kucharski, F.
    Are intraseasonal summer rainfall events micro monsoon onsets over the western edge of the South-Asian monsoon?2010In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 98, no 2-4, p. 341-346Article in journal (Refereed)
    Abstract [en]

    A thermodynamic structure leading the active phase (AP) of the Western Edge of the South-Asian Monsoon (WESAM) is investigated The APs seems to have significant contribution in the mean seasonal rainfall in the region A few days before APs the upper level warm anomaly appears over the north Hindu Kush-Himalaya (HKH) region and It is reinforced by surface heating yielding the column average warming The height anomalies are baroclinic with the low-level anticyclone being located at the east of warming The low-level anticyclone causes the moisture convergence at the core WESAM region As the region keeps warming, the height anomalies and associated low-level convergence become stronger The AP starts when the low-level moisture convergence is strong enough to overcome the preexisting stable atmospheric condition due to the upper level warming The proposed mechanism of APs has some resemblance with large scale south Asian monsoon onset, whereas conventional south Asian monsoon intraseasonal oscillations (ISOs) do not show clear relationship with APs of WESAM

  • 22.
    Syed, Faisal Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Pakistan Meteorological Department, Islamabad, Pakistan .
    Yoo, Jin Hoo
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kucharski, Fred
    Extratropical Influences on the Inter-Annual Variability of South-Asian Monsoon2012In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 38, no 7-8, p. 1661-1674Article in journal (Refereed)
    Abstract [en]

    The effects of extratropical dynamics on the interannual variations in South-Asian Monsoon (SAM) are examined. Based on NCEP/NCAR reanalysis and CRU precipitation data, a conditional maximum covariance analysis is performed on sea level pressure, 200 hPa geopotential heights and the SAM rainfall by removing the linear effects of El-Niño Southern Oscillation from the fields. It is found that two modes provide a strong connection between the upper-level circulation in the Atlantic/European region and SAM rainfall: the Circumglobal Teleconnection (CGT) and the Summer North Atlantic Oscillation (SNAO). The structures in the 200 hPa heights of both modes in the Atlantic region are similar in the Atlantic region, and their southeastward extension to South Asia (SA) also corresponds to upper-level ridges (in their positive phases) in slightly different positions. Nevertheless, the influence of both modes on SAM rainfall is distinct. Whereas a positive CGT is related to a widespread increase of rainfall in SAM, a positive SNAO is related to a precipitation dipole with its positive phase over Pakistan and the negative phase over northern India. The physical mechanisms for the influence of CGT and SNAO on SAM are related to the upper-level geopotential anomaly which affects the amplitude and position of the low-level convergence. The small displacements of the centers of these responses and the low level cold advection from the north east of SA in case of SNAO explain the differences in the corresponding SAM rainfall distributions. These findings are confirmed with the relatively high-resolution coupled climate model EC-Earth, which gives confidence in the physical basis and robustness of these extratropical variability modes and their influence on the South-Asian monsoon rainfall.

  • 23.
    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, p. 951-962Article 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.

  • 24.
    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, p. 1659-1696Article in journal (Other academic)
  • 25.
    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, p. 609-626Article 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.

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