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  • 1
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    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 30 (15). Nr. 1781.
    Publication Date: 2018-02-20
    Description: It is shown that the Arctic averaged wintertime temperature variability during the 20th century can be essentially described by two orthogonal modes. These modes were identified by an Empirical Orthogonal Function (EOF) decomposition of the 1892-1999 surface wintertime air temperature anomalies (40degreesN-80degreesN) using a gridded dataset covering high Arctic. The first mode (1st leading EOF) is related to the NAO and has a major contribution to Arctic warming during the last 30 years. The second one (3rd leading EOF) dominates the SAT variability prior to 1970. A correlation between the corresponding principal component PC3 and the Arctic SAT anomalies is 0.79. This mode has the largest amplitudes in the Kara-Barents Seas and Baffin Bay and exhibits no direct link to the large-scale atmospheric circulation variability, in contrast to the other leading EOFs. We suggest that the existence of this mode is caused by long-term sea ice variations presumably due to Atlantic inflow variability
    Type: Article , PeerReviewed
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  • 2
    Publication Date: 2019-09-23
    Description: Present global warming is amplified in the Arctic and accompanied by unprecedented sea ice decline. Located along the main pathway of Atlantic Water entering the Arctic, the Barents Sea is the site of coupled feedback processes that are important for creating variability in the entire Arctic air-ice-ocean system. As warm Atlantic Water flows through the Barents Sea, it loses heat to the Arctic atmosphere. Warm periods, like today, are associated with high northward heat transport, reduced Arctic sea ice cover, and high surface air temperatures. The cooling of the Atlantic inflow creates dense water sinking to great depths in the Arctic Basins, and ~60% of the Arctic Ocean carbon uptake is removed from the carbon-saturated surface this way. Recently, anomalously large ocean heat transport has reduced sea ice formation in the Barents Sea during winter. The missing Barents Sea winter ice makes up a large part of observed winter Arctic sea ice loss, and in 2050, the Barents Sea is projected to be largely ice free throughout the year, with 4°C summer warming in the formerly ice-covered areas. The heating of the Barents atmosphere plays an important role both in “Arctic amplification” and the Arctic heat budget. The heating also perturbs the large-scale circulation through expansion of the Siberian High northward, with a possible link to recent continental wintertime cooling. Large air-ice-ocean variability is evident in proxy records of past climate conditions, suggesting that the Barents Sea has had an important role in Northern Hemisphere climate for, at least, the last 2500 years.
    Type: Article , PeerReviewed
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  • 3
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    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 27 (17). pp. 2693-2696.
    Publication Date: 2018-02-14
    Description: The problem of the world greatest lake, the Caspian Sea, level changes attracts the increased attention due to its environmental consequences and unique natural characteristics. Despite the huge number of studies aimed to explain the reasons of the sea level variations the underlying mechanism has not yet been clarified. The important question is to what extent the CSL variability is linked to changes in the global climate system and to what extent it can be explained by internal natural variations in the Caspian regional hydrological system. In this study an evidence of a link between the El Nino/Southern Oscillation phenomenon and changes of the Caspian Sea level is presented. This link was also found to be dominating in numerical experiments with the ECHAM4 atmospheric general circulation model on the 20th century climate.
    Type: Article , PeerReviewed
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  • 4
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    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Atmospheres, 115 . D21111.
    Publication Date: 2018-02-06
    Description: The recent overall Northern Hemisphere warming was accompanied by several severe northern continental winters, as for example, extremely cold winter 2005/2006 in Europe and northern Asia. Here we show that anomalous decrease of wintertime sea ice concentration in the Barents-Kara (B-K) Seas could bring about extreme cold events like winter 2005/2006. Our simulations with the ECHAM5 general circulation model demonstrate that lower-troposphere heating over the B-K Seas in the Eastern Arctic caused by the sea ice reduction may result in strong anti-cyclonic anomaly over the Polar Ocean and anomalous easterly advection over northern continents. This causes a continental-scale winter cooling reaching -1.5°C, with more than three times increased probability of cold winter extremes over large areas including Europe. Our results imply that several recent severe winters do not conflict the global warming picture but rather supplement it, being in qualitative agreement with the simulated large-scale atmospheric circulation realignment. Furthermore, our results suggest that high-latitude atmospheric circulation response to the B-K sea ice decrease is highly nonlinear and characterized by transition from anomalous cyclonic circulation to anticyclonic one and then again back to cyclonic type of circulation as the B-K sea ice concentration gradually reduces from 100% to ice free conditions. We present a conceptual model which may explain the nonlinear local atmospheric response in the B-K Seas region by counter play between convection over the surface heat source and baroclinic effect due to modified temperature gradients in the vicinity of the heating area.
    Type: Article , PeerReviewed
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  • 5
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    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 35 . L11701.
    Publication Date: 2017-11-08
    Description: Observed multidecadal variability (30 yr running means, trends, and moving standard deviations) of the North Atlantic Oscillation (NAO) during the instrumental record is compared to that simulated by two different coupled general circulation models in extended-range control experiments. Simulated NAO exhibits strong low frequency fluctuations, even on multi-centennial time scale. Observed multi-decadal NAO variations agree well with the model variability. Trend probability distribution functions, observed and simulated, were not found to be different with statistical significance. Thus, multi-decadal NAO changes similar to those observed during the instrumental record, including the recent increase in 1965–1995, may be internally generated within the coupled atmosphere-ocean system without considering external forcing.
    Type: Article , PeerReviewed
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  • 6
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    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Atmospheres, 120 (24). pp. 12500-12513.
    Publication Date: 2019-04-04
    Description: Climate model resolution can affect both the climate change signal and present-day representation of extreme precipitation. The need to parametrize convective processes raises questions about how well the response to warming of convective precipitation extremes is captured in such models. In particular, coastal precipitation extremes can be sensitive to sea surface temperature (SST) increase. Taking a recent coastal precipitation extreme as a showcase example, we explore the added value of convection-permitting models by comparing the response of the extreme precipitation to a wide range of SST forcings in an ensemble of regional climate model simulations using parametrized and explicit convection. Compared at the same spatial scale, we find that the increased local intensities of vertical motion and precipitation in the convection-permitting simulations play a crucial role in shaping a strongly nonlinear extreme precipitation response to SST increase, which is not evident when convection is parametrized. In the convection-permitting simulations, SST increase causes precipitation intensity to increase only until a threshold is reached, beyond which further SST increase does not enhance the precipitation. This flattened response results from an improved representation of convective downdrafts and near-surface cooling, which damp the further intensification of precipitation by stabilizing the lower troposphere locally and also create cold-pools that cause subsequent convection to be triggered at sea, rather than by the coastal orography. These features are not well represented in the parametrized convection simulations, resulting in precipitation intensity having a much more linear response to increasing SSTs
    Type: Article , PeerReviewed
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  • 7
    Publication Date: 2021-01-08
    Description: The ability of state‐of‐the‐art regional climate models to simulate cyclone activity in the Arctic is assessed based on an ensemble of 13 simulations from 11 models from the Arctic‐CORDEX initiative. Some models employ large‐scale spectral nudging techniques. Cyclone characteristics simulated by the ensemble are compared with the results forced by four reanalyses (ERA‐Interim, National Centers for Environmental Prediction‐Climate Forecast System Reanalysis, National Aeronautics and Space Administration‐Modern‐Era Retrospective analysis for Research and Applications Version 2, and Japan Meteorological Agency‐Japanese 55‐year reanalysis) in winter and summer for 1981–2010 period. In addition, we compare cyclone statistics between ERA‐Interim and the Arctic System Reanalysis reanalyses for 2000–2010. Biases in cyclone frequency, intensity, and size over the Arctic are also quantified. Variations in cyclone frequency across the models are partly attributed to the differences in cyclone frequency over land. The variations across the models are largest for small and shallow cyclones for both seasons. A connection between biases in the zonal wind at 200 hPa and cyclone characteristics is found for both seasons. Most models underestimate zonal wind speed in both seasons, which likely leads to underestimation of cyclone mean depth and deep cyclone frequency in the Arctic. In general, the regional climate models are able to represent the spatial distribution of cyclone characteristics in the Arctic but models that employ large‐scale spectral nudging show a better agreement with ERA‐Interim reanalysis than the rest of the models. Trends also exhibit the benefits of nudging. Models with spectral nudging are able to reproduce the cyclone trends, whereas most of the nonnudged models fail to do so. However, the cyclone characteristics and trends are sensitive to the choice of nudged variables.
    Type: Article , PeerReviewed
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  • 8
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    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 33 (L11701).
    Publication Date: 2018-02-19
    Description: The impacts of the tropical Indian and Atlantic Oceans on ENSO are studied using a series of 500 years long GCM simulations, in which the tropical Indian and/or Atlantic Ocean SSTs are fixed. The results indicate that the tropical Indian and/or Atlantic Oceans SST anomalies substantially influence the coupling over the equatorial Pacific. In the absence of SST variability in the tropical Indian and/or Atlantic Ocean, the main ENSO period is shifted by almost one year. The total SST variance in the equatorial Pacific region is reduced if either Indian or Atlantic Ocean variability is present. At the same time the atmospheric ENSO teleconnections are damped more strongly than the SST. The results can be understood in the context of the recharge oscillator model. However, it is difficult to verify the feedback of the Indian and/or Atlantic Oceans onto ENSO only with statistical analyses of the coupled model control integration or observations.
    Type: Article , PeerReviewed
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  • 9
    Publication Date: 2023-11-08
    Description: A new mechanism for rapid climate transitions in the high latitudes is presented which involves complex ocean-sea ice-atmosphere interactions. A shutdown of the Barents Sea Inflow (BSI) which carries a vast amount of heat into the Arctic Ocean is at the heart of the mechanism. The BSI shutdown is studied in a multi-millennium integration with a global climate model forced by periodically (1000 yr) varying solar constant (+/- 2 W/m(2)). A positive feedback between the inflow and sea ice cover is revealed in the model, which triggers rapid climate changes. The BSI shutdown events are associated with strong cooling in the northern latitudes and subsequent rearrangement of the Arctic Ocean surface current system. The results reveal the existence of a bifurcation point in the Arctic climate system and demonstrate that rapid climate transitions may be caused by local feedbacks and restricted to confined areas without significant global impacts. Citation: Semenov, V. A., W. Park, and M. Latif (2009), Barents Sea inflow shutdown: A new mechanism for rapid climate changes, Geophys. Res. Lett., 36, L14709, doi: 10.1029/2009GL038911.
    Type: Article , PeerReviewed
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  • 10
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    AGU (American Geophysical Union)
    In:  Global Biogeochemical Cycles, 22 . GB1008.
    Publication Date: 2019-09-23
    Description: A simple geochemical box model for the global cycle of methane (CH4) has been developed and applied to reconstruct the evolution of atmospheric CH4 over the entire Phanerozoic. According to the model, the partial pressure of atmospheric CH4 (pCH4) increased up to approximately 10 ppmv during the Carboniferous coal swamp era. This implies a maximum radiative forcing of about 3.5 W m−2 via CH4. Through its radiative forcing, CH4 heated the average global surface temperature by up to 1°C. The elevated pCH4 values during the Permian-Carboniferous cold period may have moderated the temperature decline caused by the coeval drawdown of atmospheric CO2. Additional runs with a global carbon model indicate that the heating induced by elevated pCH4 favored the drawdown of atmospheric pCO2 via enhanced rates of silicate weathering. Simulations with a state-of-the-art climate model reveal that the effects of atmospheric CH4 on average global surface temperature also depend on the partial pressures of CO2. The CH4 climate effect is amplified by high background levels of atmospheric CO2 such that a coeval increase in the partial pressure of both greenhouse gases has a much stronger climate effect than previously anticipated.
    Type: Article , PeerReviewed
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