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Climate Variability and Prediction in High Latitudes

The Arctic sea ice and high latitude atmosphere and oceans have experienced significant changes over the modern observational era. The polar climate is crucial for the Earth’s energy and water budget, and its variability and change have direct socio-economic and ecological impacts. Thus, understanding high-latitude variability and improving predictions of high latitude climate is highly important for society. Long-term variability in ocean and sea ice are the largest sources for predictability in high latitudes. Dynamical model predictions are not yet in the position to provide us with highly accurate predictions of the polar climate. Main reasons for this are the lack of observations in high latitudes, insufficient initialization methods and shortcomings of climate models in representing some of the important climate processes in high latitudes.

This session aims for a better understanding and better representation of the mechanisms that control high latitude variability and predictability of climate in both hemispheres from sub-seasonal to multi-decadal time-scales in past, recent and future climates. Further, the session aims to discuss ongoing efforts to improve climate predictions at high latitudes at various time scales (as e.g. usage of additional observations for initialization, improved initialization methods, impact of higher resolution, improved parameterizations, novel verification approaches) and potential teleconnections of high latitude climate with lower latitude climate. We also aim to link polar climate variability and predictions to potential ecological and socio-economic impacts and encourage submissions on this topic.

The session offers the possibility to present results from ongoing projects and research efforts on the topic of high-latitude climate variability and prediction, including, but not limited to, the WMO Year of Polar Prediction (YOPP), NordForsk-project ARCPATH, MOSAiC, and the H2020-projects APPLICATE, INTAROS, BlueAction, and KEPLER.

Co-organized by AS1/CR7/OS1
Convener: Neven-Stjepan Fuckar | Co-conveners: Yongqi Gao, Helge Goessling, Torben Koenigk
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Fri, 30 Apr, 13:30–15:00

Chairpersons: Helge Goessling, Neven-Stjepan Fuckar

5-minute convener introduction

Zhengxuan Yuan et al.

The Antarctic Oscillation (AAO) is the dominant mode of the southern extratropical atmospheric mass variability which has potential influences on the Northern Hemisphere (NH). This study reveals a significantly negative correlation between the September-October (SO) AAO index and the occurrence rate of following January-February (JF) wet and cold weather in the Middle and Lower Reaches of Yangtze River Basin (MLRY) in China. The latter is quantified by a Precipitation-Temperature (PT) Index. JF PT is modulated by both northerly air flow in the lower troposphere and southerly air flow in the lower-middle troposphere. The SO AAO stimulates Southern Ocean Dipole (SOD) pattern-like SST anomalies, which induces a North Atlantic Oscillation (NAO)-like atmospheric response in the following JF through ocean-air interaction. As for the northerly flow, the JF NAO-like pattern triggers an eastward propagating wave train, influencing the intensity of East Asian Winter Monsoon (EAWM) and subsequently the northerly cold flow to MLRY. As for southerly flow, the variation of JF SOD regulates the local meridional cell, in turn modulating the Middle East Jet Stream (MEJS) along with the NAO-like pattern, influencing the intensity of precipitation and the wet and warm flow over Southern China and the adjacent regions. In addition to the tropospheric processes, the stratospheric Quasi Biennial Oscillation (QBO) serves as the ‘bridge’ for linking SOD to NH climate, inducing the JF PT response to SOD SST. To summarize, SO AAO affects the JF PT in MLRY by modulating both cold-dry northerly air flow and warm-wet southerly air flow through ocean-atmosphere interactions and stratospheric pathway.

How to cite: Yuan, Z., Qin, J., Li, S., Huang, S., Mbululo, Y., and Rehman, A.: Impact of Boreal Autumn Antarctic Oscillation on Winter Wet and Cold Weather in East Asia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-351, https://doi.org/10.5194/egusphere-egu21-351, 2020.

Steve Delhaye et al.

The retreat of Arctic sea ice for the last four decades is a primary manifestation of the climate system response to increasing atmospheric greenhouse gas concentrations. This retreat is frequently considered as a possible driver of atmospheric circulation anomalies at mid-latitudes. However, the year-to-year evolution of the Arctic sea ice cover is also characterized by significant fluctuations attributed to internal climate variability. It is unclear how the atmosphere will respond to a near-total retreat of summer Arctic sea ice, a reality that might occur in the foreseeable future. This study uses sensitivity experiments  with higher and lower horizontal resolution configurations of three global coupled climate models to investigate the local and remote atmospheric responses to a reduction in Arctic sea ice cover during the preceding summer. Recognizing that these responses likely depend on the model itself and on its horizontal resolution, and that the model’s internally-generated climate variability may obscure the atmospheric response, we design a protocol to compare each source separately. After imposing a 15-month albedo perturbation resulting in a sudden summer Arctic sea ice loss, the remote mid-latitude climate response has a very low signal-to-noise ratio such that internal climate variability dominates the uncertainty of the response, regardless of the atmospheric variable. Indeed, more than 28, 165 and 210 members are needed to detect a robust response in surface air temperature, precipitation and sea level pressure to sea ice loss in Europe, respectively. Finally, we find that horizontal resolution plays a secondary role in the uncertainty of the atmospheric response to substantial perturbation of Arctic sea ice. These findings suggest that even with higher resolution model configurations, it is important to have large ensemble sizes to increase the signal to noise ratio for the mid-latitude atmospheric response to sea ice changes.

How to cite: Delhaye, S., Fichefet, T., Massonnet, F., Docquier, D., Roberts, C., Keeley, S., Senan, R., Msadek, R., Chripko, S., García-Serrano, J., and Bretonnière, P.-A.: Role of the internal climate variability in the atmospheric response to a sudden summer Arctic sea ice loss, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2528, https://doi.org/10.5194/egusphere-egu21-2528, 2021.

Daniela Flocco et al.

Assimilation of sea ice concentration satellite products has successfully been used to initialize sea ice models and coupled NWP systems. Sea-ice thickness observations, being much less mature, are typically not assimilated. However, many studies suggest that initialization of winter sea-ice thickness could lead to improved prediction of Arctic summer sea ice. We have examined the potential for sea ice thickness observations to improve forecast skill on timescales from days to a year ahead in two state-of-the-art coupled GCMs.

Here we examine the influence of Arctic sea-ice thickness observations on the potential predictability of the sea-ice and atmospheric circulation using idealised ‘data denial’ experiments. We perform paired sets of ensembles with the HadGEM3 and EC-Earth GCMs using different initial conditions retrieved from present-day control runs.

One set of ensembles start with complete information about the sea-ice conditions and is treated as “truth”, and one set has degraded sea ice information. We investigate how the pairs of ensembles, all started in January, predict the subsequent evolution of the sea-ice state, sea level pressure and circulation within the Arctic with the aim of quantifying the value of sea-ice observations for improving predictions.

We show that accurate initialization of sea ice thickness improves the model prediction skill during the first month of simulation and that several sea ice state and atmospheric variables present a re-emergence of skill in September. Prediction skill of several oceanic variables is also observed. The two models present a good agreement in terms of the regions where they show either a skill gain or loss.

How to cite: Flocco, D., Hawkins, E., Ponsoni, L., Massonnett, F., Feltham, D., and Fichefet, T.: Sea ice and atmospheric potential predictability in coupled GCMs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2590, https://doi.org/10.5194/egusphere-egu21-2590, 2021.

Ramón Fuentes-Franco et al.

The influence of Rossby wave sources (RWS) emitted on the Northeastern Pacific Ocean (NePO) in the Northern Hemisphere during summer is analysed in the ERA5 reanalysis and a large ensemble performed with the EC-Earth3 model. Using extreme years composites of precipitation, surface temperature and geopotential height, we found a causal influence of the Rossby waves generated over the NePO on a global climate response. Both the reanalysis ERA5 and the EC-Earth3 large ensemble show that RWS triggers wave-like patterns arising from the upper troposphere NePO region. We show that an increased Rossby wave sources intensity is related with a) negative temperature anomalies over western North America, b) positive temperature anomalies over eastern North America, c) increased precipitation over Northern Europe during summer and d) sea-ice concentration decrease in the Arctic.  We also show that the North Atlantic plays a very important role hindering or permitting that Rossby waves generated in the Pacific reach the Atlantic and modulate the atmospheric conditions over Europe. Such conditions were found in ERA5 and EC-Earth3 large ensemble during colder and icier conditions over the North Atlantic.

How to cite: Fuentes-Franco, R., Koenigk, T., Docquier, D., Graef, F., and Wyser, K.: Exploring the influence of the North Pacific Ocean Rossby wave sources on interannual variability of summer precipitation and surface temperature over the Northern Hemisphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3270, https://doi.org/10.5194/egusphere-egu21-3270, 2021.

Yushi Morioka et al.

Skillful sea-ice prediction in the Antarctic Ocean remains a big challenge due to paucity of sea-ice observations and insufficient representation of sea-ice processes in climate models. This study demonstrates that the Antarctic sea-ice concentration (SIC) prediction is significantly improved using a coupled general circulation model (SINTEX-F2) in which the model’s SIC and sea-ice thickness (SIT) are initialized with the ocean/sea-ice reanalysis product (C-GLORSv7). It is found that the wintertime SIT initialization adds positive values to the prediction skills of the summertime SIC, most effectively in the Weddell Sea where the SIT climatology and variability are the largest among the Antarctic Seas. Examination of the SIT balance during low sea-ice years of the Weddell Sea shows that negative SIT anomalies initialized in June retain the memory throughout austral winter (July-September) owing to horizontal advection of the SIT anomalies by sea-ice velocities. The negative SIT anomalies continue to develop in austral spring (October-December) owing to more incoming solar radiation via ice-albedo feedback and the associated warming of mixed layer. This results in further sea-ice decrease during austral summer (January-March). Concomitantly, the model reasonably reproduces atmospheric circulation anomalies in the Amundsen-Bellingshausen Seas as well as the Weddell Sea during the development of the negative sea-ice anomalies. These results provide solid evidence that the wintertime SIT initialization benefits skillful summertime sea-ice prediction in the Antarctic Seas.

How to cite: Morioka, Y., Iovino, D., Cipollone, A., Masina, S., and Behera, S.: Improved sea-ice prediction in the Weddell Sea using sea-ice thickness initialization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3638, https://doi.org/10.5194/egusphere-egu21-3638, 2021.

Xavier Levine et al.

As climate warms sea ice loss may become a potent climate change feedback, both in the Arctic and at lower latitudes. For instance, extreme events over Europe and North America, such as drought or warm spells, have been attributed to sea ice minima in recent years. Yet a comprehensive understanding of the local or remote impact of sea ice loss on climate is lacking, with the predicted atmospheric and oceanic response to sea ice loss differing between climate studies. In particular, the impact of varying geographical distribution of sea ice loss on regional climatic changes remains uncertain.

Here, we assess the sensitivity of the atmospheric response to various patterns of sea ice loss, at a pan-Arctic or regional scale, by analyzing a set of idealised AMIP-like simulations. Depending on where sea ice is reduced, we find that climatic anomalies can vary widely among experiments, especially the zonal-mean component of the tropospheric circulation: for instance, the subpolar jet and polar cell can strengthen or weaken with sea ice loss, depending on its geographical distribution. We demonstrate that the geometry of the sea ice loss, in particular the degree to which sea ice extent changes is zonally symmetric or asymmetric, controls this disparate climatic response through an atmospheric feedback mechanism. In this feedback mechanism, changes in poleward eddy heat flux and latent heat release over the Arctic in response to a specific sea ice loss pattern can either warm or cool the Arctic troposphere. We discuss the implications of our results for interpreting the apparent discrepancies in the climate response to Arctic sea ice variability among studies.

How to cite: Levine, X., Cvijanovic, I., Ortega, P., Donat, M., and Tourigny, E.: A mechanism predicting the climate response to sea ice loss from its geometry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7251, https://doi.org/10.5194/egusphere-egu21-7251, 2021.

Caroline Holmes et al.

Results from CMIP5 have previously suggested that ensemble regression techniques or model selection may provide solutions to the challenge of making projections of future Antarctic sea ice area (SIA) in the presence of large historical biases. Here, we revisit and extend such analysis incorporating the CMIP6 ensemble, which shows modest improvements in some aspects of sea ice simulation and in particular a reduction of inter-model spread in historical SIA. We focus on the strongest forcing scenarios analysed, CMIP5 RCP85 and CMIP6 SSP5.85.

In summer (February) the historical climatology of SIA is a strong linear constraint on projections of SIA in both generations. This is because the strong forcing leads to the loss of the majority of summer SIA in each model, so that the models that start with greater SIA exhibit greater reductions. Differences between CMIP5 and CMIP6 are largely explained by the fact that, compared to CMIP6, CMIP5 contains many more models that have very large positive biases in historical SIA and do not lose the majority of ice.

In winter (September), a much smaller proportion of SIA is lost, but inter-model spread in SIA climatology still explains just under half the variance in projections of SIA change, in both CMIP5 and CMIP6. The mean historical winter climatology is similar between generations, as is the regression slope of SIA change against SIA climatology.  However, there is a greater reduction of SIA in CMIP6 than CMIP5. We find this to be statistically related to greater global mean warming in CMIP6 than CMIP5, and therefore potentially to the larger climate sensitivity in the CMIP6 ensemble.

These findings imply that, depending on season, a different balance of local (SIA climatology) and global (GMST change) drivers can be used to explain the inter-model and inter-generation spread in projections of SIA loss. They also firmly tie our ability to project Antarctic SIA loss to our understanding of the fidelity of higher CMIP6 climate sensitivity. Questions remain about whether models are correct in their simulation of Antarctic SIA sensitivity to global surface temperature.

How to cite: Holmes, C., Bracegirdle, T., and Holland, P.: Does CMIP6 better constrain projections of 21st century Antarctic sea ice loss?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7586, https://doi.org/10.5194/egusphere-egu21-7586, 2021.

Doug Smith and the PAMIP

The possibility that Arctic sea ice loss could weaken mid-latitude westerlies and promote more severe cold winters has sparked more than a decade of scientific debate, with support from observations but inconclusive modelling evidence. Here we analyse a large multi-model ensemble of coordinated experiments from the Polar Amplification Model Intercomparison Project and find that the modelled response is proportional to the simulated eddy momentum feedback, and that this is underestimated in all models. Hence, we derive an observationally constrained model response showing a modest weakening of mid-latitude tropospheric and stratospheric winds, an equatorward shift of the Atlantic and Pacific storm tracks, and a negative North Atlantic Oscillation. Although our constrained response is consistent with observed relationships which have weakened recently, we caution that emergent constraints may only provide a lower bound.

How to cite: Smith, D. and the PAMIP: Observationally constrained multi-model atmospheric response to future Arctic sea ice loss , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9633, https://doi.org/10.5194/egusphere-egu21-9633, 2021.

Lettie Roach et al.

It is broadly accepted that variability and trends in Arctic sea ice remain poorly simulated even in the most state-of-the-art coupled climate and climate prediction models. Here, we show that a modern coupled climate model (CESM1) is in fact able to reproduce the observed variability and decline in summer sea ice when winds are nudged towards values from reanalysis.  We argue that the nudged-winds framework provides a straightforward way of evaluating models by removing much of the contribution of internal variability, revealing model successes and biases. The results demonstrate the importance of atmospheric circulation in driving interannual variability in sea ice and near-surface air temperatures, particularly in the summer. Finally, we will discuss the potential role of ocean surface waves in driving variability in Arctic sea ice, based on observational analysis and new coupled modelling results.

How to cite: Roach, L., Blanchard-Wrigglesworth, E., and Bitz, C.: The roles of winds and waves in Arctic sea ice variability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10260, https://doi.org/10.5194/egusphere-egu21-10260, 2021.

Natalia Vazaeva et al.

Polar lows (PLs) are important mesoscale (horizontal diameter up to 1000 km) maritime weather systems at high latitudes, forming pole ward from the polar front. We consider the possible prognostic criteria of PLs, in particular, the kinematic helicity as a quadratic characteristic related to the integral vortex formations and the kinematic vorticity number (KVN). To calculate such characteristics we use reanalysis data and the results of numerical simulation with the WRF-ARW model (Version 4.1.) for the PLs over the Nordic (Norwegian and Barents) seas. For comparison, experimental data are used.

Our estimate of helicity is based on the connection of an integral helicity (IH) in the Ekman layer with the geostrophic wind velocity, due to the good correlation between IH and half the sum of the wind velocity squared. We have chosen IH averaged over preselected area covering the locality of PLs genesis. This area was moving along with the centre of PL during the numerical simulation.

The genesis of PLs can be divided into three stages: (i) an initial development stage, in which a number of small vortices appear in a shear zone; (ii) a late development stage, characterized by the merger of vortices; (iii) a mature stage, in which only a single PL is present. Approximately one day before PL formation, a significant increase in helicity was observed. The average helicity bulk density of large-scale motions has values of 0.3 – 0.4 ms-2. The local changes in helicity are adjacent to the front side of the PLs. The IH criterion described facilitates the identification of the PLs genesis area. For a more detailed analysis of the PL genesis, it is recommended to apply KVN, which is the additional indicator of PL size and intensity. At the moment of maximum intensity of PLs KVN can reach values of 12 – 14 units. The advantage of using KVN is also in its clear change directly in the centre of the emerging PLs, which allows to precisely indicates the limits of the most intense part of PLs.

The main challenge is to make the operational forecast of PLs possible through the selection of the prognostic integral characteristics of PLs, sufficient for PLs identification and for analysis of their size and intensity in a convenient, usable and understandable way. The criteria associated with vorticity and helicity are reflected in the PLs genesis and development quite clearly. At this time, such a claim is only a hypothesis, which must be tested using a larger set of cases. Future work will need to extend these analyses to other active PL basins. Also, it would be interesting to compare the representation of PLs by using any other criteria. It is intended to use our combined criteria as a precursor to machine learning-based PLs identification procedure where satellite image analysis and capture of particular cloud patterns are currently applied in most of the cases. It would eliminate the time consuming first stage of collecting data sets.

This work was supported by the Russian Science Foundation (project No. 19-17-00248).

How to cite: Vazaeva, N., Chkhetiani, O., and Kurgansky, M.: Prognostic criteria for Polar Lows, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12282, https://doi.org/10.5194/egusphere-egu21-12282, 2021.

Shuting Yang et al.

The subpolar North Atlantic (SPNA) is a region experiencing substantial decadal variability, which has been linked to extreme weather impacts over continents. Recent studies have suggested that the connectivity with the SPNA may be a key to predictions in high latitudes. To understand the impact of the SPNA on predictability of North Atlantic-European sectors and the Arctic, we use two climate prediction systems, EC-Earth3-CPSAI and NorCPM1, to perform ensemble pacemaker experiments with a focus on the subpolar extreme cold anomaly event in 2015. This 2015 cold anomaly event is generally underestimated by the decadal prediction systems. In order to force the model to better represent the observed anomaly in SPNA, we apply nudging in a region of the SPNA (i.e., 51.5°W - 13.0°W, 30.4°N - 57.5°N, and from surface to 1000 m depth in the ocean). Here ocean temperature and salinity is restored to observed conditions from reanalysis in both model systems. All other aspects of the setup of this pacemaker experiment follow the protocol for the CMIP6 DCPP-A hindcasts and initialized on November 1, 2014. The restoration is applied during the hindcasts from November 2014 to December 2019. Multi-member ensembles of 10-year hindcasts are performed with 10 members for the EC-Earth3-CPSAI and 30 members for the NorCPM1.

The time evolution of ensembles of the initialized nudging hindcasts (EXP1) is compared with the initialized DCPP-A hindcast ensembles (EXP2) and the uninitialized ensembles (EXP3). The prediction skills of the three sets of experiments are also assessed. It can be seen that restoring the ocean temperature and salinity in the SPNA region to the reanalysis improves the prediction in the region quickly after the simulation starts, as expected. On the interannual to decadal time scales, the areas with improved prediction skills extend to over almost the entire North Atlantic for both models. The improved skill over Nordic Seas is particularly significant, especially for EC-Earth3-CPSAI. For NorCPM, the regions with improved skills extend to the entire Arctic. Our results suggest the possible role of the SPNA as a source of skillful predictions on interannual to decadal time scale, especially for high latitudes. The ocean pathways are the critical source of skill whereas our results imply a limited role of coupled feedbacks through the atmosphere.  

How to cite: Yang, S., Tian, T., Wang, Y., Schmith, T., Olsen, S. M., and Keenlyside, N.: The role of subpolar North Atlantic as a source of predictability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13204, https://doi.org/10.5194/egusphere-egu21-13204, 2021.

Irina Gorodetskaya et al.

During the last several decades, the Antarctic Peninsula (AP) has shown a much stronger warming trend compared to the rest of the ice sheet and other land areas in the Southern Hemisphere (Jones et al, 2019). Recent studies have also highlighted that the AP has experienced both an increase in precipitation and in surface melt. Atmospheric rivers (ARs) – long corridors of intense moisture transport from subtropical and mid-latitude regions poleward - are known for prominent role in moisture transport (Gorodetskaya et al, 2020) and intense precipitation in Antarctica (Gorodetskaya et al 2014). At the same time, ARs have been also associated with major surface melt events at the AP and adjacent ice shelves (Wille et al 2019). In this study, we explore the double role of ARs, as carriers of both heat and moisture, in their impacts on precipitation (rain and snow), cloud radiative forcing and air temperature at the AP. Observations from the Year of Polar Prediction (YOPP, Bromwich et al 2020) endorsed sites/projects are used: Escudero station (the Characterization of the Antarctic Atmosphere and Low Clouds, or CAALC project) and King Sejong station (South Korean Antarctic Program projects) on King George Island, as well as Punta Arenas (southern Chile; the Dynamics, Aerosol, Cloud, And Precipitation Observations in the Pristine Environment of the Southern Ocean, or DACAPO-PESO project). These projects employed a set of ground-based remote sensing instrumentation for water vapor, cloud and precipitation observations, as well as frequent radiosonde launches during the YOPP Special Observing Period in austral summer 2018/2019. We present case studies characterizing the temporal evolution of ARs, focusing on thermodynamic and dynamic conditions accompanying the transition between snowfall and rain. Further, we demonstrate the added value of assimilating more frequent radiosonde observations in improving the forecast of weather conditions during ARs using the Polar-WRF model, including wind and precipitation prediction, which have important consequences for air, ship and station operations in Antarctica.

Bromwich, D. H., K. Werner, B. Casati, J. G. Powers, I. V. Gorodetskaya, F. Massonnet, V. Vitale, et al: The Year of Polar Prediction in the Southern Hemisphere (YOPP-SH), Bull. Amer. Meteor. Soc., doi: https://doi.org/10.1175/BAMS-D-19-0255.1.

Gorodetskaya, I.V., Silva, T., Schmithüsen, H., and Hirasawa, N., 2020: Atmospheric River Signatures in Radiosonde Profiles and Reanalyses at the Dronning Maud Land Coast, East Antarctica.Adv. Atmos. Sci., https://doi.org/10.1007/s00376-020-9221-8

Gorodetskaya, I. V., M. Tsukernik, K. Claes, M. F. Ralph, W. D. Neff, and N. P. M. van Lipzig, 2014: The role of atmospheric rivers in anomalous snow accumulation in East Antarctica. Geophys. Res. Lett.,  https://doi.org/10.1002/2014GL060881

Jones, M. E., Bromwich, D. H., Nicolas, J. P., Carrasco, J., Plavcova, E., Zou, X., & Wang, A. S.-H. (2019). Sixty Years of Widespread Warming in the Southern Middle and High Latitudes (1957-2016). J. Climate, https://doi.org/10.1175/JCLI-D-18

Wille, J.D., Favier, V., Dufour, A., Gorodetskaya, I.V., Turner, J., Agosta, C., and Codron, F., 2019. West Antarctic surface melt triggered by atmospheric rivers. Nat. Geosci. https://doi.org/10.1038/s41561-019-0460-1

How to cite: Gorodetskaya, I., Rowe, P., Kalesse, H., Seifert, P., Park, S.-J., Choi, Y., and Cordero, R.: Atmospheric rivers landfalling at the Antarctic Peninsula: the Year of Polar Prediction summer special observing period measurements for model and forecast improvement , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13616, https://doi.org/10.5194/egusphere-egu21-13616, 2021.

Helene R. Langehaug et al.

In this study we assess to what extent seven different dynamical prediction systems can retrospectively predict the winter sea surface temperature (SST) in the subpolar North Atlantic and the Nordic Seas in the time period 1970-2005. We focus in particular on the region where warm water flows poleward, i.e., the Atlantic water pathway, and on interannual-to-decadal time scales. To better understand why dynamical prediction systems have predictive skill or lack thereof, we confront them with a mechanism identified from observations – propagation of oceanic anomalies from low to high latitudes – on different forecast lead times. This observed mechanism shows that warm and cold anomalies propagate along the Atlantic water pathway within a certain time frame. A key result from this study is that most models have difficulty representing this mechanism, resulting in an overall poor prediction skill after 1-2 years lead times (after applying a band-pass filter to focus on interannual-to-decadal time scales). There is a link, although not very strong, between predictive skill and the representation of the SST propagation. Observational studies demonstrate predictability several years in advance in this region, thus suggesting a great potential for improvement of dynamical climate predictions by resolving the causes for the misrepresentation of the oceanic link. Inter model differences in simulating surface velocities along the Atlantic water pathway suggest that realistic velocities are important to better circulate anomalies poleward, and hence, increase predictive skill on interannual-to-decadal time scales in the oceanic gateway to the Arctic.

How to cite: Langehaug, H. R., Ortega, P., Counillon, F., Matei, D., Maroon, E., Keenlyside, N., Mignot, J., Wang, Y., Swingedouw, D., Bethke, I., Yang, S., Danabasoglu, G., Bellucci, A., Ruggieri, P., and Nicolì, D.: Propagation of Thermohaline Anomalies and their predictive potential in the Northern North Atlantic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15092, https://doi.org/10.5194/egusphere-egu21-15092, 2021.

Glenn Rudebusch and Francis Diebold

Based on several decades of satellite data, we provide statistical forecasts of Arctic sea ice extent during the rest of this century. The best fitting statistical model indicates that overall sea ice coverage is declining at an increasing rate. By contrast, average projections from the CMIP5 global climate models foresee a gradual slowing of Arctic sea ice loss even in scenarios with high amounts of carbon emissions. Our long-range statistical projections also deliver probability assessments of the timing of an ice-free Arctic. These results indicate almost a 60 percent chance of an effectively ice-free Arctic Ocean sometime during the 2030s—much earlier than the average projection from the global climate models. Our results are also consistent with projections from bivariate regressions of sea ice extent and carbon emissions. 

How to cite: Rudebusch, G. and Diebold, F.: Probability Assessments of an Ice-Free Arctic: Comparing Statistical and Climate Model Projections, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13126, https://doi.org/10.5194/egusphere-egu21-13126, 2021.

Leonard F. Borchert and Alexander J. Winkler

Vegetation in the northern high latitudes shows a characteristic pattern of persistent changes as documented by multi-decadal satellite observations. The prevailing explanation that these mainly increasing trends (greening) are a consequence of external CO2 forcing, i.e., due to the ubiquitous effect of CO2-induced fertilization and/or warming of temperature-limited ecosystems, however does not explain why some areas also show decreasing trends of vegetation cover (browning). We propose here to consider the dominant mode of multi-decadal internal climate variability in the north Atlantic region, the Atlantic Multidecadal Variability (AMV), as the missing link in the explanation of greening and browning trend patterns in the northern high latitudes. Such a link would also imply potential for decadal predictions of ecosystem changes in the northern high latitudes.

An analysis of observational and reanalysis data sets for the period 1979-2019 shows that locations characterized by greening trends largely coincide with warming summer temperature and increasing precipitation. Wherever either cooling or decreasing precipitation occurs, browning trends are observed over this period. These precipitation and temperature patterns are significantly correlated with a North Atlantic sea surface temperature index that represents the AMV signal, indicating its role in modulating greening/browning trend patterns in the northern high latitudes.

Using two large ensembles of coupled Earth system model simulations (100 members of MPI-ESM-LR Grand Ensemble and 32 members of the IPSL-CM6A-LR Large Ensemble), we separate the greening/browning pattern caused by external CO2 forcing from that caused by internal climate variability associated with the AMV. These sets of model simulations enable a clean separation of the externally forced signal from internal variability. While the greening and browning patterns in the simulations do not agree with observations in terms of magnitude and location, we find consistent internally generated greening/browning patterns in both models caused by changes in temperature and precipitation linked to the AMV signal. These greening/browning trend patterns are of the same magnitude as those caused by the external forcing alone. Our work therefore shows that internally-generated changes of vegetation in the northern lands, driven by AMV, are potentially as large as those caused by external CO2 forcing. We thus argue that the observed pattern of greening/browning in the northern high latitudes could originate from the combined effect of rising CO2 as well as the AMV.

How to cite: Borchert, L. F. and Winkler, A. J.: The North Atlantic Ocean as a Modulator of Vegetation Greening/Browning in the Northern High Latitudes?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15595, https://doi.org/10.5194/egusphere-egu21-15595, 2021.

Stephanie Hay and Paul Kusnher

Antarctic sea ice has gradually increased in extent over the forty-year-long satellite record, in contrast with the clear decrease in sea-ice extent seen in the Arctic over the same time period. However, state-of-the-art climate models ubiquitously project Antarctic sea-ice to decrease over the coming century, much as they do for Arctic sea-ice. Several recent years have also seen record low Antarctic sea-ice. It is therefore of interest to understand what the climate response to Antarctic sea-ice loss will be. 

We have carried out new fully coupled climate model simulations to assess the response to sea-ice loss in either hemisphere separately or coincidentally under different albedo parameter settings to determine the relative importance of each. By perturbing the albedo of the snow overlying the sea ice and the albedo of the bare sea ice, we obtain a suite of simulations to assess the linearity and additivity of sea-ice loss. We find the response to sea-ice loss in each hemisphere exhibits a high degree of additivity, and can simply be decomposed into responses due to loss in each hemisphere separately. We find that the response to Antarctic sea-ice loss exceeds that of Arctic sea-ice loss in the tropics, and that Antarctic sea-ice loss leads to statistically significant Arctic warming, while the opposite is not true.

With these new simulations and one in which CO2 is instantaneously doubled , we can further characterize the response to sea-ice loss from each hemisphere using an extension to classical pattern scaling that includes three controlling parameters. This allows us to simultaneously compute the sensitivity patterns to Arctic sea-ice loss, Antarctic sea-ice loss, and to tropical warming. The statistically significant response to Antarctic sea-ice loss in the Northern Hemisphere extratropics is found to be mediated by tropical warming and small amounts of Arctic sea-ice loss.

How to cite: Hay, S. and Kusnher, P.: The relative roles of Arctic and Antarctic sea-ice loss in the response to greenhouse warming, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16310, https://doi.org/10.5194/egusphere-egu21-16310, 2021.

Tristan Rendfrey and Ashley Payne
Climatic changes induce many significant changes to long standing weather patterns. These mechanisms interact to drive consequences that may not be immediately obvious. One such connection involves the apparent relationship between polar sea ice extent and mid-latitude precipitation timing and location. This correlation, its mechanisms, and possible influences on weather are decently understood with respect to the Northern Hemisphere. However, the analogous relation for the Southern Hemisphere has been less studied. This provides an opportunity to examine connections between polar conditions and mid-latitude weather.
We explore the teleconnection between sea ice extent and lower latitude precipitation over the Southern Hemisphere. We investigate this relationship through observations of sea ice coverage using ICESat and ICESat-2 compared with reanalysis data via MERRA-2 in order to understand the variability of sea ice extent and its impact on midlatitude precipitation over the Southern Hemisphere. This study particularly examines the importance of seasonality and regional variations of the relationship.

How to cite: Rendfrey, T. and Payne, A.: Connecting Antarctic Sea Ice and Mid-latitude Precipitation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16393, https://doi.org/10.5194/egusphere-egu21-16393, 2021.

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