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The observed relationships between anomalous Antarctic sea ice concentration (SIC) and the leading patterns of Southern Hemisphere (SH) large-scale climate variability are examined as a function of season over 1980-2008. Particular emphasis is placed on 1) the interactions between SIC, the southern annular mode (SAM), and El Niñ o-Southern Oscillation (ENSO); and 2) the contribution of these two leading modes to the 29-yr trends in sea ice. Regression, composite, and principal component analyses highlight a seasonality in SH sea ice-atmosphere interactions, whereby Antarctic sea ice variability exhibits the strongest linkages to the SAM and ENSO during the austral cold season months. As noted in previous work, a dipole in SIC anomalies emerges in relation to the SAM, characterized by centers of action located near the Bellingshausen/Weddell and Amundsen/eastern Ross Seas. The structure and magnitude of this SIC dipole is found to vary considerably as a function of season, consistent with the seasonality of the overlying atmospheric circulation anomalies. Relative to the SAM, the pattern of sea ice anomalies linked to ENSO exhibits a similar seasonality but tends to be weaker in amplitude and more diffuse in structure. The relationships between ENSO and sea ice also exhibit a substantial nonlinear component, highlighting the need to consider both season and phase of the ENSO cycle when diagnosing ENSO-SIC linkages. Trends in SIC over 1980-2008 are not significantly related to trends in either the SAM or ENSO during any season, including austral summer when the trend in the SAM is most pronounced.

The austral spring relationships between sea surface temperature (SST) trends and the Southern Hemisphere (SH) extratropical atmospheric circulation are investigated using an atmospheric general circulation model (AGCM). A suite of simulations are analyzed wherein the AGCM is forced by underlying SST conditions in which recent trends are constrained to individual ocean basins (Pacific, Indian, and Atlantic), allowing the impact of each region to be assessed in isolation. When forced with observed global SST, the model broadly replicates the spatial pattern of extratropical SH geopotential height trends seen in reanalyses. However, when forcing by each ocean basin separately, similar structures arise only when Atlantic SST trends are included. It is further shown that teleconnections from the Atlantic are associated with perturbations to the zonal Walker circulation and the corresponding intensification of the local Hadley cell, the impact of which results in the development of atmospheric Rossby waves. Thus, increased Rossby waves, forced by positive Atlantic SST trends, may have played a role in driving geopotential height trends in the SH extratropics. Furthermore, these atmospheric circulation changes promote warming throughout the Antarctic Peninsula and much of West Antarctica, with a pattern that closely matches recent observational records. This suggests that Atlantic SST trends, via a teleconnection to the SH extratropics, may have contributed to springtime climatic change in the SH extratropics over the past three decades.

Teleconnections from tropical Pacific sea surface temperature (SST) anomalies to the high-latitude Southern Hemisphere (SH) are examined using observations and reanalysis. Analysis of tropical Pacific SST anomalies is conducted separately for the central Pacific (CP) and eastern Pacific (EP) regions. During the austral cold season, extratropical SH atmospheric Rossby wave train patterns are observed in association with both EP and CP SST variability. The primary difference between the patterns is the westward displacement of the CP-related atmospheric anomalies, consistent with the westward elongation of CP-related convective SST required for upper-level divergence and Rossby wave generation. Consequently, CP-related patterns of SH SST, Antarctic sea ice, and temperature anomalies also exhibit a westward displacement, but otherwise, the cold season extratropical SH teleconnections are largely similar. During the warm season, however, extratropical SH teleconnections associated with tropical CP and EP SST anomalies differ substantially. EP SST variability is linked to largely zonally symmetric structures in the extratropical atmospheric circulation, which projects onto the southern annular mode (SAM), and is strongly related to the SH temperature and sea ice fields. In contrast, CP SST variability is only weakly related to the SH atmospheric circulation, temperature, or sea ice fields and no longer exhibits any clear association with the SAM. One hypothesized mechanism suggests that the relatively weak CP-related SST anomalies are not able to substantially impact the background flow of the subtropical jet and its subsequent interaction with equatorward-propagating waves associated with variability in the SAM. However, there is currently no widely established mechanism that links tropical Pacific SST anomalies to the SAM.

This study examines the relationship between the Atlantic Multidecadal Variability (AMV) and the wintertime atmospheric circulation of the North Atlantic in simulations of the fifth Coupled Model Intercomparison Project (CMIP5). Comparisons of internal (using preindustrial control simulations) and externally forced (using historical and Representative Concentration Pathways 8.5 simulations) simulated AMV with observations suggest that the CMIP5 models lack internally generated AMV, except for two models (GFDL-ESM2G and HadGEM2-ES). A long-term influence of the winter North Atlantic Oscillation (NAO) on the AMV is identified, but no consistent feedback of the AMV onto the atmospheric circulation is found among the models. However, GFDL-ESM2G and HadGEM2-ES show a small lagged NAO signal that suggests a driving role of the ocean on decadal fluctuations of the atmosphere, with two different potential mechanisms. HadGEM2-ES exhibits a latitudinal shift of the Atlantic Intertropical Convergence Zone that can modulate the NAO through a Rossby wave train emanating from the tropics. In GFDL-ESM2G, the AMV is associated with a decrease in storm track activity and a shift of the intraseasonal weather regimes toward the negative NAO regime. These results raise hope that some long-term predictability of the winter climate over the North Atlantic and surrounding continents could be extracted from long-term oceanic fluctuations of the North Atlantic Ocean, provided that the AMV is correctly represented in coupled ocean-atmosphere models.

[1] The spatiotemporal sensitivity of Antarctic sea ice season length trends are examined using satellite-derived observations over 1979-2012. While the large-scale spatial structure of multidecadal trends has varied little during the satellite record, the magnitude of trends has undergone substantial weakening over the past decade. This weakening is particularly evident in the Ross and Bellingshausen Seas, where a 25-50% reduction is observed when comparing trends calculated over 1979-2012 and 1979-1999. Multidecadal trends in the Bellingshausen Sea are found to be dominated by variability over subdecadal time scales, particularly the rapid decline in season length observed between 1979 and 1989. In fact, virtually no trend is detectable when the first decade is excluded from trend calculations. In contrast, the sea ice expansion in the Ross Sea is less influenced by shorter-term variability, with trends shown to be more consistent at decadal time scales and beyond. Understanding these contrasting characteristics have implications for sea ice trend attribution. Citation:Simpkins, G. R., L. M. Ciasto, and M. H. England (2013),

As the effects of regional climate change are most pronounced at polar latitudes, we might expect polar-ward migratory populations to respond as habitat suitability changes. The southern elephant seal (Mirounga leonina L.) is a pole-ward migratory species whose populations have mostly stabilized or increased in the past decade, the one exception being the Macquarie Island population which has decreased continuously over the past 50 years. To explore probable causes of this anomalous trend, we counted breeding female seals annually between 1988 and 2011 in order to relate annual rates of population change (r) to foraging habitat changes that have known connections with atmospheric variability. We found r (i) varied annually from 20.016 to 0.021 over the study period, (ii) was most effected by anomalous atmospheric variability after a 3 year time lag was introduced (R ¼ 0.51) and (iii) was associated with sea-ice duration (SID) within the seals' foraging range at the same temporal lag. Negative r years may be extrapolated to explain, at least partially, the overall trend in seal abundance at Macquarie Island; specifically, increasing SID within the seals foraging range has a negative influence on their abundance at the island. Evidence is accruing that suggests southern elephant seal populations may respond positively to a reduced sea-ice field.

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