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Graham R. Simpkins, Yannick Peings, and Gudrun Magnusdottir

Abstract

Several recent studies have connected Antarctic climate variability to tropical Atlantic sea surface temperatures (SST), proposing a Rossby wave response from the Atlantic as the primary dynamical mechanism. In this investigation, reanalysis data and atmospheric general circulation model experiments are used to further diagnose these dynamical links. Focus is placed on the possible mediating role of Pacific processes, motivated by the similar spatial characteristics of Southern Hemisphere (SH) teleconnections associated with tropical Atlantic and Pacific SST variability. During austral winter (JJA), both reanalyses and model simulations reveal that Atlantic teleconnections represent a two-mechanism process, whereby increased tropical Atlantic SST promotes two conditions: 1) an intensification of the local Atlantic Hadley circulation (HC), driven by enhanced interaction between SST anomalies and the ITCZ, that increases convergence at the descending branch, establishing anomalous vorticity forcing from which a Rossby wave emanates, expressed as a pattern of alternating positive and negative geopotential height anomalies across the SH extratropics (the so-called HC-driven components); and 2) perturbations to the zonal Walker circulation (WC), driven primarily by an SST-induced amplification, that creates a pattern of anomalous upper-level convergence across the central/western Pacific, from which an ENSO-like Rossby wave train can be triggered (the so-called WC-driven components). While the former are found to dominate, the WC-driven components play a subsidiary yet important role. Indeed, it is the superposition of these two separate but interrelated mechanisms that gives the overall observed response. By demonstrating an additional Pacific-related component to Atlantic teleconnections, this study highlights the need to consider Atlantic–Pacific interactions when diagnosing tropical-related climate variability in the SH extratropics.

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Laura M. Ciasto, Graham R. Simpkins, and Matthew H. England

Abstract

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.

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Graham R. Simpkins, Laura M. Ciasto, David. W. J. Thompson, and Matthew H. England

Abstract

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 Niño–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.

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Graham R. Simpkins, Shayne McGregor, Andréa S. Taschetto, Laura M. Ciasto, and Matthew H. England

Abstract

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.

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