Spatiotemporal Variability of the Southern Annular Mode and its Influence on Antarctic Surface Temperatures

Abstract

The Southern Annular Mode (SAM) is the predominant atmospheric variability mode in the Southern Hemisphere. In this paper, we present the spatial variability results of the SAM pattern for the period 1979-2018. The SAM-intrinsic pattern variability analysis is based on the principal component analysis (PCA), which is carried out for the ERA-Interim 500 hPa geopotential height (GPH) data set. A spatiotemporally resolved data set of SAM pattern maps (PCA loadings) is derived by projecting monthly shifted sub-sequences of SAM index values (PCA scores) on the corresponding GPH anomalies. The dominant SAM structure within single pattern fields is mapped automatically and can be interpreted as the Southern Hemisphere polar front. This data set allows an analysis of the geographical positions of the characteristic circumpolar SAM structure over four decades and shows considerable variability over space and time. Five different states of SAM patterns, which are associated with characteristic circulation anomalies during different phases of the study period, are identified. Station-based Antarctic temperature anomalies can be synoptically explained by these circulation anomalies. The overall latitudinal trend of the SAM pattern indicates an intensification of the meridional structure, especially over the East Antarctic Southern Ocean. Furthermore, we show that the SAM pattern variability is significantly correlated with the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. Composites of 500 hPa GPH anomalies during the positive and negative phases of the respective indices indicate teleconnections with Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation, and this can explain latitudinal trends of the SAM pattern.

Can Current Reanalyses Accurately Portray Changes in Southern Annular Mode Structure Prior to 1979?

“…Nevertheless, Fig. 6a does reveal that there are marked changes in the annual SAM-structure within the LATE period, consistent with the ndings of Wachter et al (2020). In particular, all the reanalyses have decades when the NPNC in the Amundsen Sea extends further north than the climatology: for example, in ERA5 this is 1980-89 to 1988-97 and 2000-09 to 2007-16.…”

“…8). As mentioned previously, the sign of the SAM-SAT relationship in some parts of Antarctica is primarily determined by the regional meridional wind anomaly associated with the SAM (e.g., Marshall and Thompson 2016;Wachter et al 2020).…”

“…Previous analyses of variations in the SAM-SAT relationships within the LATE period, have pointed to changes in the SAM structure as being responsible (e.g., Marshall et al 2011;Wachter et al 2020) and next we investigate how this varies between the EARLY and LATE periods and whether it can explain some of the differences between the reanalyses. First, we undertake a qualitative analysis comparing the changes in the Antarctic SAM-SAT relationship observed in Fig.…”

“…While this SAM-SAT spatial pattern may be the current climatological situation, several studies have described regional temporal variations across periods of several years post 1979 (Marshall et al 2011;Wachter et al 2020). These tend to be associated with changes in the phase and/or magnitude of the atmospheric wave number 3 pattern, which may in turn be linked to changes in tropical convection (Goyal et al 2021) or associated with large-scale patterns of ocean sea surface temperature (SST) variability such as the Interdecadal Paci c Oscillation (IPO) and Atlantic Multidecadal Variability (AMV: also known as the Atlantic Multidecadal Oscillation) (Wachter et al 2020). However, observations also suggest an earlier, notable shift in the Antarctic SAM-SAT relationship in the 1970s, as portrayed recently in Fig.…”

“…The SAM is the leading mode of extra-tropical SH climate variability (for a recent review, see Fogt and Marshall 2020). The 'characteristic circumpolar SAM structure' (Wachter et al 2020), which refers to the rst empirical orthogonal function (EOF) of sea level pressure (SLP) or geopotential height (or the pattern of regression or correlation between the SAM and SLP or geopotential height), broadly approximates to a zonally symmetric or annular structure, with synchronous pressure anomalies of opposite sign in the SH mid-and high-latitudes. However, it is important to note that within the climatological SAM structure there is a weak zonal wave-number 3 pattern (e.g., Raphael 2004;Goyal et al 2021) but with one of the nodes comprising an often signi cant asymmetric component, centred in the South Paci c (Fan 2007;Fogt et al 2012), and associated with teleconnections from the tropical Paci c (e.g., Ding et al 2012;Schneider et al 2012;Clem et al 2016).…”