Abstract
The study explores modes of variability in the Southern Polar Cell and their relationship with known global climate modes and Antarctic sea ice. It is found that Polar Cell is barotropic in nature and 500 hPa geo-potential height (Z500) field can satisfactorily represent variability in the Polar Cell. First, three leading Empirical Orthogonal Function (EOF) modes of Z500 account for nearly 80% of observed variability in the Polar Cell. Dominant mode (PC1500) comprises of high pressure divergence zone over Antarctica. Second leading mode (PC2500) is low pressure zone covering Amundsen-Bellingshausen Sea (ABS) similar to ABS low feature. A new climate mode called Polar Coastal Index (PCI) is defined, which describes more than 15% and close to 30% variability of circumpolar trough and ABS low, respectively. Out of four modes defined in this study, only PCI and PC2500 show linear trends and clear seasonality. Interestingly, both modes are affected by modulation of ABS low due to tropical ENSO forcing. SAM signature is present in Polar Cell as PC1500 shares large variance with it. The largest impact on sea ice comes from PC2500 followed by PC1500 in the Antarctic Dipole regions. However, this study suggests contemporary sea ice trends cannot be sustained, and can reverse given that trends in PCI and PC2500 favour a reversal. These results indicate that ENSO-driven Polar Cell variability plays a crucial role influencing Antarctic sea ice as it interacts with other climate modes and leads the combined impact at the interannual time scale.
Highlights
The atmospheric circulation in the Southern Hemisphere (SH) has a large impact on SH climate and sea ice cover through complex air-ocean-sea ice interaction
We examined the main characteristics of leading modes of Southern Polar Cell and their relationship with global climate indices along with its impact on Antarctic sea ice
Four climate modes were generated from different atmospheric variables mean sea level pressure (MSLP) (PCI) and Z500 (PC1500, PC2500 and PC3500)
Summary
The atmospheric circulation in the Southern Hemisphere (SH) has a large impact on SH climate and sea ice cover through complex air-ocean-sea ice interaction. Previous studies have identified the hemispheric-scale atmospheric variability in SH comprising of a dominant mode characterized by zonally symmetric see-saw pattern between the mid- and high-latitudes This mode appears as the leading mode of many atmospheric variables, e.g., mean sea level pressure (MSLP), geopotential heights, zonal winds [1]-[6], which has been referred to as Antarctic Oscillation (AAO) or Southern Annular Mode (SAM) and dominates the region poleward of 20 ̊S [1] [2] [7] [8]. Previous studies have investigated air mass movement in and out of Polar Cell, which is known as the semiannual oscillation (SAO; [19] [20]) that is characterised by bi-annual transfer of air masses between the Antarctic and mid-latitudes [21] It is manifested by the changes in the position and strength of the Antarctic circumpolar trough (CPT), lying between 60 ̊ - 65 ̊S, which has major impact on Antarctic climate and sea ice. In SAO studies, the choice of latitude bands (50 ̊ - 65 ̊S) selected for its computation does not capture the full variability of forces which drives intra-cell circulation in the Polar Cell.
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