On the Seasonality of the El Niño Teleconnection to the Amundsen Sea Region

The Amundsen Sea low (ASL) is a quasi–stationary low pressure system that affects climate in West Antarctica. Previous studies have shown that the El Niño Southern Oscillation modulates the position and strength of the ASL with the strongest teleconnection found in austral winter despite the amplitude of ENSO events generally being largest in austral autumn/summer. This study investigates the mechanisms behind the seasonality of the El Niño teleconnection to the Amundsen Sea region (ASR) using experiments with the HadGEM3 climate model forced with an idealised fixed El Niño sea surface temperature anomaly present throughout the year. The seasonality of the El Niño–ASR teleconnection is found to originate from seasonal differences in the large-scale zonal winds in the south Pacific sector. In austral winter, the region of strong absolute vorticity near ∼30◦S provided by the subtropical jet, in combination with the changes to upper tropospheric divergence due to the El Niño perturbation, acts as an anomalous Rossby wave source that is largely absent in austral summer. Furthermore, in austral summer the poleward propagation of tropically-sourced Rossby waves into the ASR is inhibited by the strong polar front jet in the south Pacific sector, which leads to Rossby wave reflection away from the ASR. In austral winter, Rossby waves are able to propagate into the ASR forming part of the Pacific South America pattern. The lack of the Rossby wave source in the tropical Pacific and the absence of favourable conditions for wave propagation explains the weaker El Niño–ASR teleconnection in austral summer compared to austral winter.