On the Seasonality and Linearity 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. This study investigates the mechanisms behind the seasonality and linearity of the El Niño teleconnection to the Amundsen Sea region (ASR) using experiments with the HadGEM3 climate model forced with an El Niño sea surface temperature anomalies of amplitudes between 0.75 and 3 K.

The seasonality of the El Niño–ASR teleconnection originates 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 associated with 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. The lack of the Rossby wave source in the tropical Pacific and the absence of favorable conditions for wave propagation explains the weaker El Niño–ASR teleconnection in austral summer compared to austral winter.

In austral winter, the El Niño–ASR teleconnection behaves linearly for El Niño amplitudes up to 2.25 K but weakens for stronger forcing (3 K). The anomalous Rossby wave source in the subtropical South Pacific increases monotonically with El Niño amplitude. However, a Rossby wave reflection surface originally located in the western South Pacific sector extends progressively eastward with increasing El Niño amplitude, reducing wave propagation into the ASR. The wave reflection surface is associated with curvature in the upper tropospheric zonal winds that intensifies as the subtropical jet strengthens under El Niño forcing. In contrast, the El Niño–ASR teleconnection in austral summer, which more closely resembles the Southern Annular Mode, increases linearly for El Niño amplitudes up to 3 K. The results explicitly demonstrate that a linear approximation of the El Niño–ASR teleconnection is reasonable based on the range of El Niño amplitudes observed in recent history.