A Shifting Tripolar Pattern of Antarctic Sea Ice Concentration Anomalies During Multi-Year La Niña Events

Abstract

A 2,200-year CESM1 pre-industrial simulation is used to contrast Antarctic sea ice concentration (SIC) variations between the first and second austral winters of multi-year La Niñas. The typical SIC anomaly pattern induced by single-year La Niñas appears only during the second austral winter of multi-year La Niñas. A similar pattern, but zonally shifted compared to the typical one, is found during the first winter and exhibits a tripolar pattern with anomaly centers over the Ross, Amundsen-Bellingshausen, and Weddell Seas. The shift is a result of the pre-onset conditions associated with multi-year La Niñas that excites unique atmospheric circulation modes during the first winter. The distinct zonally-shifted SIC anomaly pattern is observed in four of the six multi-year La Niña events during the period 1979–2020. These results suggest that it is helpful to separate La Niñas into single and multi-year events to better understand the La Niña impacts on Antarctic climate.