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

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 Ninas. The typical SIC anomaly pattern induced by single-year La Ninas appears only during the second austral winter of multi-year La Ninas. 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 Ninas 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 Nina events during the period 1979-2020. These results suggest that it is helpful to separate La Ninas into single and multi-year events to better understand the La Nina impacts on Antarctic climate.

Automated summary

By comparing the variations in Antarctic sea ice concentration (SIC) between the first and second austral winters of single-year and multi-year La Ninas, it was found that the SIC anomaly patterns differ between the two winters. The first winter exhibits a unique zonally-shifted pattern, while the second winter follows a more typical SIC anomaly pattern. This zonally-shifted pattern during the first winter is caused by the pre-onset conditions associated with multi-year La Ninas.