A comparison of the impacts of two consecutive double-peaked La Niña events on Antarctic sea ice in austral spring

Abstract Among 9 La Niña events since 1980, there are 7 double-peaked La Niña events which typically persist for two years and peak twice in the two consecutive boreal winters. In the study, the individual impacts of the first and second peak episodes of such La Niña on the Antarctic sea ice in austral spring (September to November) were compared. The results suggest a difference. The first episode induces a tripolar distribution of sea ice concentration (SIC) with negative anomaly in the Bellingshausen Sea sandwiched with positive anomalies in the Ross Sea and the northeastern Weddell Sea. The second causes a SIC reduction in most parts of the Southern Ocean except for the eastern Ross-western Amundsen Seas where an increase is observed. Mechanistically, the first episode forces one single Rossby wave train propagating southeastward, causing a strong cyclone anomaly over the eastern Ross-Amundsen-Bellingshausen Seas along with a weak anticyclone over the Weddell Sea. In comparison, the second La Niña excites two branches of Rossby wave trains emanating from the southeastern tropical Indian Ocean and the central equatorial Pacific, respectively, which induce three anomalous anticyclones and two anomalous cyclones over the Southern Ocean. These different atmospheric circulation anomalies shape their different sea ice distributions between the two La Niña episodes through both dynamic and thermodynamic processes. The modeling results from CAM5 verify these differences.