Processes leading to second-year cooling of the 2010–12 La Niña event, diagnosed using GODAS

Abstract

Isopycnal analyses were performed on the Global Ocean Data Assimilation System (GODAS) to determine the oceanic processes leading to so-called second-year cooling of the La Niña event. In 2010–12, a horseshoe-like pattern was seen, connecting negative temperature anomalies off and on the Equator, with a dominant influence from the South Pacific. During the 2010 La Niña event, warm waters piled up at subsurface depths in the western tropical Pacific. Beginning in early 2011, these warm subsurface anomalies propagated along the Equator toward the eastern basin, acting to reverse the sign of sea surface temperature (SST) anomalies (SSTAs) there and initiate a warm SSTA. However, throughout early 2011, pronounced negative anomalies persisted off the Equator in the subsurface depths of the South Pacific. As isopycnal surfaces outcropped in the central equatorial Pacific, negative anomalies from the subsurface spread upward along with mean circulation pathways, naturally initializing a cold SSTA. In the summer, a cold SSTA reappeared in the central basin, which subsequently strengthened due to the off-equatorial effects mostly in the South Pacific. These SSTAs acted to initiate local coupled air-sea interactions, generating atmospheric-oceanic anomalies that developed and evolved with the second-year cooling in the fall of 2011. However, the cooling tendency in mid-2012 did not develop into another La Niña event, since the cold anomalies in the South Pacific were not strong enough. An analysis of the 2007–09 La Niña event revealed similar processes to the 2010–12 La Niña event.