An Incursion of Off-Equatorial Subsurface Cold Water and Its Role in Triggering the "Double Dip" La Niña Event of 2011

Based on Global Ocean Data Assimilation System (GODAS) and NCEP reanalysis data, atmospheric and oceanic processes possibly responsible for the onset of the 2011/12 La Niña event, which followed the 2010/11 La Niña even——referred to as a "double dip" La Niña——are investigated. The key mechanisms involved in activating the 2011/12 La Niña are illustrated by these datasets. Results show that neutral conditions were already evident in the equatorial eastern Pacific during the decaying phase of the 2010/11 La Niña. However, isothermal analyses show obviously cold water still persisting at the surface and at subsurface depths in off-equatorial regions throughout early 2011, being most pronounced in the tropical South Pacific. The negative SST anomalies in the tropical South Pacific acted to strengthen a southern wind across the equator. The subsurface cold water in the tropical South Pacific then spread northward and broke into the equatorial region at the thermocline depth. This incursion process of off-equatorial subsurface cold water successfully interrupted the eastern propagation of warm water along the equator, which had previously accumulated at subsurface depths in the warm pool during the 2010/11 La Niña event. Furthermore, the incursion process strengthened as a result of the off-equatorial effects, mostly in the tropical South Pacific. The negative SST anomalies then reappeared in the central basin in summer 2011, and acted to trigger local coupled air-sea interactions to produce atmospheric-oceanic anomalies that developed and evolved with the second cooling in the fall of 2011.