Anomalous tropical ocean circulation associated with La Niña Modoki

Abstract

El Niño Modoki is a variant of El Niño characterized by warming around the dateline flanked by anomalous cooling in the east and west. However, the opposite phase (La Niña Modoki) has received little attention because the prominent cooling of sea surface temperature (SST) during major La Niña events is observed in the central Pacific, and thus, it is difficult to define the two different types of cold events from the SST anomaly pattern. Here we demonstrate that cold events in 2000 and 2008 can be clearly distinguished from traditional La Nñia events using surface currents derived from satellite observations. During 2000 and 2008, anomalous zonal currents in the equatorial Pacific demonstrate divergence, with westward currents west of the dateline and eastward currents east of it. These currents are opposite to the circulation pattern during the 2004 El Niño Modoki event. An empirical orthogonal function (EOF) analysis of surface currents for the period 1993-2009 shows a circulation anomaly pattern similar to that in 2000, 2004, and 2008 in the second EOF. The first EOF is consistent with traditional El Niño/La Niña events and does not exhibit a current reversal along the equator. Our results also indicate that strong cyclonic (anticyclonic) circulation anomalies occur in the tropical western Pacific around 5°N-15°N during the 2004 (2000 and 2008) El Niño (La Niña) Modoki events and during the strong traditional El Niño (La Niña) events of 1997 (1998). These circulation anomalies are related to an SST gradient in the western and central Pacific.