Special Sea‐Level and Circulation Anomalies in the Philippine Sea During the 2006/2007 and 2009/2010 El Niño Events

Abstract

The Philippine Sea (PS) tends to show sea‐level falling and cyclonic upper‐layer circulation anomalies during the developing stage (spring and summer) of El Niño, with notable influences on the circulation, climate, and biosystems of downstream regions. These changes show diversity across individual events, partly associated with El Niño‐Southern Oscillation complexity and bringing uncertainties to scientific attribution and prediction. Here, we focus on the positive sea‐level anomalies (SLAs) and anticyclonic circulation anomalies in the boreal spring and summer of 2006 and 2009, which are opposite in sign to the changes observed during other El Niño events. Correspondingly, exceptional changes were seen in downstream regions, such as the enhanced Indonesian throughflow in the Makassar Strait. Our analysis highlights the persistent equatorial easterly wind anomalies during the preceding winter, which were likely favorable for these special changes in 2006 and 2009. This is confirmed by sensitivity experiments of a simplified ocean model. The results show that the negative off‐equatorial wind stress curls in the western and central North Pacific play the key role in causing positive SLAs in the PS through evoking downwelling Rossby waves, and equatorial winds played a secondary role. Further analysis indicates that such special changes of the PS tend to take place in late‐onset weak El Niño events following the La Niña conditions.