The relationship between post 1997/1998 Westerly Wind Events and recent lack of ENSO related cold-tongue warming

Westerly Wind Events (WWEs) in the western equatorial Pacific have previously been shown to significantly affect warming of the eastern Pacific cold-tongue. During and prior to the large ENSO of 97/98, WWEs were known to be statistically linked to substantial (up to 3 degrees C) warming of the cold-tongue. Results from post 97/98 WWEs, however, show significantly smaller WWE-related cold tongue warming. During this latter period, warm equatorial Pacific sea surface temperature anomalies have tended to be trapped near the dateline rather than extending all the way across the Pacific as in a classical ENSO composite. Here, we revisit the relationship between WWEs and cold-tongue warming using in situ and operational forecast winds and in situ and satellite based sea surface temperature (SST). Results show significant differences in the basin scale zonal wind anomalies associated with WWEs that occurred before and after 97/98. Although the post-97/98 composite WWE westerly anomalies are very similar to their predecessors, conditions east of the WWE region are different; there are enhanced equatorial easterlies. These differences can account for the changes in the average cold-tongue warming associated with pre and post 97/98 WWEs. General ocean circulation model experiments are conducted to explore the extent to which the observed changes in the character of post 97/98 WWEs can explain the recent behavior of cold-tongue SSTA