An SST anomaly dipole in the northern subtropical Pacific and its relationships with ENSO

Jin‐Yi Yu,W Timothy Liu,Carlos R Mechoso

doi:10.1029/1999gl011340

Abstract

This study examines the links between tropical and subtropical sea surface temperature (SST) anomalies in the Pacific Ocean during ENSO (El Niño‐Southern Oscillation) events. A long‐term simulation by the UCLA coupled atmosphere‐ocean general circulation model is used. It is found that a zonally‐oriented SST anomaly dipole in the subtropical Pacific develops almost simultaneously with and is closely related to tropical ENSO events. The dipole is located east of the dateline between 20°N and 40°N and consists of an anomaly center off the coast of the North America and another anomaly center with opposite sign further to the west. It is demonstrated that this dipole feature is primarily driven by anomalous surface heat fluxes associated with the altered atmospheric circulation during ENSO events.

Highlights

Over the past few years, the atmosphericcomponent of the UCLA coupled atmosphere-oceagneneralcirculationmodel (CGCM) has been extensivelyrevised andThe sea surfacetemperature (SST) anomaly field during the 1997-98 warm E1 Nifio-SouthernOscillation (ENSO) eventincludeda dipole-typepatternin the Pacificeastof the datelinebetween20øNand40øN[Liu et al, 1998].The dipoleconsistedof centersof anomalous warming along the coast of California and anomalous cooling further to the west, as is clearly seen in the map of meanSST anomaliesfor May 1997(Fig. 1)
Large westerly wind stressanomaliesare simulated to the west of the maximum SST anomaliesin all major warm events(not shown)
The Simulated SST Anomaly Dipole warmer SSTs developed in association with reduced evaporativecoolingby warm surfacesoutherlieseast of the anomalous cyclonic center, while colder SSTs appeared west of the center due to enhancedsurfac evaporation

Summary

Introduction

Over the past few years, the atmosphericcomponent of the UCLA CGCM has been extensivelyrevised and. The 53-yearlongsimulationanalyzedfor this study produces warm equatorial SST events approximatelyevery years[Yu and Mechoso2000]. Large westerly wind stressanomaliesare simulated to the west of the maximum SST anomaliesin all major warm events(not shown). Proposeda cause-effectrelationship between those atmospheric and oceanic anomalies. The Simulated SST Anomaly Dipole warmer SSTs developed in association with reduced evaporativecoolingby warm surfacesoutherlieseast of the anomalous cyclonic center, while colder SSTs appeared west of the center due to enhancedsurfac evaporation. YU ET AL.- SST ANOMALY DIPOLE IN THE NORTHERN SUBTROPICS AND ENSO revealsa dominanttimescaleof about 48 months(not shown).There are two other distincttimescalesin the spectrum(72 monthsand 143months),but onlythe 48-. The 48-month timescaleis closeto the simulated ENSO period, which suggeststhe existenceof tropical-subtropicallinks dur-

Relationship with ENSO

Findings

Summary and Discussion