Understanding the responses of sea surface temperature to the two different types of El Niño in the western North Pacific

Abstract

In this study, we examine the physical processes of the response of sea surface temperature (SST) in the western North Pacific to the two different types of El Niño, the Central Pacific El Niño and the Eastern Pacific El Niño. While a center of anomalous warm SST is observed in the eastern tropical Pacific in the case of the Eastern Pacific El Niño, it is located in the central tropical Pacific for the Central Pacific El Niño. We investigate the detailed SST response in the Kuroshio Extension region, where the anomalous SST is warm (cool) in the Central Pacific El Niño (Eastern Pacific El Niño) during the boreal fall and winter. We diagnose the SST tendency to identify the roles of the surface net heat fluxes and Ekman transport.During the fall and winter, the atmospheric teleconnections over the North Pacific are quite different between the two types of El Niño, which might be due to changes in atmospheric diabatic forcing in relation to rainfall in the tropical Pacific. Our results suggest that the physical processes, which are responsible for the western North Pacific SST in response to the two different types of El Niño, are basically similar. That is, Ekman heat transport plays a key role in warming and cooling of the SST anomalies in the Kuroshio Extension region in the Central Pacific El Niño and Eastern Pacific El Niño, respectively. In contrast, the net surface heat fluxes act to weaken the SST anomalies. This result indicates that changes in the anomalous atmospheric circulation over the North Pacific, which is directly associated with Ekman currents at the upper levels, are important to determine the responses of SST in the Kuroshio Extension region to the two types of El Niño.