Establishment of the Low-Level Wind Anomalies over the Western North Pacific during ENSO Development*

Abstract

The anomalous low-level anticyclone over the western North Pacific presents a link between El Nino and the western North Pacific-East Asian climate. During a La Nina event, however, the low-level wind anomalies over the western North Pacific reverse sign. The low-level wind anomalies move eastward from South Asia and are established over the western North Pacific in the fall of the El Nino (La Nina) developing year. The anomalous low-level anticyclone associated with El Nino is a response to suppressed convection. This suppressed convection is mainly induced by a cooling tendency associated with the vertical average of the anomalous horizontal advection of moist static energy that is defined as the mechanism of the horizontal advection of moist static energy. The El Nino-related sea surface temperature (SST) anomalies, over the eastern Pacific in particular, create negative meridional gradients of temperature and moisture over the western North Pacific. When the winter monsoon starts to dominate the Asian region, the northerly component of the winter monsoon transports low moist static energy air into South Asia that suppresses convection and induces the anomalous low-level anticyclone. Associated with the mean state of temperature and moisture, the meridional components of the anomalous low-level anticyclone transports cold and dry air to the southward branch and warm and moist air to the northward branch of the anomalous low-level anticyclone. This cooling-warming pattern of the anomalous horizontal advections of moist static energy tends to move the anomalous low-level anticyclone eastward. The mechanism of the horizontal advection of moist static energy implies a phase-locking behavior of the anomalous low-level anticyclone with the seasonal cycle of the mean state over the western North Pacific-East Asian region.