Role of moist energy advection in formulating anomalous Walker Circulation associated with El Niño

Abstract

The role of meridional moist energy advection in formulating anomalous Walker Circulation during El Niño events is investigated using observational data and linearized GCM. During the El Niño mature phase, the warm pool is extended to the central Pacific, so that the magnitudes of equatorial SST become uniform over the western-central Pacific. In contrast to the uniform SST distribution over the western-central Pacific, the maximum of equatorial convective activities are zonally displaced from the western to the central Pacific. That is in line with the fact that the strong anomalous descending motions are located over the western Pacific without local negative SST anomalies. It is demonstrated here that suppressed convection over the western Pacific is associated with cold-dry moist energy advection due to low-level equatorward winds. The meridional cold-dry moist energy advection leads the vertical warm-wet moist energy advection and adiabatic warming processes (descending motion) to balance the moist energy budget. It is shown that this meridional wind is a part of a cyclonic Rossby wave in response to the anomalous convection induced by El Niño SST. This mechanism, suggested here, is validated by using a linearized GCM. The model results show the downward branch of the anomalous Walker Circulation over the western Pacific is significantly reduced when the moist energy advection is not allowed. Copyright 2007 by the American Geophysical Union.