Asymmetric Responses of Tropical Precipitation during ENSO

Abstract

In response to the zonally symmetric El Niño?Southern Oscillation forcing, hemispherically asymmetric tropical precipitation anomalies associated with the Hadley circulation are found. In boreal spring after an El Niño peak phase, positive tropical precipitation anomalies occur in the Southern Hemisphere, while negative precipitation anomalies are found in the Northern Hemisphere. This zonal asymmetry is more apparent in the El Niño decaying phase than in the El Niño growing phase. The maximum amplitude of this zonal asymmetry lags one season behind the maximum SST anomalies over the tropical eastern Pacific. This lagged response of the asymmetry is mainly because of the tropical precipitation outside the tropical eastern Pacific, which is associated with the SST and tropospheric temperature anomalies outside the tropical eastern Pacific. A combination of the effect associated with the anomalous gross moist stability and the effect of the horizontal moist static energy (MSE) transport is responsible for the asymmetry. The above effects are associated with the seasonal migration of the Hadley circulation. Warm SST and tropospheric temperature anomalies increase the low-level moisture in the Tropics. In the effect associated with anomalous gross moist stability, the tropical precipitation over the ascending branch of the Hadley circulation is enhanced because of the decrease of effective moist stability, which is induced by the increase of low-level moisture. This enhancement associated with the Hadley circulation reduces the low-level moisture over the descending branch and creates a meridional moisture gradient. In the effect of the horizontal MSE transport, the tropical precipitation anomalies over margins of the ascending branch is reduced by dry advection from the descending branch, which is associated with mean Hadley circulation.