Relation between SSTs and predictability of seasonal mean precipitation over the western tropical Pacific

Abstract

This study investigates the difference of the predictability of seasonal mean precipitation with Japan Meteorological Agency (JMA) Atmospheric General Circulation Model (AGCM), using two types of prescribed sea surface temperature (SST). One set of seasonal prediction simulations is called "SMIP" (which stands for "Seasonal prediction Model Intercomparison Project"). In the SMIP, observed SSTs are prescribed. The other set is called "HINDCAST". In the HINDCAST, SST anomalies at initial time are assumed to persist during the forecast period. The December-January-February (DJF) averaged precipitation predictability over the tropical Pacific in SMIP is higher than that in HINDCAST, as was expected. However, it was found that the June-July-August (JJA) averaged precipitation predictability over the western tropical Pacific in SMIP was lower than that in HINDCAST. In th e western tropical Pacific, there is a negative correlation between the observed precipitation anomalies and SST anomalies in JJA. The observed precipitation anomalies in early summer (May-June-July) are well correlated with the SST anomalies in spring (March-April-May). The simulated precipitation anomalies are strongly influenced by local SST anomalies in the same period. Because of this observed lag-correlation between precipitations and SSTs, and the property of simulated precipitation by the model, JJA averaged precipitation predictability over the western tropical Pacific in HINDCAST is higher than that in SMIP. © 2005, Meteorological Society of Japan.