Joint impact of North Pacific Victoria mode and South Pacific Quadrapole mode on Pacific ITCZ summer precipitation

Abstract

This study investigates the joint impact of the boreal late winter–early spring North Pacific Victoria mode (VM) and South Pacific Quadrapole (SPQ) mode on the following summer Pacific Intertropical Convergence Zone (ITCZ) precipitation. Here, both observational data and historical simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) models show that the precipitation anomalies are enhanced when the VM and the SPQ are in phase but are weakened when they are out of phase. The strength of the anomalous vertical velocity (ω) and outgoing longwave radiation also tend to be stronger (weaker) in same-sign (opposite-sign) events. One possible reason for this is that when the VM and the SPQ are in (out of) phase, westerly anomalies over the western–central tropical Pacific are greatly increased (reduced) due to the interactions between preceding North and South Pacific sea surface temperature anomalies, thereby enhancing (reducing) convective upward motion over the western and central Pacific. In addition, an empirical linear regression model is also developed to forecast Pacific ITCZ summer precipitation based on both the late winter–early spring VM and SPQ indices. The combination of the two precursors in North and South Pacific show higher prediction skills than the single VM or SPQ model, which means it is necessary to consider their joint impact when we focus on the Pacific ITCZ precipitation.