Impacts of the East Asian winter monsoon on winter precipitation variability over East Asia-western North Pacific

Abstract

The boreal winter precipitation variability over East Asia and the western North Pacific is often attributed to the El Niño-Southern Oscillation (ENSO). The present study investigates the independent impacts of the East Asian winter monsoon (EAWM) on winter precipitation anomalies over East Asia and the western North Pacific. It is revealed that anomalous EAWM is accompanied by a south-north dipole pattern of precipitation anomalies in the above regions. During strong EAWM years, the enhanced northeasterly winds induce anomalous convergence and divergence over the tropical and subtropical regions, respectively, leading to anomalous ascent and above-normal precipitation over the southern South China Sea-Philippine Sea and anomalous descent and below-normal precipitation over eastern China-subtropical western North Pacific. Opposite convergence, vertical motion and precipitation anomalies are induced in the above regions during weak EAWM years. In the observations, both ENSO and EAWM contribute to the south-north dipole precipitation anomaly pattern with a larger contribution from ENSO and EAWM for the tropical and subtropical precipitation anomalies, respectively. Atmospheric model experiments with climatological annually varying sea surface temperature forcing confirm the independent role of the EAWM in the formation of the south-north dipole precipitation anomaly pattern. A moisture budget diagnosis shows that the dynamic effect associated with vertical motion is dominant in the formation of the above precipitation anomaly pattern in both the observations and model simulations. The horizontal moisture transport has an additional contribution to the formation of subtropical precipitation anomalies.