The mechanisms of the subseasonal zonal oscillation of the western Pacific subtropical high in 10–25-day period and 25–50-day period

Abstract

This study investigates the evolution characteristics and the mechanism, including the convection–circulation interaction and the air–sea interaction, of the subseasonal zonal oscillation of the western Pacific subtropical high (WPSH) in 10–25-day period and 25–50-day period. Accompanied with the westward extension of the WPSH, a suppressed convection propagates northwestward over the western Pacific with an anticyclonic circulation anomaly at its northwest. A warm sea surface temperature anomaly (SSTA) lags the anomalous convection, showing near-quadrature phase relationship in temporal, and the magnitude of SSTA in 25–50-day period is greater than that in 10–25-day period. Further analysis shows that in both two periods the westward extension of the WPSH is mainly attributed to the convection–circulation interaction. The suppressed convection-induced cooling stimulates the divergence anomaly at the northwest of the suppressed convection, causing anticyclonic anomaly through the horizontal divergence effect. In turn, the anticyclonic anomaly motivates the downward dry advection in planetary boundary layer (PBL) preceding the suppressed convection, and thus favors the propagation of the suppressed convection. Through such interaction processes, the convection and circulation anomalies are combined and propagate together, leading to the zonal oscillation of the WPSH. Moreover, it is also found that the significance of air–sea interaction in two periods is distinct due to different magnitudes of the SSTA. In 10–25-day period, the ocean has no obvious feedback on the atmospheric circulation, while in 25–50-day period, the oceanic anomaly can fully develop and impact on the atmosphere more effectively.