Contributions of Scale-interaction to the Decadal Change in Tropical Cyclone in Autumn over the western North Pacific

The activity of tropical cyclones in autumn (September to November) over the western North Pacific (WNP) has experienced a decadal change in 1997 detected by the Bayesian change-point analysis. The frequency of tropical cyclones occurrence increases significantly along 10°–20°N over the WNP during the decade before the change point occurrence (1987–1996). Meanwhile, the lifespan and amplitude of tropical cyclones in 1987–1996 are higher than these in the latter decade of 1997–2006. The local thermodynamic conditions (sea surface temperature and mid-tropospheric humidity) contribute insignificantly to the enhanced tropical cyclone activity, while the dynamic effect associated with atmospheric internal scale-interaction plays an important role. During the earlier decade (1987–1996), enhanced synoptic-scale eddy (SSE) kinetic energy was observed over the region with increased tropical cyclone genesis, suggesting that the active SSEs would favor the generation of tropical cyclones. The diagnosis of a new SSE budget equation, which partitioned the original field into seasonal mean circulation, intraseasonal oscillation (ISO) and SSE, shows that both of the anomalous cyclonic mean circulation and asymmetric ISO activity (skewed toward a cyclonic phase) contribute positively to the enhanced SSEs at the lower troposphere in autumn 1987–1996 via enhanced barotropic energy conversions related to the mean–SSE and ISO–SSE interactions. By contrast, the weakened monsoon trough along with the reduction in probability of ISO cyclonic phases lead to the decreased barotropic energy conversion from background flows to SSEs, resulting in reduced tropical cyclone geneses in autumn during 1997–2006.