Formation of long-lasting inactive and active multiple tropical cyclone events in the western North Pacific

Long-lasting inactive and active multiple tropical cyclone (MTC) events in the western North Pacific (WNP) during July-August 1982-2020 were investigated. It is found that the significant difference between the inactive and active events lies on large-scale circulation conditions and tropical SST anomalies in the Pacific and Indian Ocean. A negative phase of the Pacific Meridional Mode (PMM), an Indian ocean warming and a local suppressed convective phase are associated with the long-lasting inactive MTC events, and vice versa. A further diagnosis of the environmental parameters showed that dynamic parameters such as mid-level vertical motion and the upper-level divergence and thermodynamic parameters such as vertically integrated water vapor content and low-level specific humidity play important roles in regulating the inactive and active MTC events. A separation of interannual and intraseasonal components indicates that the extreme MTC events are influenced by both the timescale motions. While the former is primarily through the SSTA-induced low-level anticyclonic and descent anomalies, the latter is via the fluctuation of the boreal summer atmospheric intraseasonal oscillation.