Decadal-Scale Modulation of Atmospheric Circulation around the Onset of the Western North Pacific Summer Monsoon

Decadal-scale modulation (DSM) appears in the atmospheric circulation around the onset (July 20) of the western North Pacific summer monsoon (WNPSM). This work examined the DSM associated with the onset precipitation increase (OPI), using global reanalysis datasets from 1979 to 2010 (32 years). The years 1985-1993 had positive decadal anomalies, and the onset precipitation increased with the development of specific meridional circulation similar to the so-called Pacific-Japan pattern. In the subsequent negative decadal anomaly in the years 1994-2000, the atmospheric circulation was distinguished by a smaller OPI, southwestward extension of the WNPSM, and an anomalous wave train emanating northeastward from the Philippines. The patterns in the following positive decadal anomaly in the years 2001-2007 were similar to those in the preceding negative period but with opposite signs. These changes in the DSM were coherent with decadal sea surface temperature anomalies in the region of 28-29C in the western North Pacific before the onset due to vertical instability under the trade wind inversion.