Covariability between the Baiu Precipitation and Tropical Cyclone Activity through Large-Scale Atmospheric Circulations

Abstract

Interannual variations of both the Baiu precipitation and tropical cyclone (TC) activity in the western North Pacific (WNP) are controlled by large-scale atmospheric circulations associated with the El Niño/Southern oscillation (ENSO) and the tropospheric biennial oscillation (TBO) of the Asian monsoon. This work examines covariability between the Baiu precipitation and the TC activity in the WNP through the ENSO and the TBO.
In years when sea surface temperature anomalies (SSTAs) are negative in the eastern tropical Pacific with respect to the ENSO, the number of TCs increases near the Philippines in the Baiu season, June and July. On the other hand, in years of negative SSTAs in the eastern tropical Pacific related to the TBO, the strength of TCs is enhanced to the southeast of Japan. Each of these two TC activities enhances a cyclonic circulation there, which shifts the axis of monsoon southwesterlies and contributes to form the peculiar Baiu precipitation anomalies. These modifications are, however, dependent on the phase of the ENSO and the TBO. In years of positive SSTAs in the eastern tropical Pacific, the anomalous TC activity is small and sometimes has opposing effects on the atmospheric circulations of the ENSO and TBO. Thus, the covariability is strong between the Baiu precipitation and TC activity in the WNP in the specific phase of the ENSO and the TBO, although both covary in large-scale atmospheric circulations.