Influence of three types of boreal summer intraseasonal oscillation on summer precipitation over the Yangtze River Valley

Abstract

Previous studies have demonstrated that boreal summer intraseasonal oscillation (BSISO) active in the Indian Ocean can affect precipitation over the Yangtze River Valley (YRV). In this study, the impacts of three types of BSISO modes on precipitation over the YRV were investigated and found to vary significantly. First, moisture budget analysis indicated that the process responsible for the precipitation anomalies in all BSISO modes is moisture convergence, while moisture advection is insignificant. In the canonical mode, precipitation anomalies are negative (positive) at pentad −2 (−1 to 3) owing to a BSISO-related cyclone over the northwestern Pacific (anticyclone moving from the subtropical western to northwestern Pacific) accompanied by enhanced (suppressed) convection propagating northeastward. Comparatively, in the eastward expansion mode, precipitation anomalies are positive (negative) at pentad −2 to 0 (1 to 3) via a BSISO-related anticyclonic (cyclonic) circulation over the northwestern Pacific induced by the teleconnection effect of enhanced (suppressed) convection over the Indian Ocean. Different from the previous two BSISO modes, in the northward dipole mode, precipitation anomalies are negative (positive) at pentad −2 to 0 (1 to 3) owing to a BSISO-related cyclonic (anticyclonic) circulation, which moves from the South China Sea to the northwestern Pacific because of the propagation of enhanced (suppressed) convection.