Coordinated influences of the tropical and extratropical intraseasonal oscillations on the 10–30-day variability of the summer rainfall over southeastern China

This study explores the spatial variations and physical mechanisms of 10–30-day rainfall anomalies over southeastern China based on daily station-observed rainfall data for the period 1979–2015. Empirical orthogonal function analysis shows that the dominant spatial distribution of 10–30-day rainfall anomalies is a monopole pattern over the south of the middle and lower reaches of the Yangtze River Valley (SMLY). Lead-lag composites reveal that the evolution of such a monopole pattern depends on the coordinated influences of 10–30-day atmospheric intraseasonal oscillations (ISOs) from the tropics and mid-high latitudes. In the upper troposphere, the southeastward-propagating Rossby wave train from the mid-high latitudes, which presents as anomalous anticyclones and cyclones alternating over eastern Europe to southeastern coastal area of China, induces strong ascents (descents) over the SMLY via vorticity advection. Circulation anomalies associated with tropical ISO over East Asia/Western North Pacific trigger a vertical cell with strong updraft (downdraft) over the SMLY and downdraft (updraft) to the south, further enhancing the ascents (descents) over the SMLY, forming the wet (dry) phases of 10–30-day rainfall anomalies. Moreover, due to the meridional non-uniformity of ISO-related diabatic heating along the Indian Ocean longitudes, an anticyclone (cyclone) is generated over the central Indian–northern Bay of Bengal, which tends to anchor the anomalous ascents (descents) over the SMLY through its interaction with the intraseasonal Rossby wave from mid-high latitudes, thus favoring the persistence of wet (dry) phases of the 10–30-day SMLY rainfall anomalies.