A case study of the impact of boreal summer intraseasonal oscillations on Yangtze rainfall

The impact on Yangtze rainfall of the boreal summer intraseasonal oscillations (BSISOs) over the entire Asian summer monsoon region during summer 1996 was investigated using the APHRODITE gridded rainfall and the NCEP–DOE reanalysis II products. Wavelet analyses suggest that the ISOs of Yangtze rainfall were regulated mainly by both 30–60- and 10–25-day oscillations, respectively linked to BSISO1 and BSISO2 activity. Phase locking of the wet phases of these two ISOs resulted in a prolonged wet episode from late June to mid-July. The circulation evolution of the BSISO1 mode showed that active convection accompanied by strong convergence of anomalous zonal winds first developed over the equatorial Indian Ocean, with suppressed convection over the South China Sea (SCS)–Philippine Sea and with active convection over the Yangtze Basin. The triple convection anomaly that aligned meridionally in the East Asian sector arose from a local meridional–vertical cell associated with a Rossby wave-like coupled circulation–convection system. The opposite flow patterns occurred during the dry phase of Yangtze rainfall. The composite BSISO2 cases demonstrated a weak convective anomaly initially appeared around the Maritime Continent, with a huge anomalous anticyclone accompanied by suppressed convection over the SCS–Philippine Sea. The low-level convergence of the anomalous southwesterlies on the northwestern side of the anticyclone and the consequent ascent led to positive rainfall anomalies over the Yangtze Basin. When the entire SCS was dominated by an anomalous cyclone resulting from interaction with upstream systems, convection over the Yangtze Basin was suppressed.