Influences of northward propagating 25-90-day and quasi-biweekly oscillations on eastern China summer rainfall

摘要

The present study reveals that rainfall anomalies associated with the 25-90-day (quasi-biweekly) tropical convective activity propagates northward (northwestward) into northern China (the Yangtze River). Enhanced rainfall over southern China results directly from enhanced convection propagating northward from the South China Sea (SCS)-western north Pacific (WNP), strengthened northward water vapor transport and intensified upper-level divergence. The anomalous anticyclone over SCS-WNP, which is induced by local lower sea surface temperature, provides a favorable condition for transporting more moisture to southern China. Stronger subtropical westerly jet and South Asia high strengthen upper-level divergence. The northward propagating 25-90-day oscillation over eastern China reaches farther north than the quasi-biweekly oscillation (QBWO). The intensity of QBWO in summer rainfall has significantly strengthened over southern China after 1993 but not for the 25-90-day oscillation. This is mainly contributed by an interdecadal increase in rainfall over southern China and the Yangtze River Basin in the wet phases of QBWO. The interdecadal enhancement of rainfall associated with the QBWO is attributed to three factors. One is lower tropospheric convergence caused by anomalous anticyclone over the WNP and anomalous cyclone over Korean Peninsula and Japan. The second is upper tropospheric divergence resulting from strengthening of subtropical westerly jet and the South Asia high. The last is enhanced ascent over South China through meridional vertical circulation.