Interannual variability in the onset of the South China Sea summer monsoon from 1997 to 2014

Abstract

Using observed and reanalysis datasets, the South China Sea summer monsoon (SCSSM) onset process is analyzed in each year from 1997 to 2014. Regional mean (5–20°N, 110–120°E) 850 hPa zonal wind, precipitation, and SST are used as indices to describe SCSSM onset. Three distinct onset types are identified: among the 18 years studied, nine are normal onset years, which are characterized by a well-established westerly wind and associated precipitation over the South China Sea (SCS); eight are intermittent onset years, in which monsoon precipitation does not occur continuously following the establishment of the westerly wind over the SCS; and one year, 2014, is a delayed onset year, in which the western Pacific subtropical high dominates over the SCS after the seasonal transition and prevents the monsoon onset. A comparison of the first two types suggests that a positive SST gradient in the northern Indian Ocean and local SST warming in the SCS are two key factors in the normal SCSSM onset type. With regard to the influence of the El Niño–Southern Oscillation background, there are four late onset years (1997, 1998, 2007, and 2010) that coincide with El Niño events, but only two early-onset years (1999 and 2012) out of the six years featuring La Niña events. Further analysis suggests that the zonal thermal contrast across the Indian and western Pacific oceans modulates monsoon onset in La Niña years.

Keywords

• South China Sea
• summer monsoon onset
• air–sea interaction