The role of equatorial waves in the onset of the 1998 South China Sea summer monsoon

The observed sequence of events leading to the onset of the summer monsoon in the South China Sea (SCS) is described, with a particular focus on conditions during the South China Sea Monsoon Experiment (SCSMEX) in May-June 1998. During SCSMEX, SCS monsoon onset occurred within the context of a multitude of scale interactions within the ocean-atmosphere system on intraseasonal time scales. Results from the 1998 SCSMEX case study and an independent 25-year composite illustrate that SCS monsoon onset is frequently preceded by the development of an eastward-propagating Madden-Julian Oscillation (MJO) in the Indian Ocean and the subsequent emanation of a convectively coupled Kelvin wave into the Pacific. Since both the MJO and Kelvin waves generate westerly surface winds in their wake, it is suggested that these waves may accelerate or trigger the monsoon onset process in the southern SCS. A detailed analysis of the Kelvin wave that propagated through the SCS during SCSMEX shows that it was responsible for a large portion of the surface wind shift leading to monsoon onset in 1998.