Definition of South China Sea Monsoon Onset and Commencement of the East Asia Summer Monsoon*

Abstract

The climatological mean summer monsoon onset in the South China Sea (SCS) is a remarkably abrupt event. However, defining onset dates for individual years is noticeably controversial. The controversies and complications arise primarily from a number of factors: the intermittent southward intrusion of cold fronts into the northern SCS, the bogus onset in late April before the establishment of tropical monsoons over Indochina, and active intraseasonal oscillation. In this paper, a simple yet effective index, U SCS , the 850-hPa zonal winds averaged over the central SCS (5Њ-15ЊN and 110Њ-120ЊE), is proposed for objectively defining the SCS monsoon onset. This onset index depicts not only the sudden establishment of the tropical southwesterly monsoon over the SCS but also the outbreak of the rainy season in the central-northern SCS.In this paper the East Asian summer monsoon (EASM) is defined as the broadscale summer monsoon over East Asia and the western North Pacific region (0Њ-40ЊN, 100Њ-140ЊE). It is shown that the seasonal transition of EASM can be objectively determined by the principal component of the dominant empirical orthogonal mode of the 850-hPa zonal winds, U EOF1 . It is found that the local index U SCS represents U EOF1 extremely well; thus, it can be used to determine both the SCS onset and the commencement of the broadscale EASM. The result suggests that the SCS summer monsoon onset indeed signifies the beginning of the summer monsoon over East Asia and the adjacent western Pacific Ocean. Evidence is also provided to show the linkage between the two salient phases of EASM: the local onset of the SCS monsoon and the local onset of the mei-yu (the rainy season in the Yangtze River and Huai River basin and southern Japan).

ENSO and the South China Sea summer monsoon onset

“…Wang et al (2004) suggested a wind index -the 850-hPa zonal winds averaged over the region 5°-15°N, 110°-120°E (the precise domain is 3.75°-16.25°N, 108.75°-121.25°E) -to quantify changes in the SCS circulation before, during, and after the onset. The OLR data are used as a proxy for deep convection because of its strong relationship with tropical rainfall (Wu and Zhang, 1998), which is a direct reflection of the summer monsoon.…”

“…Further, the SCSSM onset indicates the beginning of the EASM (Wang et al, 2004). An early or late SCSSM onset could affect the precipitation pattern of the EASM to some extent.…”

Convective and stratiform rainfall and heating associated with the summer monsoon over the South China Sea based on TRMM data

“…Qiao et al (2002) showed that there are huge differences in precipitation over four sub-regions (105-110 E, 0-20 N; 110-120 E, 0-5 N; 110-120 E, 5-10 N; 110-120 E, 10-20 N), and the precipitation over the region of 110-120 E and 10-20 N can best represent the summer monsoon precipitation over the South China Sea. Wang et al (2004) also suggested that the rainfall averaged over the same region can be used as a rainfall index to describe the SCSSM onset because it increases concurrent with the reversal of the 850 hPa zonal. Therefore, we will choose 110-120 E and 10-20 N as our computation and analysis domain.…”

“…Its onset and intraseasonal variability, however, are the focus in the recent years (Chang and Chen 1995;Wang et al 2004;Mao and Chan 2005). With the field observation of the South China Sea Monsoon Experiment (SCSMEX) in 1998, much better data were collected for the studies of the SCSSM (Lau et al 2000;Ding et al 2004).…”

Combining regional moist static energy and ENSO for forecasting of early and late season Indian monsoon rainfall and its extremes

“…For those that successfully predict a variety of atmospheric conditions associated with monsoons [e.g., Wang et al, 2009], skill in predicting monsoon rainfall is insignificant. In lamenting the failures to predict monsoon failures in and 2004, Gadgil et al [2005 argued that no progress in monsoon prediction had been made for seven decades and stated: "It is clear that no 'quick fix' solutions are available to tackle the problem of predicting the summer monsoon rainfall over the Indian region. "…”