Mechanisms Responsible for the Maintenance of the 1998 South China Sea Summer Monsoon.

Abstract

This study examines the dynamic and thermodynamic characteristics associated with the evolution of the SCSSM of 1998 using the data from the South China Sea Monsoon Experiment (SCSMEX). The objective is to identify the physical processes that determined the active and inactive periods of the 1998 SCSSM.The intraseasonal variations in the activity of the 1998 SCSSM are found to be mainly controlled by the 30-60-day oscillation, but modified by the 10-20-day mode. Definition of the active and inactive period is thus based on the OLR anomaly and its 30-60-day filtered component, giving two active (25-30 May, 23 June-5 July) and two inactive periods (6-18 June, 13-26 July).An examination of the circulation features suggests that the two modes operated very differently. After the onset of the SCSSM, a northward propagation of the 30-60-day mode into the SCS dictates the active and inactive phases of the SCSSM. During an active (inactive) phase, positive (negative) relative vorticity and convergence (divergence) associated with a low-level trough (ridge) propagated into the SCS, initiating (suppressing) the convection. The divergence field at the upper level was in phase with the low-level relative vorticity. However, the direction of propagation associated with the 10-20-day mode was different. A westward propagation of this mode into the SCS caused changes in the convection over the region during the first half of the 30-60-day mode. A second oscillation apparently propagated from the midlatitudes into the SCS to initiate or suppress convection. The dynamic conditions in terms of the vorticity and divergence are similar to those for the 30-60-day mode. The atmosphere over the SCS was also found to be generally favourable for the development of deep convection after the onset of the SCSSM.

The East Asian summer monsoon: an overview

“…Unit: ms À1 for wind and m 2 s À2 for kinetic energy (Mu and Li, 2000) 30-60-day mode occurred concurrently with bursting of the westerly monsoon at 850 hPa in this region (Mu and Li, 2000). Also based on the data from the SCSMEX in 1998, Chan et al (2002) have shown that the onset and maintenance of 1998 SCS summer monsoon were controlled by the 30-60-day oscillation and further modified by the 10-20-day mode. Chen and Chen (1995) previously indicated that the onset of the 1979 SCS summer monsoon occurs under the condition of a phase-lock between the 30-60-day and the 10-20-day modes over the Northern SCS.…”

Intraseasonal oscillations and the South China Sea summer monsoon onset

“…Since the focus here is on the conditions prior to the SCSSM, this latter part will not be explored further. For more details of how the ISO modulates the monsoon active/break cycle, the reader is referred to the recent studies of Chan et al (2002) and Mao and Chan (2005). cyclonic anomalies in the circulation (Figure 3), which indicates contributions from the released latent heat of condensation.…”

West Pacific subtropical high double ridges and intraseasonal variability of the South China Sea summer monsoon

“…Based on this selection criterion, 8 cycles are isolated for 3/6 mode, and 33 cycles for 1/2 mode, as well as 9 cycles of the 30-60-day oscillation and 32 cycles of the 10-20-day oscillation selected for dual mode during the period of 1979-2005. Then, each cycle is grouped into several different phases for composition, as in Chan et al (2002). That is, phase 3 is the peak of the active period, while phase 7 is the peak of inactive period.…”

“…They further suggested that the SCS summer monsoon in 1979 was adjusted by the northward-propagation of the 30-60-day monsoon trough-ridge, and the SCS summer monsoon break occurred simultaneously with a phase lock in the northern SCS between the 30-60-day oscillation and the 10-20-day oscillation. According to the data from South China Sea Monsoon Experiment, Chan et al (2002) examined the evolution of the SCS summer monsoon in 1998 and found that the maintenance and break of the SCS summer monsoon in that year were controlled by the 30-60-day oscillation, and were further modified by the 10-20-day oscillation.…”