Evolution of the Large Scale Circulation, Cloud Structure and Regional Water Cycle Associated with the South China Sea Monsoon during May-June, 1998.

Abstract

This paper studies the evolution of the South China Sea (SCS) monsoon during May-June 1998, to elucidate relationships among the large scale circulation, organization of convection, cloud structures, and fluctuations of the regional water cycle of the SCS. Primary data used include field observations from the South China Sea Monsoon Experiment (SCSMEX), and the satellite rain products from the Tropical Rainfall Measuring Mission (TRMM). Prior to the onset of the SCS monsoon, enhanced convective activities associated with the Madden and Julian Oscillation were detected over the equatorial Indian Ocean in early May while the SCS was under the influence of the West Pacific Anticyclone with prevailing low level easterlies and suppressed convection. Subsquently, a bifurcation of the MJO convection near 90 E led to the development of strong convection over the Bay of Bengal, which spawned low-level westerlies across Indo-China and contributed to the initial build-up of moisture and convective available potential energy over the northern SCS. The onset of the SCS monsoon occurred around May 18-20, and appeared to be triggered by the equatorward penetration of extratropical frontal disturbances, originating from the continental regions of East Asia.Analysis of TRMM microwave and precipitation radar data revealed that during the onset phase, convection over the northern SCS consisted of squall-type rain cells embedded in meso-scale complexes similar to extratropical systems. The radar Z-factor intensity indicated that SCS clouds possessed a bimodal distribution, with a pronounced signal (>30 dBz) at a height of 2-3 km, and another one (>25 dBz) at the 8-10 km level, separated by a well-defined melting level signaled by a bright band at around 5-km level. The most convectively active phase of the SCS monsoon, as measured by the abundance of convective and stratiform hydrometeor types, inferred from the radar vertical profile, was found to occur when the large scale vertical wind shear was weakest. The fluctuation of the water cycle over the northern SCS was found to be closely linked to the largescale dynamical and SST forcings. Before onset and during the break, the northern SCS was relatively warm and served as a moisture source (E À P > 0) to the overlying atmosphere. During the active phase, the northern SCS was cooled, providing a strong sink (E À P f 0) for atmospheric moisture, with the primary source of moisture coming from regions further west over Indo-China and the eastern Indian Ocean. Vigorous water recycling by convective systems in the northern SCS occurred during the mature phase of the SCS monsoon, with precipitation efficiency (defined as the ratio of the surface precipitation to the sum of large scale moisture convergence and surface evaporation from the ocean) approaching 96%. Westward transport of moisture from Indo-China into, and northward transport out of, the northern SCS provided the main source of moisture for the torrential rain over the YRV in mid-June 1998. The present results suggest...

Relationship between the Onset of the South China Sea Summer Monsoon and the Structure of the Asian Subtropical Anticyclone

“…Fong and Wang (2001) supported this idea based on their analyses of the climate mean 850-hPa winds. In fact, the climatological onset date of the Asian summer monsoon found by Lau and Yang (1997) and Webster et al (1998) also did not indicate that the SCSSM first occurs in the Asian summer monsoon regime. The onset date of the SCSSM in a particular year can also differ among different researchers (e.g., Dai et al 2000;He et al 2001).…”

“…An important component of East Asian summer monsoon is the summer monsoon over the South China Sea (SCS), which He et al (2001) considered to be the connecting gear that links the East Asian monsoon and South Asian monsoon systems. The onset of the SCS summer monsoon (SCSSM) is signified by a large-scale wind shift in the lower troposphere from easterly to westerly, and an outburst of deep convection over the SCS (Lau et al 1998;Ding and Li 1999;Chan et al 2000;Lau et al 2002). Climatologically, the onset of the SCSSM is around mid-May (Tao and Chen 1987;Chang and Chen 1995;Lau and Yang 1997;Hsu et al 1999).…”

“…Based on the observational datasets available during SCSMEX, many researchers have examined the 1998 SCSSM onset and considered that it occurred around 20-25 May (Chan et al 2000;Ding and Liu 2001;Xu and Chan 2001;Lau et al 2002;Liu et al 2002). Unfortunately, such comprehensive data are not available every year for studying the climatological features associated with the SCSSM onset.…”

Spatial and Temporal Distribution of Latent Heating in the South Asian Monsoon Region

“…The dynamical response of the tropical atmosphere depends strongly on both the vertical and horizontal distribution of LH (e.g., Hartmann et al 1984;DeMaria 1985;Lau and Peng 1987;Nakazawa 1988;Sui and Lau 1989;Emanuel et al 1994;Yanai et al 2000;Chiang et al 2001). These studies show that improved vertical distributions of LH allow a better representation of tropical east-west circulations (e.g., the Walker circulation) by models other than those using simple heating functions, such as Webster (1972) and Gill (1980).…”

Seasonal dependence of the predictable low-level circulation patterns over the tropical Indo-Pacific domain

“…The tropical Indo-Pacific domain is one of the key regions controlling the global climate variability (Ramage 1968;Neelin et al 1998;Torrence and Webster 1998;Lau et al 2000Lau et al , 2002Wang et al 2000;Wang and Zhang 2002;Kosaka and Xie 2013). The large-scale atmospheric circulation over this region plays an important role in not only influencing the variability of the tropical climate systems but also transferring their impacts remotely through teleconnections (Wang and Zhang 2002;Cai et al 2011;Sun and Zhou 2013).…”