Abstract
In the South China Sea (SCS), 17% of tropical cyclones (TCs) formed in the late season (November−January) were associated with a strong northeasterly monsoon. This study explores the effects of northeasterly strength on TC formation over the SCS. The Weather Research and Forecasting (WRF) model is used to simulate the disturbances that develop into TCs (formation cases) and those that do not (non-formation cases). Two formation (29W on 18 November 2001 and Vamei on 26 December 2001) and two non-formation (30 December 2002 and 9 January 2003) cases are simulated. To address the importance of upstream low-level northeasterly strength to TC formation, two types of sensitivity experiments are performed: formation cases with increased northeasterly flow and non-formation cases with decreased northeasterly flow. If the strength of the northeasterly is increased for the formation case, the stronger cold advection reduces the convective instability around the disturbance center, leading to the weakening of the simulated disturbance. If the strength of the northeasterly is decreased for the non-formation case, the simulated disturbance can develop further into a TC. In summary, strength of the upstream low-level northeasterly flow does affect the environmental conditions around the disturbance center, resulting in the change of TC formation probability over the SCS in the late season.
Highlights
The South China Sea (SCS) is a unique basin of the western North Pacific (WNP)
In the SCS, the relative percentage of tropical cyclones (TCs) that form in the late typhoon season is higher than that in the WNP, especially in December
Most of the previous studies focused on the specific favorable environmental conditions for TC formation in the SCS, such as the shear vorticity and the warm ocean water [6,30], but the unfavorable conditions for TC formation are rarely discussed
Summary
The South China Sea (SCS) is a unique basin of the western North Pacific (WNP). The SCS, surrounded by lands, has warm sea surface temperature in the whole year and is affected by the synoptic-scale systems and longer timescale systems from the Maritime Continent [1,2]. The averaged 200-hPa divergence over the southern SCS is stronger for the formation cases They indicated that the strong northeasterly surge provided large low-level background vorticity at lower latitude favorable for the formation of the initial vortex, which is pointed out by Chang et al (2003) [11]. For TC formation, the sensitivity experiments of the environmental vorticity [17], trade-wind surge [18], and moisture patterns [19] have been conducted Building on these studies, this study uses the WRF model to simulate the formation and non-formation cases associated with strong northeasterly flow in the SCS, and performs the sensitivity experiments to examine the importance of upstream low-level northeasterly strength for TC formation. FFiigguurree77. .TTimimeeseserireises(h(h) )ofotfhtehemminiinmimumumcecnetnratrlaslusrufarcfeacperepsrseusrseur(heP(ha)Pfao)rftohrethcoenctoronltroulnrsuannsdasnednssietnivsiittyiveixtypeerxipmeernimtsefnotrs (fao)r0(1a2)90W12a9nWd a(bn)dV(abm) Veia.mCeuir.vCeusrwveitshwdiitfhfedreifnfterceonlot rcsorloeprsrerseepnrtesthenetctohnetrcoolnrturonlarnudn danifdferdeinfftesrenstisteivnistyitievxitpyereixmpenritms.ents
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