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Tropical Cyclone Genesis Over the South China Sea ⁎ Guihua Wang A, , Jilan Su A, Yihui Ding B, Dake Chen A,C

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Tropical Cyclone Genesis Over the South China Sea ⁎ Guihua Wang A, , Jilan Su A, Yihui Ding B, Dake Chen A,C Available online at www.sciencedirect.com Journal of Marine Systems 68 (2007) 318–326 www.elsevier.com/locate/jmarsys Tropical cyclone genesis over the south China sea ⁎ Guihua Wang a, , Jilan Su a, Yihui Ding b, Dake Chen a,c a State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, State Oceanic Administration, Hangzhou, China b National Climate Center, China Meteorological Administration, Beijing, China c Lamont-Doherty Earth Observatory of Columbia University, New York, USA Received 29 June 2005; received in revised form 10 October 2006; accepted 11 December 2006 Available online 22 December 2006 Abstract The South China Sea (SCS) is among areas in the Northwest Pacific most frequented by tropical cyclones (TCs) with intensity reaching a tropical storm or stronger. It is also an area of significant TC genesis. In this study, TC genesis in SCS and its monsoonal variability for 1948–2003 are analyzed. Altogether, in May–September (southwest monsoon period) 157 TC geneses have occurred north of 12°N in SCS, while in October–December (northeast monsoon period) 64 out of 65 TC geneses have happened south of 18°N. It is found that the monsoonal characteristics of the SCS basically determine the region of TC genesis in each monsoon season. Winter TC genesis in the SCS happens over the region where the marine environment satisfies the four criterions on, respectively, the sea surface temperature (SST), mid-troposphere relative humidity, vertical shear of the horizontal winds and low-level atmospheric vorticity. During the summer, as the two criterions on SST and the mid-troposphere relative humidity are satisfied for the whole SCS, TC genesis occurs in the region where both the low-level vorticity and the vertical shear satisfy the criterion. In addition, there is likely more TC genesis in the winter during the onset of La Nina, and more TC genesis in the summer following the onset of El Nino. © 2006 Elsevier B.V. All rights reserved. Keywords: Tropical cyclones; Genesis; Monsoon; Air–sea interaction; South China Sea 1. Introduction established progressively over the SCS, first appearing over its northern part in September, reaching its central The South China Sea (SCS), the largest semi- in October and covering the entire SCS in November. enclosed marginal sea in the Northwest Pacific with an The winter monsoon gradually diminishes in April. area about 3.5×106 km2 (Fig. 1), is under the influence Most of the oceanic characteristics of the SCS are of the East Asia Monsoon (Liang, 1991). Onset of the influenced by the monsoon. mild southwesterly summer winds over the SCS usually As in many research works, here we define the occurs suddenly around mid-May in its southern and tropical cyclone (TC) as a tropical mesoscale cyclonic central part and soon expands to the entire SCS in June. weather system with its strength of a tropical storm or In contrast, the strong northeast winter monsoon is stronger (i.e., typhoon). Globally, the Northwest Pacific, including the SCS, has the most TCs. Based on data ⁎ Corresponding author. spanning over 1968–1988 (Neumann, 1993), the annual E-mail address: [email protected] (G. Wang). average numbers of TC and typhoon generated in the 0924-7963/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2006.12.002 G. Wang et al. / Journal of Marine Systems 68 (2007) 318–326 319 Fig. 1. SST climatology (in °C) and the initial positions of TCs (solid triangle) over the SCS for 1948–2003 (P1: Luzon Strait; P2: Taiwan Strait.). a: Southwest monsoon period; b: northeast monsoon period. Northwest Pacific are 25.7 and 16.0, respectively. SCS dry ambient air (Lighthill, 1998). If the relative is among the areas in the Northwest Pacific most often humidity at 500 mb (RH) is lower than 40%, frequented by TCs. In addition, SCS itself is also an area formation of TCs will be inhibited (Gray, 1968); of significant TC genesis. Our TC dataset shows that, (4) Small vertical shear of the horizontal winds: among the annual average of 10.3 TCs passing through important for the TC development for allowing the SCS, 3.5 originated within the SCS. For typhoons, the heat released by condensation to concentrate in the corresponding numbers are 6.0 and 1.3, respectively. a vertical column (Fink and Speth, 1998). Gray TC is a tropical mesoscale weather system operating (1968) proposed to characterize the vertical shear on the feedback between the enthalpy fluxes and winds at with |Vz|, the magnitude of the horizontal wind the ocean surface (Emanuel, 2003). It may be regarded as difference between the upper and lower tropo- a heat engine with heat-intake over the ocean, a nearly sphere, i.e., 200 mb and 850 mb, respectively. Local −1 adiabatic work-output in the eyewall and heat-loss near |Vz| greater than 8 ms is generally unfavorable for the stratosphere (Lighthill, 1998). In this system, TC TC development (e.g., Goldenberg et al., 2001); genesis depends critically on the conditions at the air–sea (5) An atmospheric background with relatively large surface and on the surrounding atmospheric environment. cyclonic low-level vorticity: producing initially a As summarized by Emanuel (2003), there are 6 necessary, frictionally forced low-level convergence of mass or conducive, oceanic and atmospheric environmental and water vapor, and consequently leading to an conditions for TC genesis, namely, upward vertical motion in favor of TC genesis (Gray, 1968); and (1) High sea surface temperature (SST): an ocean (6) A non-negligible Coriolis force, usually at latitudes with SST at least 26 °C to provide sufficient beyond 5° from the equator, to maintain the latent-heat input to fuel the winds of a TC cyclonic circulation. (Lighthill, 1998). However, Chen (1981) has found that an SST above 27 °C is a pre-requisite The first 3 are thermodynamic conditions, while the for TC genesis over the SCS. This will be the last 3 are dynamical ones. In addition, an external value we shall adopt for this study; disturbance is also needed to trigger the TC genesis. (2) Sufficiently large depths of the ocean mixed layer; Previous studies have shown that TC genesis over the (3) An atmospheric environment characterized by large SCS happens predominantly at its northern basin in values of mid-troposphere relative humidity: nec- summer (Liang, 1991), while in winter it favors the essary for sustained development of a TC. southern SCS (Liang et al., 1998). Liang (1991) suggested Otherwise, the extremely dense cloud eyewall of that many external disturbances from atmosphere circu- the TC will be dried out by upflow of the entrained lation over the SCS are responsible for the TC genesis. 320 G. Wang et al. / Journal of Marine Systems 68 (2007) 318–326 The lifespan of these TCs is relatively short, lasting only since 1992 but have close to 14% discrepancy in TC 3–4 days. Statistical parameters of their life history are identity before 1992. This may be due to different data listed in Table 1. Though previous studies presented the sources or different methods of analysis used to construct possible linkage between TCs and large scale tropical the two datasets. Data in both files include 6-hourly circulation (for example, Chu, 2004; Harr and Chan, positions of each TC, as well as its associated minimum 2005), how the ocean and monsoon environments affect center pressure and maximum wind speed. We choose SCS TCs have not been explored in detail. from both datasets all the TCs that have occurred over the In this paper, detailed analysis of the seasonal and SCS, defined for this purpose as the area (0–22.5°N, interannual environmental conditions of the SCS for TC 98.5–120.5°E), in 1948–2003. genesis is carried out, with special attention to their The wind and relative humidity product are from the connection with the monsoon characteristics. External NCEP data, available from 1948 to present. They have a disturbances for the development of the TCs are also one-month temporal resolution and a 1.875° Gaussian addressed briefly. grid spatial resolution. The SST data are from Had1SST, a dataset provided by the Hadley Center of the UK Met 2. Data Office, which has a one-month temporal resolution and a 1°×1° spatial resolution (Rayner et al., 2003). Our TC dataset covers the period of 1948–2003. The To study the growth of TCs from initial tropical year 1948 is chosen because it is the starting year of the disturbances, we use the cloud satellite images from the National Centers for Environmental Prediction (NCEP) StretchedVisibleandInfraredSpinScanRadiometer wind data. The TC data are derived from two TC datasets, (S-VISSR) Data provided by the Institute of Industrial one provided by the National Oceanic and Atmospheric Science, University of Tokyo. The available S-VISSR Administration (NOAA) of USA and the other the Japan data is from 1997 to 2003 with an hourly time res- Meteorological Agency (JMA). The former dataset spans olution. Tropical disturbances are defined as a distinctly 1945–1998 and the latter 1953–2003. For the period of organized cloud pattern of a spatial scale of 100 to 1948–1952 or 1999–2003, only the NCEP or JMA 300 km, with a corresponding wind pattern in the range product, respectively, is used. The two products are of 100 to 600 km. As both Gray (1968) and Liang merged for the period 1953–1998 because of the (1991) have demonstrated, the tropical disturbances can difference in these two datasets. The JMA data covers be traced well from satellite pictures and wind fields. only TCs as defined in this study, while the NOAA data includes, in addition to TCs, also some but not all tropical 3.
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