Tropical Cyclones in the South China Sea
by Arnold Doray
Tropical Cyclones (also known as Typhoons (in the western Pacific), Hurricanes (Atlantic), or Tropical Revolving Storms) occur all year round over the northern South China Sea. However, the "Typhoon Season" is taken to be from the Autumn transition (Oct) to the first half of the Northeast monsoon (Nov-Dec), when they occur most frequently in the South China Sea.
Storms that occur during the Southwest monsoon usually form well east of Luzon. These system initially move to the northwest towards Taiwan, but later recurve sharply to the northeast into southern Japan. Such storms indirectly influence the South China Sea by causing "surges" in the Southwest monsoonal flow, which would cause high seas, strong winds and squalls over the South China Sea.
Later in the year, from October to December, Tropical Cyclones track westwards, crossing the Philippines into the South China Sea. These westward moving storms occur progressively farther south towards the end of the season. The figure shows typical "early" and "late" tracks.
The figures below show actual plots of cyclone tracks and intensities for Mar - Sep and Oct - Feb respectively. Notice that during the Southwest Monsoon (Mar - Sep), most storms recurve towards Japan. However, some storms still affect the northern South China Sea during this period. During the Northeast monsoon, the storm tracks change dramatically. There are significantly less storms recurving towards Japan, and the storms tracking westwards into the South China Sea also track farther south.
Tropical Cyclones are mainly confined to above latitude 10°N. However, rare storms can occur even as far south as the Sulu Sea. These storms (also known as "monsoonal lows") start their life in the central South China Sea and track eastwards over northern Borneo and into the Sulu Sea (see figure). Although these systems are usually weak, they bring squalls, which can have a devastating effect on offshore structures, besides disrupting operations.
On very rare occasions, Tropical Cyclones can develop within 2 degrees of the equator. Only one such storm is known to have occurred within the last 100 years. Typhoon Vamei (2001) developed in the southern South China Sea and moved southwestwards, making landfall over Singapore. (This storm is visible in the Oct - Feb storm plot.)
Tropical Cyclone Categories
Tropical Cyclones are categorised according to maximum sustained wind speed close to the storm's centre. The scale used for the northwest Pacific, including the South China Sea comes from the Japanese Meteorological Agency, which classifies storms based on their 10-minute winds (ie, wind speeds averaged over 10 minutes):
| Tropical Disturbance | 15 - 23 knots |
| Tropical Depression | 24 - 33 knots |
| Tropical Storm | 34 - 47 knots |
| Severe Tropical Storm | 48 - 63 knots |
| Typhoon | 64 - 128 knots |
| Super Typhoon (variable classification) | > 128 knots |
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