Squalls in the South China Sea

Robert Chang

Squalls refer to a sudden increase in wind speeds. The World Meteorological Organisation's definition of a squall is: "A sudden increase of wind speeds by at least 16 knots, the speed rising to 22 knots or more and lasting for at least one minute.". Squalls are associated with thunderstorms and heavy showers and they can last for several minutes but they normally last for less than 10 minutes.

Over the southern South China Sea region, squalls of 50 to 60 knots are not uncommon with peak gusts up to around 70 to 80 knots. Thus squalls can be very destructive. Squalls can occur all year round but are most likely during the Southwest Monsoon and inter-monsoonal periods.

Sumatras

During the Southwest Monsoon, squalls known as Sumatras are liable to be experienced in the Straits of Malacca. These squalls nearly always occur at night and early morning though similar squalls may occur in the daytime.

The wind direction during a Sumatra is generally between southwest and northwest, the wind usually shifting suddenly to this quadrant at the same time as the wind increases. Sumatras are often violent, the wind sometimes exceeding gale force.

These storms do not usually last more than two hours, but in exceptional cases they may continue for six to eight hours. Thunder, lightning and torrential rain nearly always accompany a Sumatra.

Effects & Prediction

Squalls are especially dangerous during sensitive operations like off/onloading, heavy lifts, floatovers, or during the jack-up/down phases of a rig. Severe structural damage usually results if squalls occur during such operations.

Fortunately, squalls are easily visible on weather radar or the visible channel of geostationary satellites. These data, together with an analysis of surrounding upper air soundings allow trained forecasters to determine the likelihood or strength of squalls affecting offshore operations.

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