Offshore Vietnam and Thailand

Robert Chang

The East Coast of Vietnam

Light to moderate (< 15 knots) winds cover the area for much of the year except during the height of the Northeast and Southwest Monsoons. During the Northeast Monsoon, wind speeds increase to mostly fresh-strong (17-27 knots) from November to January. Occasional strong surges in the Northeast Monsoon can cause wind speeds to increase to gale-strong gale (35-45 knots) lasting for up to a few days. The Southwest Monsoon over the area is somewhat weaker than the Northeast Monsoon with mostly fresh (17-21 knots) winds from July to August, increasing to strong-near gale (25-33 knots) at times.

Wind directions are predominantly from north to northeast from October through to March. South to southwest winds dominate from May to September with variable winds in April and September.

Average significant wave heights are generally around 1 to 2m rising to around 2 to 2.5m from November to January. Heavy swells ( > 4m) during strong surges in the Northeast Monsoon can cause significant wave heights to rise to more than 6m for one to a few days. During the Southwest Monsoon, squally periods can cause significant wave heights to rise briefly to around 4m.

The Southern Coast of Vietnam

Average wind speeds over the area are mostly light to moderate (< 15 knots) throughout much of the year except during the Northeast Monsoon period. During the Northeast Monsoon, wind speeds increase to moderate-fresh (11-21 knots) for a significant proportion of the time. Surges in the Northeast Monsoon can cause wind speeds to increase to strong-gale force (25-35 knots) occasionally over the monsoon period.

Wind directions are mainly northly to easterly during the period from November to January becoming mainly northeastly to southeasterly from February to April. Winds are mostly southeasterly to westerly during May through to September. October is characterised by variable winds.

Wave heights are generally low during much of the year with short period (< 7sec) waves dominating. However, during the Northeast Monsoon, swells become moderate (2-4 m) for a significant period of time. During strong surges in the Northeast Monsoon, swell can become heavy (> 4m) for one to a few days with significant wave heights exceeding 5m.

The Gulf of Thailand

Light to moderate winds (< 15 knots) dominate over the gulf for much of the year. Winds increase to fresh-strong ( 17-27 knots ) during surges in the Northeast Monsoon. Periods of fresh-strong winds also occur during the Southwest Monsoon.

Wind direction is mainly from northeast to east from November to January. In the months of February and March winds are mainly from the east over the southern half of the gulf turning to southerly over the northern half. Variable winds dominate in April with mostly southerly winds in May. West to southwest winds dominate from June to September becoming variable in October.

Average significant wave heights are generally low for much of the year with short period (< 7sec) waves prevailing. Average significant wave heights rise to around 1 to 1.5m in November-December and also from July to September. Significant wave heights can rise to around 3m during periods of intensified monsoon winds.

Tropical Cyclones

The most significant weather event over the southern South China Sea region is the occurrence of tropical cyclones. Tropical cyclones affecting the southern South China Sea either develop in situ or they move in from the western Pacific. Tropical cyclones develop from tropical disturbances in the low pressure region of the monsoon trough. Therefore the tropical cyclone season over the southern South China Sea occurs during the passage of the monsoon trough southwards during October-early December and northwards in April-May.

The highest probability of tropical cyclone occurrence in the southern South China Sea and Gulf of Thailand is in the month of November with lower probabilities in October and early December. In comparison to year end months, the probability of tropical cyclone occurrence is much lower in April-May. Two notable tropical cyclones affecting offshore southern Vietnam and Gulf of Thailand are Typhoon Sally (30 Nov to 05 Dec 1972) and Typhoon Gay ( 31 Oct to 5 Nov 1989). Significant tropical cyclones affecting offshore eastern Vietnam are Typhoon Faith (08 Dec to 15 Dec 1998) and Typhoon Lingling ( 06 Nov to 12 Nov 2001).

The usual direction in which tropical cyclones move in the southern South China Sea is between west and southwest with some moving in a west to northwest direction.

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