Sailing the Tropics- Weather Guide

by Ken Batt and Bruce Buckley, Bureau of Meteorology on 22 Jun 2006 Sailing in the tropical waters of the Australasian region, from Cocos Keeling Islands in the west to the Solomon Islands in the east, is usually a relatively tranquil experience weather-wise. Local wind effects, such as land and sea breezes, and mountain-induced winds, tend to dominate close to land. None the less, the wise sailor should be aware of some weather features that can range from frustrating through to downright dangerous.

Tales of old talk about the doldrums. This is a near equatorial zone where the trading ships of old were often stranded for days with fickle winds that would refuse to settle into a steady wind direction. This is an area forecasters refer to in various ways, with the monsoon trough being one common term. To explain what is happening in this zone we must look closer to the poles - at the bands of high pressure that normally circles the globe somewhere around 30o to 40o of latitude away from the equator.

The world’s steadiest winds - the Trade Winds - spiral equatorwards out of these often large high pressure systems. In the southern hemisphere the winds spiral anticlockwise out of the high to produce the south east trade winds. In the northern hemisphere the winds spiral out of the highs in a clockwise direction to produce the north east trade winds on their equatorward side. In the South China Sea region this airstream is likely to produce showers and storms when the flow is vigorous and is referred to locally as the north east monsoon The broad area of low pressure where the winds from both hemispheres meet will often produce a region of light and variable winds - the doldrums. This light wind regime is most prevalent during the period near the equinoxes when the trade winds tend to be weaker in both hemispheres.

When the trade winds cross the equator and move into the other hemisphere they are deflected by the earth’s rotation. By this time the winds are moisture laden and the atmosphere can be quite unstable. The very warm equatorial waters further increase this instability. Those sailing in northern Australian waters will hear of winds and wet weather known as the north west monsoon. This is the wind regime that dominates the wet season of tropical Australia, which typically runs from November through to March. Areas of very intense rain and thunderstorms are likely, particularly near the region (known to forecasters as the Inter-Tropical Convergence Zone, or ITCZ) where the trade winds collide with the monsoonal winds. These thunderstorms are generally guaranteed to provide a tremendous light show but, apart from the odd squall line that heralds the arrival of the storm, they are usually much less dangerous than the thunderstorms that occur in the mid-latitudes. The north west monsoon is typically a moderate to fresh breeze. However, if a tropical low forms or if the high pressure system in the northern hemisphere is very strong, the north west monsoon can develop to gale strength.

If you sail through south east Asia during the period from May to September you are likely to encounter the south west monsoon. It is formed by the deflection of the south east trades crossing into the northern hemisphere. The combination of numerous islands in the Malaysian- Indonesian- Phillipines area and very warm seas makes the south west monsoon even wetter than the north west monsoon of the southern hemisphere.

This brings us to the real danger of the tropics - the very intense tropical lows known in the Australian region as tropical cyclones and in Asian waters as typhoons. These systems, which are given names once they produce winds that reach gale force strength, can rapidly intensify to produce winds and seas that no sailing craft and few powered vessels can hope to survive. They are to be avoided at all costs. Classified as severe once the mean wind speed reaches 60 knots, the most intense tropical cyclones can produce mean wind speeds in excess of 120 knots with seas of 20 metres height or more. These only exist further than 6 o from the equator, reaching maximum intensity between 15 o and 25 o from the equator. If you plan to sail in Australian tropical waters between November and April, particularly in the Coral Sea, Arafura Sea and Timor Sea, closely follow the weather bulletins issued by the Weather Services or the various nations in the region. In the northern hemisphere, the Pacific Ocean east of the Philipines, the South China Sea and the Bay of Bengal are particularly prone to these monsters.

There are a few other features of tropical weather that are worth noting. In the eastern Coral Sea during the wet season there is often a very persistent and large area of thunderstorms with a substantial cloud band streaming away to the south east. So persistent was this north west to south east orientation that the islanders of old used to use the cloud band as a navigation aid. This cloud band is a result of middle level convergence of winds tied in with an east-west circulation across the Pacific Ocean known as the Walker Circulation. This is the predominant atmospheric circulation associated with the El Nino - La Nina phenomena that have been much talked about in recent years. The cloud band is much more persistent in La Nina years than it is in El Nino years.

On the other side of Australia the weather tends to be strongly influenced by low pressure systems that form over the Kimberley and Pilbara regions, caused by the very high temperatures in these areas during the summer months. The Kimberley low becomes a part of the monsoonal trough during the wet season with considerable thunderstorm activity the result, but the Pilbara low generally remains dry. These lows also affect the winds for a few hundred kilometres offshore, with a region of fairly consistent south westerly winds being established offshore from the north west Western Australian coastline.

The other feature of note are surges in the trade winds. These arise from sudden strengthening of the mid latitude high pressure systems, normally following the passage of a moderate or strong frontal system. Watch the weather maps regularly because these surges may arrive in the tropics a day or two after the cold front has passed through well to the south (in the southern hemisphere). There will seldom be a well defined change marked on the weather maps, but you should notice the isobar spacing tighten to the south of you (or north if you are in the northern hemisphere) before the surge arrives.

Tracking changes in the weather in the tropical regions requires a different approach to that used elsewhere in Australian waters. The synoptic weather map commonly used is an analysis of pressure fields. However, the pressure changes in tropical areas are mostly slight, making these charts of limited use. Note that for equal isobaric spacing, the wind will be stronger in the tropics than outside of the tropics (extra-tropics). An old rule of thumb used by tropical mariners goes like this- Take the last two digits of the central pressure of the high pressure system exerting the ridge along say, the tropical Queensland coast, and that will roughly be the average surface wind speed. For example, if the central pressure is 1030hPa, then the average wind speed should be around 30 knots. Wind field charts, called streamline charts, which show the prevailing wind flows, are normally used instead of isobaric charts. These are available from the Bureau’s web-site and are broadcast as a part of the radio facsimile weather chart product set. Special attention also needs to be taken of all forms of tropical weather bulletins as these may provide the earliest indication of a developing tropical low.

Forewarned with this information your tropical sailing should be a safe and satisfying experience.