WEATHER PATTERNS IN THE INDIAN OCEAN
The great advantage of the Indian Ocean, as compared with the Atlantic and Pacific, is that instead of constant trade winds throughout the year, the heating of the Asian land mass in the northern summer results in a reversal of the wind. It is therefore possible to time your voyages and sail east, as well as west, with a free wind. You can make good passages all over the Indian Ocean, cruising from one side to the other in accordance with the changing monsoons.
The Indian Ocean can broadly be divided into four horizontal bands. The North Indian Ocean runs down to the Equator, then the top of the South Indian Ocean runs from the Equator to 10°S, the middle band of the South Indian Ocean runs from 10°S to 30°S, and the bottom band stretches down from 30°S into the Southern Ocean.
Band 1 (North Indian Ocean). The NE monsoon runs from November to March. The further north you are, the earlier it is likely to set in. It is supposed to blow at 10 – 15kts, though is often less in the Malacca Straits and on the coast of Thailand. It may blow from a more northerly direction on the Asian side, working its way round to the east somewhere near 60°E. The SW monsoon runs from June to September, and blows more strongly, often at gale force. The two monsoons are separated by periods of unstable weather in April/May and October/November. During these months the weather is generally typical of the Doldrums, except that these are also the dangerous times for cyclones.
Band 2 (Equator down to 10°S). The SE trades blow from April through to October. The SE trade is mostly steady, but can blow freshly.Between November and March the northern hemisphere’s NE monsoon crosses the Equator, but is deflected, and becomes the NW monsoon. The NW monsoon tends to be light and unreliable.
Band 3 (Between 10°S and 30°S). The SE trades blow from Western Australia to Madagascar all year round. The SE zone moves north in the southern winter to a northern limit of about 2oS during August. In the southern summer the SE zone moves south. In January they blow between 12°S and 30°S. Wind strengths are supposed to be 10–15kts in summer and 15–20kts in winter, but the experience of many yachts is that the trades here can blow at Force 7 for days on end.
Band 4 (Below 30°S). Below 30°S the prevailing winds are westerly.
Convergence Zone; There is often a trough or line of convergence lying along the Cocos, Chagos, Seychelles routes. Sailing in this should not be underestimated. It is hard tiring work with constantly changing wind strengths and a lot of rain. The Indian Ocean has confused seas, lots of squalls and often strong winds.
Cyclones: Tropical revolving storms form in both hemispheres of the Indian Ocean, but at different times of the year. North of the equator, the worst times are late May to mid-June, and late October to the end of November. None have been recorded in February or March, but they can form in the Bay of Bengal at the end of December and early January. Those that form in the Bay of Bengal often trend north-west and then north towards Bangladesh, but sometimes carry on westward into the Arabian Sea, or stall over India. Those that form in the Arabian Sea, usually in May and June, generally head north, and then curve either west towards the Gulf of Aden or east towards India.
South of the Equator, the cyclone season extends from the beginning of November to the end of May.
Indian Ocean: Two HF transmitters in Australia broadcast automated weather information; Charleville in QLD and Wiluna in WA. The same forecast in text can be received from Catalogues in SAILMAIL.
Volunteer Marine Rescue (VMR) stations throughout Aus. broadcast the same info on VHF for their area of interest. Some locations have repeater stations to extend their VHF coverage.
Voice Services Automated voice broadcasts occur from both transmitters covering their respective areas east and west. Weather warnings for all areas are given on the hour. Coastal forecast are valid out to 60 nm off the coast. Met area l0 high seas broadcasts cover ocean areas.
For more info go to www.bom.au/marine/australia.
The central southern Indian Ocean is covered by the French Meteo from Mauritius. The best way to get this is with a Sat C receiver. This information combined with the GRIBS is useful, especially concerning the troughs.
GRIB files get less accurate nearer the equator. The version with rain forecasts is very useful and gives a good idea of the position of the troughs.
Piracy There are areas on both sides of the Indian Ocean in which piracy is reported to be rife. The worst areas are the Malacca Straits, between the Indonesian islands close to Singapore, around the Horn of Africa (particularly the Somali coast), off the Yemeni coast and all the way up the Red Sea. The SE Asian zones seem to hold more dangers for big ships than for yachts. Yachts may be considerably more vulnerable in the Red Sea, particularly its southern approaches.
Yachts have been boarded by armed gangs in fast dinghies, and have been robbed of cash and items such as VHF radios and outboard motors. There have been a number of reported incidents in recent years, including incidences of kidnapping. The risk of violence to yachts should not be underestimated. It would seem prudent for yachts to travel in groups and maintain close contact when cruising these areas. Maintaining a regular VHF contact between yachts also makes the various international Naval Patrol and commercial vessels in the area aware of your presence and movements. They are generally very supportive.
￼￼For yachts leaving Fremantle to cross to South Africa the acceptable first leaving date is the 1st May. Cyclones can happen even this late but they are very rare. Yachts coming from the north of Australia are unlikely to be arriving in the Indian Ocean until well after the end of the cyclone season.
• World Cruising Routes, by Jimmy Cornell, published by Adlard Coles Nautical.
• Cruising Guide to SE Asia, vols 1 and 2, by Stephen Davies and Elaine Morgan, published by Imray Laurie Norie
• The Indian Ocean Cruising Guide, by Rod Heikell, published by Imray Laurie Norie
• The Seychelles, by Alain Rondeau, published by Imray Laurie Norie