At 1,600km x 600km Madagascar is the world’s fourth largest island. It scores big too, catching the same swell as its esteemed neighbours Mauritius and Réunion in the south-west Indian Ocean – and prevailing trade winds at these latitudes mean ...
One particular climatic feature sets the Indian Ocean apart: all the spots enjoy a subtropical or tropical climate and, apart from the ocean regions around South Australia, Indian Ocean coasts are exclusively warm water areas. Of course there are regional differences (like a warmer tendency to the north) and cloudy, rainy days with more windchill, but water temperatures between 23°C and 30°C and daytime highs from 25°C to well over 30°C mean no-one really gets cold!
A large part of Madagascar with its southern Toliara region, and the Mascarenhas Archipelago, to which La Réunion and Mauritius belong, lie almost permanently under the influence of the SE trades. In the southern hemisphere, predominantly south-easterly winds develop between the subtropical high pressure zone, from which the air-mass flows, to the equatorial low pressure (or ‘tropical convergence zone’). Here, the solar-heated air rises high into the atmosphere to descend again like a blanket in the subtropical high pressure zone – and so the cycle repeats. There’s nothing in the middle of the Indian Ocean to disturb this phenomenon – only Madagascar in the west blocks its progress, producing a wind buffer on the east coast. But the south profits from this as the air-mass is forced around the cape, strengthening the wind and improving the weather too. The SE trades blow most reliably in wintertime (Apr-Nov) between 15° and 25° South. From June to October in particular, it dominates the west of the Indian Ocean, towards the end it weakens and develops an east to north-easterly component.
Meanwhile, the Cocos Islands out east receive it well into December, when weather becomes more changeable. Occasional low-pressure troughs reach Madagascar, and cyclones can develop from January to March – high summer in the southern hemisphere. Tropical storms develop where large areas of ocean warm beyond 26°C for extended periods of time, commonly in summer when the sun’s almost directly overhead in the outer tropics near the Tropic of Capricorn. The solar rays quickly evaporate the water, building towers of cloud that rise to very high altitudes. Finally the Earth’s rotation spins them south-west into the southern Indian Ocean. Cyclones can create good swell, but you don’t want to be too close – the winds are crazy, gusting well over 100 knots with flooding rains.
Fortunately, trade wind season coincides with the best waves as winter storms in the Roaring Forties (40-50° South) send quality groundswell unhindered through the Indian Ocean, predominantly from the south-west. Swell heights of 3-5m with periods of 12-18 seconds aren’t rare. In summer the quantity and size are smaller but the islands in the southern Indian Ocean still see some action.
North of the equator, the Asian monsoon system controls the weather. In summertime (May-Oct) the landmass from Saudi Arabia to north India is super-heated, creating large heat lows. These draw warm, damp air-masses in from the Indian Ocean, resulting in the dependable SW Monsoon that even extends to areas near the equator like the Maldives and Arugam Bay on Sri Lanka. On the small, low-lying Maldivian islands there is no topographical strengthening so the wind’s generally light. In contrast, south-east Sri Lanka often scores Force 6-7. The SW wind is strongest on the Horn of Africa where it accelerates along the Somali coast. Force 8 or more is a daily occurrence in summer, and 50+ knots is possible with venturi effects like those on the Yemeni island of Socotra. The strength backs off further north, hence it’s not so extreme in Oman but you’ll always be powered up. Unlike further east, in the SW Monsoon it’s dry over much of the Arabian Desert while the strong wind drives respectable 4-5m windswell into the Arabian Sea.
The same quality swell that lights up the islands down south also hits the Maldives and Sri Lanka, but although the sets get more defined it dwindles to 2-3m on its long journey. In winter the wind does a u-turn as dry NE Monsoon from massive high pressure regions over the chilly northern landmass of the Hindu Kusch and Himalayas blows towards the tropical low pressure zone. It’s much weaker and less constant, but there’s little swell anyway. In the intervening months as the wind direction changes there’s mainly weak and erratic winds – apart from cyclones tearing up the Bay of Bengal in Apr/May and the Arabian Gulf too in Oct/Nov.
The Cocos-Keeling Islands are a far-flung Australian horse-shoe of low-lying coral islets in the Indian Ocean, 2700km north-west of Perth and 900km west of their nearest neighbour the Christmas Islands. First sighted in 1609 by Captain William ...