Palk Straight Intermonsoon
Sri Lanka may be said to enjoy four annual seasons. These are not the spring, summer, fall and winter experienced at high latitudes, but two rainy seasons – known as the northeast and southwest monsoons – and two more or less dry intermonsoonal periods. These seasons are not experienced equally in all parts of Sri Lanka; the barrier of the island’s central massif dries out the monsoon winds as they pass over it, so that the northeast and southwest monsoon rains, particularly the latter, fall chiefly upon their respective portions of the country.
In any year, the start or end of a season may vary by up to one month.
December to February: Northeast Monsoon
The relatively mild winds of the northeast monsoon deposit moisture from the Bay of Bengal all over Sri Lanka, but particularly on the north-eastern slopes of the central mountains – swelling the streams that drain them and raising freshwater outflows from rivers in the north and east of the country to their highest annual level. Cool, dry winds blowing from the Indian subcontinent bring calm seas to the Gulf of Mannar and the southern coastal waters.
March & April: First Intermonsoonal
This is the best season to see migratory and nomadic whales and dolphins in Sri Lanka, partly due to weather conditions and partly to the fact that many species of whale and dolphin move through the coastal waters of the island at this time.
The first intermonsoonal is not, strictly speaking, a dry season; typical conditions include brief afternoon or early evening thunderstorms, mostly occurring in the southwest. It is also the warmest season of the year. Mornings bring light variable winds and mostly calm seas – ideal conditions for marine mammal watching.
May to September: Southwest Monsoon
Of greater duration, and given to stronger winds and twice as much precipitation as the relatively mild northeast monsoon, the southwest monsoon has considerable oceanographic impact and greatly influences the incidence, composition and behaviour of marine life in the northern Indian Ocean.
Strong south-westerly winds, pregnant with moisture from the open ocean, bring rough seas to Sri Lanka’s southern and western shores. Moving inland, the winds encounter the slopes of the central highlands. Here they unload nearly the whole of their cargo of rain, generating heavy outflows of fresh water and organic material from rivers in the south and west; meanwhile, the leeward hill slopes and the eastern, central and northern plains receive little, if any precipitation. During this period, storms may occur at any time of day or night.
October & November: Second Intermonsoonal
This is a season of periodic squalls, particularly during the afternoon or evening, and occasional tropical cyclones. The influence of the Bay of Bengal weather system is critical; when depressions and cyclones occur in that region, the whole of Sri Lanka experiences strong winds and heavy rains.
Oceanic currents mix nutrient-rich water from the depths with more highly oxygenated surface water, giving rise to ‘blooms’ of plankton – the tiny organisms that live at the base of the marine food chain, nourishing and causing the proliferation of all other forms of oceanic life. These currents also transport nutrients and the organisms that feed on them from location to location, creating zones of relative abundance and scarcity. By understanding the action of currents and their influence on the movement of food resources, we gain a better understanding of marine animal behaviour.
Seasonal currents in the northern Indian Ocean carry water back and forth between the Bay of Bengal and the Arabian Sea. From May to September, the Southwest Monsoon Current flows eastward and anticlockwise, while the Northeast Monsoon Current flows westward and clockwise between November and February.
The term ‘sea conditions’ relates to the behaviour of winds and waves. It is measured by the Beaufort Scale, whose units (BS) relate wind speed to observed conditions in the open ocean. These are often very different from conditions near shore, and it can be dangerous to use one as a guide to the other. It is also important to remember that there is usually a time-lag between a change in the wind and the resulting change in sea conditions.
Sea conditions of BS3 and above (more than 6 knots) make commercial whale-and-dolphin watching expeditions impractical and uncomfortable. During the southwest monsoon, sea conditions between BS4 and BS7 affect the region from the Gulf of Mannar, south and east to Panama. The less fierce northeast monsoon creates conditions of, typically, between BS3 and BS5 from Hambantota to Bay of Bengal inlet.
The seas around Sri Lanka are ‘microtidal’, meaning that the difference between high and low tide is less than a metre. There are two high and two low tides every day, their timing and magnitude influenced by the relative alignment of the sun and the moon, tidal patterns in the deep ocean, the shape of the coastline and the depth and topography of the adjacent sea bottom. Every tidal cycle lasts about 12hr 25min, giving a ‘tidal day’ of 24hr 50min – and thus putting back the times of low and high water by fifty minutes every day.
Tidal patterns in the Laccadive Sea indicate that high tide initially sets in Kalpitiya and then moves southward and then around to the Eastern seaboard. As such tide times vary around Sri Lanka. For example, Colombo to Trincomalee is 6 hrs later and Bar Reef to Kalpitiya in Dutch Bay is 1.5 hrs later.