The monsoons are seasonal reversing wind, atmospheric pressure and rainfall systems that dominate insular Southeast Asia and the Indian Ocean generally.
Two systems prevail. Firstly, the wet, warm mid-year summer monsoon which blows from the southwest in the Indian Ocean, then the colder, drier winter monsoon with north-easterly winds. The onset of the summer monsoon can be very blustery, and west-facing harbours in India and Southeast Asia generally close for this season.
According to seabed analysis in the South China Sea, the monsoon has been blowing for over 30 million years, but the onset, intensity and duration vary considerably over time and according to geographic location.
Thunderheads build up in September during the transition period between monsoon seasons at Saparua, near Ambon.
In the Spice Islands, the two seasons are less distinct than further west in Indonesia, because they lie on (the Moluccas) or very near (Ambon and the Bandas) the equator, and intervening landmasses–particularly the island of Borneo–have an impact on wind direction. They are also seasonally influenced by the trade winds, which have less of an impact as you go west.
By April or May each year, surface air in the high-pressure belt of the sub-tropical zone flows towards the low-pressure zone of the equatorial belt, and is spun to the west by the earth’s rotation, producing southeasterlies in the southern hemisphere and northeasterlies in the northern. These wind systems are called the ‘trades’ because they were predictable seasonal systems ideal for the wind-driven sailing ships of old. As they approach the low latitudes, they gain moisture and lose power, entering the area of the often-windless tropical doldrums that sailors hate.
The Spice Islands generally lie in these doldrums and their average monthly windspeed is always less than 10 knots, though gusts can reach 20 at any time through the year.
From the Coral Sea off Australia’s east coast, these south-easterly trade winds howl through the Torres Strait separating Australia from New Guinea and continue on, falling in strength as they pass through the Spice Islands. From April, they blow for a good six or seven months and are called the East Monsoon.
By September, a high-pressure system over Siberia produces cold dry winds that gradually follow the East Asian coastline down from the north, jump the Malayan peninsula and extend right across the Indian Ocean, all the way to Africa. This takes time to progress south, but normally hits Ternate and Ambon in the Spice Islands in December or January. This is known as the northern, or winter monsoon, and it is a beautiful but light sailing breeze.
Between the monsoons, in January and again in June/ July there are unsettled periods of thunderstorms, squalls and troubled weather. It is not the time to be sailing. It can be the time of spectacular thunderstorms, with great columns of cumulonimbus rearing up, accompanied by thunder, lightning and sudden stormy winds.
Ambon gets the bulk of its nearly two metres of annual rainfall from May to August, topping 400 mm per month, while the daily temperature varies from 24 to 29 degrees all year, along with a near-constant 80% humidity. Ternate by comparison gets fairly constant rainfall all year round, totalling a metre and a half, but has generally cooler nights–down to an average of 22.
In an area of plenty of weird and amazing natural phenomena–rumbling volcanos, tectonic instability, incredible natural biodiversity, the teeming seas of the Coral Triangle, not to mention being the only places on the face of the earth bestowed with spices like nutmeg and cloves –the rhythm of the monsoons is just another amazing characteristic of these remote and remarkable Spice Islands.
