The Tropics, defined as the region between the tropics of Cancer and Capricorn, have a climate dominated by the large scale convection associated with the Inter-Tropical Convergence Zone (the ITCZ). This moves with the seasons, according to which latitude is closest to the sun. At the equinoxes, the sun is closest to the equator, at the December solstice, the sun is over the Tropic of Capricorn and at the June solstice, it is over the Tropic of Cancer. The most rapid ascent of hot air, associated with the formation of towering cumulonimbus clouds, is found at the ITCZ. These are often the source of the heavy rains and violent thunderstorms of the tropics.

Intertropical Convergence Zone. Source: Wikimedia Commons

Tropical climates are normally hot and humid and usually show far less seasonality in temperature than extra-tropical climates. On the other hand, other climate features, such as rainfall and wind patterns, can show pronounced regularity, such as the monsoons.

The most dramatic weather systems found in the tropics are tropical cyclones: called hurricanes in the Atlantic, Caribbean and eastern Pacific, cyclones in the Indian Ocean and typhoons in the western North Pacific. They are low pressure systems, typically 200 to 2,000 km across, with wind speeds greater than 120 km/hour. They consist of deep cumulonimbus clouds, up to 12 km high, spiralling around a central, clear eye where air is descending. They form over warm tropical oceans, but cannot form equatorwards of 5°, as the Coriolis force is too weak. They rapidly decay when they move over land and are cut off from their source of warm water.

The model we are using isn’t great at producing hurricanes, mostly because the grid is too coarse for the relevant processes to operate.

The monsoon is another important feature of the tropical climate, and is a result of land/sea differences and the seasons. Continental land masses cool down and heat up faster than oceans because their thermal heat capacity is lower. This means that, in winter, the air above the continents is colder than the air above the oceans. The same processes which cause the large scale atmospheric circulation then operate, and there is ascent over the oceans, descent over the continents and surface level flow from the continents to the oceans. In the summer the reverse happens. The seasonal reversing winds are called the monsoon (derived from the Arabic word for season, mausim), and most affect the Indian Ocean and western tropical Pacific. The monsoon pattern interacts with the large scale atmospheric circulation and is affected by the orography (the shape of the land surface, for example the Himalayas), which together produces a complicated weather pattern in south-west Asia.