Global Distribution and the Role of Atmospheric Circulation

Where tropical cyclones begin

Tropical cyclones do not form evenly across the world's oceans. Their source areas are mainly warm tropical waters between approximately 5° and 30° north and south of the equator. A source area is where a storm develops; its track is the route it subsequently follows. A cyclone can therefore affect places well beyond its source area.

These storms have different regional names but develop through the same basic processes. They are called hurricanes in the Atlantic and north-east Pacific, typhoons in the north-west Pacific, and generally cyclones in the Indian Ocean and South Pacific.

The Atlantic includes storms that can affect the Caribbean and USA, but it accounts for less than 15% of global tropical cyclone activity. Pacific cyclones can affect Mexico, south-east Asia, north-east Australia and South Pacific islands. Indian Ocean cyclones can affect India, Bangladesh, north-west Australia, parts of east Africa, Madagascar and Mauritius. Tropical cyclones in the South Atlantic are very rare: being within tropical latitudes does not by itself guarantee suitable conditions.

How atmospheric circulation creates favourable source areas

Global atmospheric circulation produces a broad belt of low pressure near the equator. Here, warm air rises. At the surface, the north-east trade winds approach this belt from the northern hemisphere and the south-east trade winds approach from the southern hemisphere. Where these winds converge, air is forced upwards.

Over a warm tropical ocean, evaporation supplies the air with water vapour. Sea-surface temperatures normally need to be around 27°C or higher to provide a plentiful supply of warm, moist air. Rising air cools, allowing water vapour to condense into clouds and produce thunderstorms. Condensation releases heat, helping the air continue rising.

Most tropical thunderstorms do not become cyclones. However, clusters of thunderstorms can become organised around a developing surface depression. More air flows towards the low-pressure centre, supplying further warm, moist air. This connects the tropical low-pressure belt and converging trade winds to cyclone formation: they provide a setting in which rising air and thunderstorms can develop.

Two other conditions matter. Low wind shear allows the storm clouds to grow upwards without being strongly disrupted by changes in wind with height. The developing system must also be sufficiently far from the equator for the Coriolis effect to provide rotation. This effect is too weak at the equator, which explains why the warmest equatorial waters are not automatically cyclone source areas.

Why cyclone occurrence is seasonal

The requirement for warm ocean water helps explain why tropical cyclones are concentrated in the warmer part of the year. Conditions are favourable for longer in some ocean basins than in others, so there is no single worldwide cyclone season.

In the northern hemisphere, most tropical cyclones occur from June to November, with a peak in September. In the southern hemisphere, the main season is usually November to April. These contrasting seasons reflect the opposite timing of the hemispheres' warmer months. They are broad patterns rather than absolute limits.

The main ocean-basin seasons show the regional differences:

The north-west Pacific is an important exception to a simple summer-only pattern: occasional typhoons occur outside its main season. The broad southern-hemisphere season also does not mean storms cannot occur in May; the basin seasons extend beyond the general November–April pattern.

How circulation steers cyclone tracks

The atmosphere influences both where cyclones begin and where they travel. Tropical cyclones initially tend to move westwards and slightly towards the poles, steered by tropical easterly winds. “Easterly” means the wind comes from the east, so it carries storms towards the west.

Many storms then move far enough polewards to enter the middle latitudes, where westerly winds dominate. These winds can turn their tracks eastwards. A northern-hemisphere storm may therefore travel westwards, curve northwards and then turn north-eastwards. In the southern hemisphere, a comparable track curves southwards and then towards the south-east. These are common patterns, not fixed routes followed by every storm.

As a cyclone moves polewards, its forward movement can become faster. This movement of the whole system is different from the winds spinning around its centre.

When interpreting a cyclone-track map, first identify where tracks begin and their approximate latitudes. Then follow individual tracks to see their direction and whether they curve polewards and eastwards. Use dates or the map's key to compare seasons. A map combining many years of tracks shows the overall distribution, but does not mean that all those storms occurred together or that every storm followed the same route.