Causes of Tropical Storms and El Niño/La Niña

Warm oceans provide the energy for tropical storms

A tropical storm is an organised, rotating low-pressure system with strong winds and heavy rainfall. Hurricanes, typhoons and tropical cyclones are regional names for these systems. Their energy comes from warm ocean water rather than from the rotation itself.

Strong solar heating warms tropical oceans. Tropical storms generally develop where sea temperatures are about 27°C or above, with warm water extending below the surface so that the storm has a continuing energy supply. Evaporation transfers water vapour and energy from the ocean into the air above it.

Warm, moist air rises, helping to create low pressure near the surface. Surrounding air flows towards this lower-pressure area, bringing in more moisture. In the tropics, converging trade winds near the Intertropical Convergence Zone (ITCZ) encourage air to rise. This rising air is part of the Hadley-cell circulation and produces thunderstorms, some of which can develop into an organised tropical storm when conditions are suitable.

Rising air produces torrential rain and strengthens the storm

As moist air rises, it cools. Water vapour condenses into droplets, forming towering cumulonimbus clouds. These deep storm clouds produce heavy rainfall.

Condensation also releases heat into the surrounding air. This heating supports further uplift and helps maintain the low-pressure system. More warm, moist air is drawn in from the sea surface, supplying further energy and moisture. This is a reinforcing process: the storm draws on the ocean's warmth to maintain vigorous uplift, rainfall and winds.

The difference in pressure between the storm's very low-pressure centre and the surrounding atmosphere drives air inwards. A steep pressure gradient—a large pressure change over a short distance—produces particularly strong winds.

Rotation and storm structure explain the weather extremes

Earth's rotation produces the Coriolis effect, which deflects moving air and helps the inward-flowing winds spiral around the low-pressure centre. Tropical storms rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The effect is too weak at the equator for storms to develop their organised rotation there; they usually form roughly between 5° and 30° north or south.

Storms also need low wind shear: winds must not change too greatly in speed or direction with height. Strong wind shear can disrupt the developing system rather than allowing its storm clouds to remain organised above the surface circulation.

The strongest winds and heaviest rainfall occur in the eyewall, a ring of deep clouds and rapidly rising air around the centre. Inside it is the eye, where air sinks, limiting cloud formation. The eye is relatively calm and often dry, despite lying at the centre of an intense low-pressure system. Conditions therefore change sharply as the eyewall passes over a place.

When a storm moves over land or cooler water, its supply of heat and moisture is reduced. The reinforcing process weakens, so the storm eventually loses strength.

ENSO links the Pacific Ocean to changing rainfall

Tropical storms concentrate energy into an individual weather system. El Niño and La Niña involve changes across a much larger area: the equatorial Pacific Ocean and the atmosphere above it. Together with neutral conditions, they form the El Niño Southern Oscillation (ENSO).

ENSO is a natural, irregular cycle. It involves interactions between sea temperatures, ocean currents, winds and atmospheric pressure. These changes shift where moist air rises and rain falls, and influence atmospheric circulation far beyond the Pacific.

To understand the two phases, first picture neutral conditions. Easterly trade winds blow from east to west, pushing warm surface water towards Indonesia and northern Australia in the western Pacific. Warm, moist air rises there, producing cloud and rain. Near South America in the eastern Pacific, colder water rises towards the surface through upwelling. Air tends to descend over this cooler eastern region, giving relatively drier conditions.

El Niño shifts rainfall away from the western Pacific

During El Niño, the easterly trade winds weaken and may sometimes reverse. Warm surface water spreads eastwards, upwelling is reduced, and the central and eastern equatorial Pacific become warmer than usual. Ocean warming and changes in winds can reinforce one another, allowing the event to develop.

The main zone of rising moist air and rainfall shifts eastwards. The western Pacific, including Indonesia and Australia, can receive less rainfall than usual as rising air and rain-producing clouds become less frequent there. If this rainfall shortage persists, drought can develop. Parts of the eastern tropical Pacific become wetter instead: the rainfall has shifted, not simply disappeared.

La Niña strengthens the usual contrast

During La Niña, easterly trade winds are stronger than usual. They push more warm surface water westwards and strengthen cold-water upwelling in the eastern Pacific. The central and eastern equatorial Pacific become cooler than average.

Rising moist air and rainfall become more concentrated in the western Pacific, so Indonesia and Australia often become wetter. Rainfall decreases over the central tropical Pacific, making it relatively drier.

Changes in Pacific heating also alter wider atmospheric circulation. La Niña can increase the risk of drought in the southern United States, where changed circulation patterns can reduce rainfall. This shows why drought linked to ENSO is not confined to the shores of the Pacific.

A rainfall shortage becomes drought when it persists

Drought means rainfall stays below the normal amount for a place for an extended period. A naturally dry region is not necessarily experiencing drought; the comparison is with its usual rainfall.

ENSO can increase drought risk by shifting rain-producing circulation away from a region. During a prolonged rainfall shortage, rivers, reservoirs and groundwater receive less replenishment while water use continues. High temperatures, where they occur, can increase evaporation and make shortages worse.

Neither ENSO phase guarantees drought in a particular country. Effects vary with season, location and the strength of the event, and other weather influences also matter. The key connection is changing ocean temperatures and winds → changing atmospheric circulation → reduced rainfall in some regions → possible drought.

Tropical storm causes

• Warm tropical ocean, about 27°C or above, supplies heat and moisture through evaporation.
• Moist air rises, cools and condenses: cumulonimbus clouds produce heavy rain.
• Condensation releases heat, supporting further uplift and drawing in more moist air.
• A steep pressure gradient drives strong winds; the Coriolis effect produces rotation.
• Usually form roughly 5°–30° north or south, with low wind shear.
• Eyewall: strongest winds and heaviest rain. sinking air, relatively calm and dry.