Changing Distribution of Drought and Tropical Cyclones

Weather hazards change in both place and time

The distribution of a weather hazard describes where it occurs. Its timing matters too: a region may face a much greater risk in one season than another. Changes over time are temporal changes; changes in location or the area affected are spatial changes.

There are two important timescales to distinguish. Seasonal changes repeat through the year, such as a tropical wet season followed by a dry season. Longer-term changes alter patterns over years or decades, such as a region experiencing more severe drought as temperatures rise. A seasonal pattern and a longer-term trend can operate together.

Why tropical rainfall moves with the seasons

Tropical temperatures often vary less through the year than temperatures in the middle latitudes. However, rainfall can vary greatly, creating distinct wet and dry seasons.

Near the Equator, the trade winds meet in the Intertropical Convergence Zone (ITCZ), also called the tropical rain belt. Warm, moist air rises, cools and condenses, forming clouds and heavy rain. The belt shifts towards the region receiving the strongest solar heating: generally northwards during Northern Hemisphere summer and southwards during Southern Hemisphere summer.

A tropical location can therefore receive heavy rain when the rain belt is nearby, then become much drier when it moves away. Where sinking air dominates, cloud formation and rainfall are suppressed. This explains why rainfall patterns change across tropical regions during the year rather than everywhere having a wet season at the same time.

Seasonal wind changes also produce monsoons. A monsoon is a large-scale seasonal wind pattern, not a single storm. In affected regions, the summer monsoon brings moist air and substantial rainfall, while the winter monsoon brings much drier conditions. The arrival, duration and strength of the rainy season are therefore important to water supplies and farming.

When a dry season becomes drought

Drought is a shortage of water that builds up over time. It can last from weeks to several years. A normal dry season is not automatically a drought: rainfall and water availability must be considered against what is usual for that region.

If seasonal rains arrive late, end early or are unusually weak, they may not replace the water lost during the dry season. Soil moisture declines, crops have less water available and rivers or reservoirs may fall. If poor rainy seasons follow one another, the shortage can become prolonged.

Different measures reveal different parts of this process:

• Meteorological drought: rainfall is below the regional average.
• Agricultural drought: insufficient rainfall or soil moisture affects farming and crop growth.
• Hydrological drought: water levels in rivers, streams or reservoirs are low.

These do not necessarily begin or end together. Rain may return before depleted water stores recover. Equally, low reservoir levels can reflect water use as well as rainfall shortages.

Drought also connects with desertification, the degradation of land in dry regions. Prolonged water shortages can reduce vegetation and make land more vulnerable to degradation. However, drought is a climatic water shortage, whereas desertification is a change in the condition of the land; they are not interchangeable terms.

Longer-term changes in tropical drought

Climate change can alter both rainfall patterns and the rate at which water is lost. Some regions may become wetter while others become drier, so there is no single drought trend for the whole of the tropics.

Higher temperatures can increase evaporation from the ground. This can intensify soil drying even where the change in rainfall is small. As soil becomes drier, the air above it may heat further, increasing evaporation and worsening the shortage.

There is evidence of reduced precipitation across significant parts of Africa and South America. Agricultural and ecological drought have also increased in many regions, including much of Africa and eastern Brazil. Ecological drought means that insufficient water affects the natural environment, not just farming.

Future projections suggest that the area affected by drought will expand as global temperatures rise. Under high greenhouse gas emissions, affected regions could extend across Central America and the Caribbean, most of South America and most of Africa. These are projections, not a claim that all these places already experience the same trend.

The overall conclusion is that warming can increase drought risk, but its effects vary by location and by the type of drought measured. Rainfall, temperature, land use and water demand all matter. It is therefore harder to attribute changing river or reservoir levels to climate change alone than simply to recognise that a water shortage exists.

Where tropical cyclones occur

Tropical cyclones are intense, rotating low-pressure systems that develop over warm tropical seas. They generally form between about 5° and 30° north and south of the Equator. Warm ocean water supplies energy, while the Coriolis effect helps the storm rotate. Very close to the Equator, this effect is too weak for tropical cyclones normally to form.

The names differ by region, but the basic weather system is the same:

Cyclone activity is not evenly distributed between these regions. Atlantic storms, including those affecting the Caribbean and USA, usually account for less than 15% of global tropical cyclone activity.

Why cyclone seasons differ

Cyclones are seasonal because the conditions needed for their development, including warm seas, vary through the year. The broad Northern Hemisphere season is June–November, peaking in September. The broad Southern Hemisphere season is November–April. However, individual ocean basins have important differences:

These are periods of typical activity, not guarantees that a storm will occur. The north-west Pacific is especially important because tropical cyclones can occur there throughout the year.

How cyclone tracks change the areas at risk

Tropical cyclones initially tend to move westwards and slightly towards the poles, carried by tropical easterly winds. Many eventually reach the middle latitudes, where westerly winds can turn their tracks eastwards. This produces a curved path rather than a straight route towards the Equator.

A storm may therefore form over an ocean far from the coast it eventually affects. Its track determines whether it remains at sea, makes landfall or threatens several coastal regions in succession. More than one storm can be active in the same ocean basin at once.

When interpreting a cyclone-track map, first locate the latitude bands where tracks begin. Then follow any direction arrows to identify westward movement and poleward curvature. Compare the concentration of tracks between ocean basins and check the dates covered by the map. A map combining many years shows a broad pattern of past activity; it does not predict the route of the next storm.

Changes in time and place

• Temporal: seasonal changes or longer-term trends.
• Spatial: changes in location or area affected.

Tropical drought

• ITCZ shifts north and south seasonally → tropical wet and dry seasons.
• Monsoon wind changes produce contrasting rainy and dry periods.
• Late, weak or shortened rains can cause drought; repeated poor rainy seasons can prolong it.
• Meteorological drought concerns rainfall; agricultural drought concerns crop water availability; hydrological drought concerns water stores and flows.
• Warming increases evaporation; rainfall changes vary regionally. Evidence includes increasing agricultural/ecological drought in much of Africa and eastern Brazil.
• Drought can contribute to desertification, but water shortage and land degradation are different processes.