Distribution and Frequency of Tropical Storms and Droughts

Where, how often and how severe?

Two places can experience the same hazard but have very different patterns of risk. Distribution describes where a hazard occurs; frequency describes how often it occurs within a particular period. Neither tells us its intensity or how long it lasts.

For example, a region could experience fewer tropical storms in a year but still suffer a particularly powerful storm. A drought could become longer or more severe without there being more separate drought events. These distinctions matter when assessing whether hazards have changed over time.

Tropical storms follow warm ocean belts

Tropical storms are rotating, intense low-pressure systems with strong winds and heavy rainfall. They develop over warm tropical oceans, generally between about 5° and 30° north and south of the equator. Formation is particularly common between about 5° and 15°.

Warm seawater supplies the heat and moisture needed by a storm. Sea temperatures of around 27°C or above favour development. However, tropical storms do not normally form directly over the equator: the Coriolis effect, which helps air rotate, is too weak there. This explains why their distribution forms belts on either side of the equator rather than a single belt along it.

The same broad type of weather system has different regional names:

• Hurricanes occur in the Atlantic, including the Caribbean Sea and Gulf of Mexico, and in the north-east Pacific off Mexico.
• Typhoons occur in the north-west Pacific, including the South China Sea.
• Cyclones occur in the Indian Ocean, including the Bay of Bengal, and around northern Australia.

These names describe regional differences in terminology, not three unrelated hazards. The important geographical pattern is their concentration over warm tropical oceans and the surrounding coastal regions.

Tropical storm frequency varies by ocean and season

On average, more than 80 tropical storms form worldwide each year, but they are not evenly distributed. The Pacific experiences the largest number, followed by the Indian Ocean and then the Atlantic. The north-west Pacific is a particularly active storm region.

Storm activity is seasonal because oceans become warm enough to support development at different times of year. Northern hemisphere activity is generally concentrated between June and November, while southern hemisphere activity is generally concentrated between November and April. Exact seasons vary between ocean basins.

These are broad seasonal patterns, not guarantees that storms only occur within those months. Likewise, an annual average does not mean that the same number forms every year. Conditions vary between years, producing more or less active seasons.

Have tropical storms changed over time?

The broad distribution remains linked to warm tropical oceans, but storm activity varies between regions and years. An increase in one ocean basin does not necessarily mean an increase worldwide.

There is no clear detected trend in the overall global frequency of tropical storms. It would therefore be misleading to conclude simply that global warming has already made tropical storms more frequent everywhere.

Future projections also distinguish the number of storms from their strength. The global number may decrease, while storm intensity and associated rainfall may increase. Warmer oceans provide energy for storms, but temperature is not the only factor controlling whether they develop.

Ocean warming may also allow suitable conditions to extend to higher latitudes. This is a possible future change in distribution, not evidence that every tropical storm region has already shifted polewards. A careful judgement separates present observations from possible future changes.

Drought is a shortage relative to a place’s normal conditions

Drought develops when rainfall remains below the usual level for a region over an extended period. It can build over weeks, months or years. The UK drought of 1975–1976 lasted about 16 months, showing that drought is not restricted to permanently dry countries.

A desert’s normally low rainfall is not automatically a drought. Drought means a shortage compared with the conditions normally expected in that place. Consequently, both relatively wet and relatively dry regions can experience it.

Rainfall shortages can first dry the soil and later reduce river flows and reservoir levels. This helps explain why different measures of drought do not always show the same pattern:

• Meteorological drought concerns below-average rainfall.
• Agricultural drought concerns insufficient soil water for crops.
• Hydrological drought concerns low water supplies in rivers, reservoirs and other stores.

Drought can also affect ecosystems: this is called ecological drought. High temperatures can worsen water shortages by increasing evaporation, even where the rainfall change is relatively small.

Drought has a widespread but uneven distribution

Drought-prone areas include Australia, the Middle East, central and southern Africa, parts of North America and eastern South America. Regions around desert margins are especially vulnerable to prolonged water shortages, but drought also occurs elsewhere, including Europe and the UK.

Unlike tropical storms, drought is not confined to a narrow belt over warm oceans. Its distribution depends on rainfall patterns and water availability. Its frequency also varies between regions, so there is no single annual count that usefully describes drought everywhere.

Drought changes differ between regions

Observed changes are not uniform. Agricultural and ecological droughts have increased across many regions, including much of Africa, western North America, the Mediterranean and central western Europe, southern Australia and eastern Brazil. Hydrological drought has also increased in the Mediterranean, with evidence of increases in western Africa, East Asia and southern Australia.

These patterns do not mean that every region, or every type of drought, has increased. River and reservoir levels are also affected by water use and land changes, making their trends harder to explain using rainfall alone.

With further warming, the area affected by drought is projected to expand, and several regions are expected to experience more frequent and severe droughts. Changing rainfall patterns can reduce water inputs, while higher temperatures increase evaporation and dry soils further.

For the UK, projections indicate hotter, drier summers, with stronger summer drying in southern areas, alongside wetter winters. Wetter winters do not automatically prevent summer drought: when rain falls, how it is stored and how much water is lost or used all matter. The overall conclusion is therefore regional and qualified: drought risk is increasing in many places, but not in the same way everywhere.

Tropical storms: distribution and frequency

• Develop over warm tropical oceans, usually 5°–30° north and south; not normally at the equator.
• Sea temperatures around 27°C or above favour formation.
• Hurricanes: Atlantic and north-east Pacific. Typhoons: north-west Pacific. Cyclones: Indian Ocean and northern Australia.
• More than 80 per year worldwide on average; Pacific has the most, followed by Indian Ocean and Atlantic.
• Broad seasons: June–November in the north; November–April in the south.

Tropical storms: change over time

• No clear detected trend in overall global frequency; regional and annual variation occur.
• Future global frequency may decrease, while intensity and rainfall may increase.
• Suitable conditions may extend to higher latitudes as oceans warm.

Drought: distribution and change

• Below-average rainfall relative to a region’s normal conditions; not simply a naturally dry climate.
• Risk areas include Australia, the Middle East, central/southern Africa, parts of North America and eastern South America.