5.4A - The Causes of Drought
The causes of drought, both meteorological (short term precipitation deficit, longer trends, ENSO cycles) and hydrological.
The Four Types of Drought (Meteorological and Hydrological most important)
Drought is defined in meteorological terms as a shortfall or deficiency of water over an extended period, usually at least a season.
This differs from hydrological drought, where there is reduced stream flow, lowered groundwater levels and reduced water stores.
Agricultural drought is when agricultural activity is greatly impacted by drought.
This can lead to food shortages, famine and starvation (socio-economic drought)
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Causes of Meteorological Drought
The physical causes of drought are only partially understood. They lie somewhere in the complex interactions between atmosphere, oceans, cryosphere, biosphere and the land, which produces the climates of the globe.
Droughts can range from short-term and localised precipitation deficits to longer-term trends that are part of climate change.
Research suggests that sea surface temperature anomalies are an important causal factor in short-term precipitation deficits.
Droughts can range from short-term and localised precipitation deficits to longer-term trends that are part of climate change.
Research suggests that sea surface temperature anomalies are an important causal factor in short-term precipitation deficits.
El Niño Southern Oscillation (ENSO)
Temperature anomalies provide the key to ENSO, which, in turn, is thought to trigger the occurrence of droughts.
Normal conditions in the Pacific Basin
At the same time, the area of warmer water further west, near Australia and Indonesia, is replaced by cooler water.
El Niño events usually occur every 3-7 years, and usually last for 18 months. El Niño events seem to trigger very dry conditions throughout the world, usually in the second year. For example, the monsoon rains in India and South East Asia often fail.
La Niña
La Niña events may sometimes, but not always, follow an El Niño event. They involve the build-up of cooler-than-usual subsurface water in the tropical part of the Pacific. This situation can lead to severe drought conditions, particularly on the western coast of South America.
Very strong air circulation and very warm water moving east-west.
Normal conditions in the Pacific Basin
- In a non-El niño year, the trade winds blow from east to west along the equator
- The air pushes the warm water westerwards
- Thermocline, upwelling,
- Warm, moist air rises, cools and condenses, forming rain clouds
- The trade wind pattern is disrupted - it may slacken or even reverse and this has a knock-on effect on the ocean currents
- Air circulation loop reversed
At the same time, the area of warmer water further west, near Australia and Indonesia, is replaced by cooler water.
El Niño events usually occur every 3-7 years, and usually last for 18 months. El Niño events seem to trigger very dry conditions throughout the world, usually in the second year. For example, the monsoon rains in India and South East Asia often fail.
La Niña
La Niña events may sometimes, but not always, follow an El Niño event. They involve the build-up of cooler-than-usual subsurface water in the tropical part of the Pacific. This situation can lead to severe drought conditions, particularly on the western coast of South America.
Very strong air circulation and very warm water moving east-west.