Causes of Droughts
- Droughts are an extended period of time when there is below average rainfall
- There are three types of droughts:
- Meteorological
- Hydrological
- Agricultural
- Meteorological drought occurs when there is rainfall deficit and the length of the dry period, it is caused by:
- Changes in atmospheric circulation leading to:
- A lack of precipitation as a result of short-term changes or longer-term trends
- The lack of rainfall is often combined with high temperatures which increases evaporation
- Changes in atmospheric circulation leading to:
- Agricultural drought is when there is a water deficiency in the soil, which leads to:
- Crop failure
- Reduced biomass
- Hydrological drought occurs when there is a lack of water stored on the surface and underground in lakes, rivers, reservoirs and aquifers
El Niño Southern Oscillation (ENSO)
- The ENSO cycle is the movement of a warm water mass in the equatorial Pacific Ocean
- It occurs due to the changes in the trade winds, atmospheric circulation and ocean currents
- There are two phases:
- El Niño
- La Niña
- El Niño phases occur more frequently than La Niña
El Niño
- An El Niño occurs when the sea temperatures are 0.5°C above average
- This leads to warmer than average weather in the eastern Pacific
- The peak is usually in December
- It occurs every 2-7 years
- The temperature of the ocean off the coast of Peru rises an average of 6-8°C, causing thermal expansion and sea level rise
- Water off the coast of Australia and Indonesia is cooler and precipitation is reduced leading to droughts in Australia
Causes of El Niño
- In a non-El Niño year there is descending air over the eastern Pacific and rising warm moist air over the western Pacific
- In an El Niño year the trade winds are reduced or reverse (going west to east) leading to a reversal of the conditions and rising air over the eastern Pacific and descending air over the western Pacific
El Niño
La Niña
- La Niña occurs when sea temperatures fall below average
- This brings cooler and drier than average weather in the eastern Pacific
- La Niña occurs every 3-5 years
Causes of La Niña
- Stronger than usual eastward trade winds and ocean currents
- These bring cold water to the surface through upwelling
- Both El Niño and La Niña impact the global climate due to the changes they cause in the jet stream
- El Niño has the most significant impacts with:
- Increased rainfall and flooding in South America, Africa and the south of the US
- Drought in Australia and Southeast Asia
- Has been linked to a higher risk of colder winters in the UK
- La Niña leads to:
- Increased rainfall in Australia and Southeast Asia
- Drought conditions in the south of the US
- Increases the risk of tropical storms in the Atlantic