5.4A Causes of drought

• 3 Types of drought:

  • Meteorological 
  • Hydrological
  • Agricultural 
• Meterological 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
• Agricultural drought = when there is a water deficiency in the soil, which leads to:

  -> Crop failure

  -> Reduced biomass
• Hydrological drought = when there is a lack of water stored on the surface and underground in lakes, rivers, reservoirs and aquifers

  
• El nino Southern oscilaition (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:

  • 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

  
• 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