Geography Case Studies

Saturday, May 13, 2006

El Nino Southern Oscillation (ENSO)

Topic: Weather & Climate; ENSO

MD

The oceans are a major influence on world climate because of their large heat storage capacity. Changes in ocean temperatures therefore affect air masses greatly. An example of how this happens is the El Nino Southern Oscillation.

Normal Conditions
Under normal atmospheric conditions, pressure rises over the eastern Pacific Ocean (off the west coast of South America) and falls over the western Pacific towards Indonesia and the Philippines. Descending air over the eastern Pacific gives dry, clear conditions – the Atacama Desert is extremely arid. The warm, moist air rising over the western Pacific causes heavy convectional rainfall.

This movement of air creates a circulation cell called the Walker cell, in which the upper air moves from west to east and the surface air moves from east to west (forming the trade winds). These trade winds push surface water westwards so that sea level in the Philippines is 60cm higher than at the coast of Panama. They also promote the east-west equatorial current, which carries surface water towards Indonesia whilst it heats. This leads to ocean temperatures in the western Pacific reaching about 28*C. This warm water is pushed away from the eastern Pacific and is replaced by an upwelling of cold water (which is nutrient-rich). This is below 20*C, and provides a rich plankton supply that supports Peru’s fishing industry. Therefore a moderate temperature gradient exists across the Pacific.

El Nino Event
El Nino events occur every 3-4 years, appearing just after Christmas and usually lasting 12-18 months. In an El Nino event, pressure and precipitation trends reverse in the equatorial Pacific; and winds/atmospheric circulation and ocean currents either fail or reverse also. Pressure rises over the western Pacific, and the descending air over Indonesia and SE Asia causes drier conditions than normal, sometimes drought and forest fires. Pressure falls over the eastern Pacific, causing much wetter conditions in places (such as Peru) which usually experience desert conditions.

The inter-tropical convergence zone over the equatorial Pacific migrates southwards, which causes the trade winds blowing from east to west to weaken, and often reverse. The reversal of the trade winds means that warm surface water is pushed eastwards, so sea level falls in the western Pacific and rises in the eastern Pacific. The 28*C band of warm water extends much further east than normal, causing the eastern Pacific waters to become up to 6*C warmer than under normal conditions. The cold upwelling also reduces or fails altogether. The resulting warmer water in the eastern Pacific contains less oxygen (and lacks nutrients from the cold upwelling), meaning plankton populations die and Peru's fishing industry is damaged.

As well as drier conditions in SE Asia and Australasia and wetter conditions on the west coast of South America, El Nino has other, global effects:

  • It causes severe droughts in the Sahel, southern Africa and the Indian subcontinent.
  • It causes extremely cold winters in central North America.
  • It causes stormy conditions with floods in California.
  • It causes exceptionally wet, mild, windy winters in the UK and NW Europe.
  • It is believed to suppress the hurricane season in the Caribbean.
The 1997-98 El Nino event was the largest on record. Examples of its effects can be found in Peru and Kenya.
PERU - The equivalent of 6 months of normal rainfall was received every day for 12 days in early March, causing flash flooding (which killed 300 and destroyed 13,000 homes), destroying infrastructure, ruining crops and disrupting schooling.
KENYA - During what is usually the six-month 'dry season' Kenya received over 1000mm of rainfall - 50 times the usual. Roads and railways were destroyed, causing the derailment of the Nairobi-Mombasa train, a severe accident. Over 500 people died from malaria as receding floodwaters left mosquito-spawning stagnant pools.

Evidence collected by meteorologists to identify the start of an El Nino event include:
  • Weakening of the Walker circulation and trade winds along the equatorial Pacific.
  • Cooling of the western tropical Pacific.
  • The waters off the western South American coast warm by 2-8*C.
  • Increased cloudiness over the central equatorial Pacific.

La Nina Event
La Nina is the opposite to El Nino, but doesn't always accompany it. It occurs less frequently than El Nino (the last La Nina event was 1988-89), meaning its effects are less well understood and harder to predict (there is less evidence). When La Nina does arrive, it occurs either just before or just after El Nino.

The region of low pressure over the western Pacific becomes even lower, and the region of high pressure over the eastern Pacific becomes even higher. Rainfall increases in SE Asia and Indonesia (in 1988-9, Bangladesh experienced extreme flooding), causing storms and floods. In South America drought arises, and ocean temperatures can fall by 8*C as the cold upwelling strengthens and warm surface water in the Pacific is restricted to the extreme west.

La Nina strengthens the trade winds which push warm surface water further west, and causes higher than normal sea levels in the western Pacific. The temperature gradient across the Pacific Ocean is strengthened, aided by the stronger equatorial undercurrent and increased cold upwelling to the east. There is a suggested link between La Nina events and increased hurricane activity in the Caribbean. La Nina was predicted in 1998 after El Nino ended and ocean temperatures off the coast of Peru fell by 8*C. In October 1998, hurricane Mitch became the most destructive storm for 200 years. There is also a suggested link between La Nina and disruption of the jet streams which results in more cold and stormy weather in the UK.

Visit http://www.bom.gov.au/lam/Students_Teachers/elnanim/elani.shtml for an animation summarising the ENSO.

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