What are El Nino and La Nina?

El Niño (Spanish for “The Boy”) and its partner La Niña (“The Girl”) are naturally occurring oscillations in the atmosphere and ocean across the equatorial Pacific. Spanish names are used for these globally important phenomena because the first observations of El Niño and La Niña were made on the coast of South America. They cause the sea level to change in the Pacific Ocean by as much as 50cm, and the sea temperature to change by as much as 5°C.

There are many complicated atmospheric and oceanic mechanisms (based on the transfer of heat around the Pacific Ocean) that cause the Pacific to oscillate between El Niño and La Niña. The time taken for a full cycle from El Niño, to La Niña and back again varies from 2 to 7 years, with an average of about 4 years. El Niños are usually strongest early in the Southern Hemisphere summer (January).

During normal conditions, easterly winds blow across the equator.  This pushes warm surface water (heated as it travels from east to west across the Pacific at the equator) towards the Western Pacific, around Papua New Guinea, the Solomon Islands, and the Federated States of Micronesia.  The warm surface water in the western Pacific makes the atmosphere moist and warm.  This causes the air to rise (convection) and drop its moisture as rain.
normal-only
[Source: NOAA Pacific Marine Environmental Laboratory]

During a La Niña event, such as experienced during 2010-2011, the easterly winds across the equator strengthen. This leads to even warmer surface ocean temperatures and to higher atmospheric humidity in the western Pacific.  In turn, this causes increased atmospheric convection and therefore heavier than usual rainfall in the Western Pacific Islands.
nina-only
[Source: NOAA Pacific Marine Environmental Laboratory]

Because the warm surface water is pushed to the West, La Niña also leads to colder sea surface temperatures in the Eastern and Central Pacific, around Tuvalu, Kiribati, Tokelau and Samoa. Colder sea surface temperatures lead to lower atmospheric humidity, less convection and therefore lower rainfall, possibly even drought.

Warmer water in the Western Pacific increases the number of cyclones West of the dateline. This means that countries such as the Solomon Islands, Vanuatu and Fiji are more vulnerable to cyclones during La Niña conditions.

During La Niña

Strengthened easterly tradewinds.

In the Western Pacific

In the Central and Eastern Pacific

Warm Sea Surface Temperature, relatively deep warm pool.

Cool Sea Surface Temperature on the equator

Low Atmospheric Pressure

High Atmospheric Pressure

Heavy Rainfall

Low Rainfall

Higher Sea Level *

Lower Sea Level *

Increased cyclone activity

Decreased cyclone activity

 

During an El Niño event, the easterly winds across the equator weaken. The warm water that usually resides in the Western Pacific moves further east, towards the dateline. Depending on the strength of the El Niño, this warm water can travel all the way across the Pacific to the South American coast. This leads to reduced rainfall in the Western Pacific, and sometimes severe drought in Indonesia and Australia. However, there is increased rainfall in the Central and Eastern Pacific[i].
nino-only
[Source: NOAA Pacific Marine Environmental Laboratory]

Warmer water in the central pacific also increases the number of cyclones that occur east of the dateline. This particularly affects the Cook Islands and French Polynesia.

During El Niño

Weakened easterly tradewinds

In the Western Pacific

In the Central and Eastern Pacific

Shallower warm pool, cooler Sea Surface Temperature

Warmer Sea Surface temperature

High Atmospheric Pressure

Low Atmospheric Pressure

Lower Rainfall

High Rainfall

Lower Sea Level

Higher Sea Level

Decreased cyclone activity

Increased cyclone activity

 

El Niño and La Niña have far reaching effects beyond the borders or the tropical Pacific Ocean. The warm waters in the Western Pacific during a La Niña trickle through the narrow passages north of Australia and south of Indonesia into the Indian Ocean. This affects weather patterns in places like Thailand and Western Australia. Warm waters during strong El Niños reach up the coast of North American and down the coast of South American to cause heavy rainfall on the west coast of the United States of America, in the usually dry deserts of Chile.

Fast winds in the jet-streams that encircle the earth at high altitudes are redirected by the atmospheric component of El Niño and La Niña. This means that these tropical Pacific Ocean processes have an impact on such distant processes as snowfall in the Himalayas and rainfall in Madagascar.

The Pacific Ocean soaks up a vast amount of heat during a La Niña event, and dumps this back into the global atmosphere during an El Niño. This can cause global surface average temperatures to decrease by up to 0.5°C during a La Niña, and we can expect to see global temperatures increase in the near future after the recent event.

 
Indices of La Niña and El Niño.
To easily see what the state of the Pacific Ocean is in, some simple indices have been developed;

The ENSO (El Niño Southern Oscillation) index compares the air pressure in Tahiti (in the Eastern Pacific) to the air pressure in Darwin (Western Pacific).  These two values are easy to observe, and give a quick measure on the strength of the easterly winds across the equator.  To find the most recent ENSO values, have a look at http://www.cgd.ucar.edu/cas/catalog/climind/soi.html .

The difference in ocean temperature from usual across the equatorial pacific is used as another index of El Niño and La Niña. The Niño 3 index monitors the temperature across a box in the Eastern Pacific, while the Niño 4 index uses a box in the central-western Pacific.  The Niño 3.4 index uses a box that overlaps these.

ninoareas c
The areas of the Pacific ocean used to define the Nino indices.  [Source: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/nino_regions.shtml ]

There are also, more complicated ways which look at the way both the ocean and the atmosphere are behaving to describe what state the Pacific ocean is in.  These include the Multi-variate ENSO Index (MEI), see http://www.esrl.noaa.gov/psd/enso/mei/ and the Coupled ENSO Index (CEI).


What State is the Pacific Ocean in Now?
A lot of effort has gone into observing the ocean and atmosphere around the Pacific.  This is done with moored buoys (the TAO / TRITON array across the eqautor), satellites (to look at sea surface temperature, sea surface height and winds) and the Argo array (for a wider view of subsurface ocean temperature and salinity).

Here are some links to see what the current data is suggesting.

http://www.esrl.noaa.gov/psd/enso/enso.current.html

http://www.esrl.noaa.gov/psd/enso/mei/
http://www.esrl.noaa.gov/psd/map/images/sst/sst.anom.gif

What is Forecast to happen in the next few months?
Predicting what the rainfall and temperature will be like in a few months is crucial for effectively managing our resources.  Some examples are;
  • What crops should we plant?
  • Will a colder ocean bring more fish?
  • Should we top up our water reservoirs so we have water to drink?
  • Should we empty our reservoirs so they can cope with heavy rain?
  • Will forest fires be more likely this summer?
  • Will cyclones be more likely this summer?
  • etc.
The Island Climate Update gives an idea of what is expected in the Pacific.  This information is based on many sophisticated computer models that simulate what the earth's atmosphere and ocean will do in the next few months.


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