El Niño – Return of the ‘Enfant Terrible’

A composite picture showing a heat map of the Earth, highlighting the location of the temperature anomaly in the southern hemisphere, with superimposed on it the face of a young child having a tantrum. Image: NaturPhilosophieEl Niño: A Temperature Anomaly

Still in its early stages, El Niño has the potential to cause extreme and even devastating weather around the World.  According to climate graphs, we have reached a 0.6 value for the ENSO.  It’s a 60% probability.  El Niño is now officially back.

The El Niño effect is under way.  Fishermen who work their trade in the waters of the Pacific Ocean, off the coast of Peru and Ecuador, have known for centuries about El Niño.  Every few years, during the months of December to January, fish in the coastal waters off these countries virtually vanish, causing the fishing business to come to a standstill.

A planisphere showing the pattern of formation of El Niño in the southern hemisphere.‘El Niño’ weather event happens every 5 years on average.  The phenomenon arises from natural variations in local ocean temperatures.

The vast tropical Pacific Ocean receives more sunlight than any other region on Earth.  Much of this sunlight is stored in the ocean in the form of heat

Although El Niño leaves Europe largely unscathed, it wreaks havoc around the Southern Hemisphere.  El Niño causes droughts, floods and has been linked to an increased incidence of war and social unrest.  And yet it is surprisingly hard to predict.

 

El Niño Southern Oscillation (ENSO) – Part I

El Niño comes along about every two to seven years as part of a complex natural cycle between atmosphere and ocean.  Essentially, El Niño involves the warming of the Pacific Ocean.

During an El Niño, the physical relationships between wind, ocean currents, oceanic and atmospheric temperature, and biosphere break down into destructive patterns that are second only to the march of seasons in their impacts on weather conditions around the World.  Predicting the ENSO implies a grounded understanding of its dynamics.

The Pacific trade winds blow from East to West, dragging the warm surface waters westward, where they accumulate into a large, deep pool just east of Indonesia, and northeast of Australia.  At the same time, the deeper, colder waters in the eastern Pacific are allowed to rise to the surface, creating an east-west temperature gradient along the equator, known as the thermocline tilt.

 

The Walker Circulation

Two Walker diagrams illustrating the circulation pattern of the El Niño weather phenomenon.
Walker Circulation Diagrams  Source: Australian Government Bureau of Meteorology

 

Atmospheric circulation on the equatorial plane – the Walker circulation – is closed by ascent of warm moist air over the warm pool (associated with deep convection and precipitation), westerly winds aloft, and subsidence in the high-pressure zone of the eastern Pacific.  In the ocean, westward flow in the surface SEC is in part compensated by a return flow to the east in the thermocline, i.e. the Equatorial Under Current (EUC).

This current flows down the zonal pressure gradient associated with the east-west tilt of the thermocline and provides a source of water for upwelling in the East.

Around Spring  in the Northern Hemisphere, the trade winds tend to lose strength.  Less water is pushed westward and waters in the central and eastern Pacific begin to heat up several degrees and the thermocline tilt diminishes.  But the trade winds are usually replenished by the Asian summer monsoon, and the delicate balance of the thermocline tilt is maintained.

At times, the trade winds do not replenish, or reverse direction to blow from West to East.  When this happens, the ocean responds in a several ways:

  • The warm surface waters from the large, warm pool east of Indonesia, begin to move eastward.
  • The natural spring warming in the central Pacific continues and spreads eastward through Summer and Autumn.
  • Beneath the water surface, the thermocline along the equator flattens as the warm waters at the surface effectively act as a 300-foot-deep cap preventing the colder, deeper waters from upwelling.

 

Warming of Pacific Regions

As a result, the large central and eastern Pacific regions warm up over a period of about 6 months into an El Niño.  On average, waters warm by 3° to 5°F, but in places the waters can peak at more than 10°F higher than normal (up from temperatures in the low 70s Fahrenheit, to the high 80s).

In the eastern Pacific ocean, as temperatures increase, the water expands, causing sea levels to rise anywhere from several inches to as much as a foot.  In the western Pacific, sea level drops as much of the warm surface water flows eastward.

Last March, NOAA scientists announced that El Niño  had arrived, but the weather anomaly was described as “weak”.  However, Australia’s Bureau of Meteorology said models suggested El Niño could gather strength from September and become a ‘substantial’ event, although it was too early to determine how strong it might ultimately be.

 

A planisphere from the National Oceanic and Atmospheric Administration detailing the circulation pattern of the weather phenomenon. The caption reads: "El Niño Arrives in 2015".
Source: NOAA (National Oceanic and Atmospheric Administration)

Never the Same Twice!

Every El Niño weather event is different.  But once it starts models can predict how it might develop over the next 6-9 months with reasonable accuracy.

Predicting El Niño is a difficult feat.  In the tropical Pacific Ocean, scientists operate a network of buoys that measure temperature, currents and winds.  This data, together with information from satellites and meteorological observations, is fed into complex computer models designed to predict an El Niño.

In 2009-2010, the last El Niño was a moderate to strong event.

Other recent El Niño events took place from 2002-2003 (moderate), 2004-2005 (weak), 2006-2007 (weak to moderate).

The models cannot predict the precise intensity or duration of an El Niño.  They cannot predict the areas likely to be affected.  At least, not for more than a few months ahead.

Researchers are still trying to refine their models and observational work to give more advance notice.

Five years ago, a strong El Niño event was linked with poor monsoons in Southeast Asia, droughts in southern Australia, the Philippines and Ecuador, blizzards in the US, heat-waves in Brazil and extreme flooding in Mexico.  Another strong El Niño was expected during last year’s record-breaking temperatures, but it failed to materialise.

The last very strong El Niño was 1997-1998 and was known for providing heavy rainfall in the West, especially the state of California. 

 

The Hottest El Niño?

A graph showing the recovery of the commodity sector performance 12-months post El Niño.
Source: S&P Dow Jones Indices and http://ggweather.com/enso/oni.htm

A strong El Niño event is likely to increase prices of staple foods, such as rice, coffee, sugar, soybeans and cocoa, say scientists.

Current models for El Niño point to an event on a similar scale to that of 2009/2010, and potentially as strong as the 1997/1998 event, which was extreme.

The warming of parts of the ocean is likely to have a knock-on effect worldwide.  Pacific temperatures are 2-3 degrees above normal.  Global temperatures are very high due to the evolution of El Niño.

The consequences of El Niño for Europe and the UK are less clear to establish, although they can lead to “dry” British winters with heavy snowfall.

Catastrophic natural weather events are happening right now as we are experiencing a deadly heat-wave in India, floods in Houston, Oklahoma and Mexico, and drought in California.

The ENSO Recent Evolution, Current Status and Predictions Report prepared by the Climate Prediction Centre/NCEP of NOAA stated on May 26, 2015 that El Niño conditions are present and there is an approximately 90% chance that El Niño conditions will continue through Northern Hemisphere summer 2015, and a greater than 80% chance it will last through 2015.

A photograph showing the effect of the drought on Asia's largest upper lake. A woman is seen crossing a large deserted and completely arid area, while transporting water in a container balanced on her head.In cases like California, where El Niño may bring much wanted rain, the elevated ocean temperatures are welcome.  The consistency in temperature increases has not been sufficient to shift growing seasons, however, so more destructive to crops than helpful, and may indeed disrupt global food markets.

El Niño events are expected to become more extreme as global temperatures rise.  Modelling studies over the last two years have led to further insights on extreme El Niños, and found that their frequency is projected to double under greenhouse warming after 2050.