Introduction

Impacts of the ENSO

History of ENSO Research

Understanding the
Mechanisms of ENSO
Atmosphere
Oceans

Viewing ENSO

Numerical Models
and ENSO Predicition

Teaching and Learning
Resouces

About This Module

Understanding the Mechanisms of ENSO

 

  • Low pressure over Indonesia (large pressure gradient)
  • Strong trades
  • Weak counter current
  • Upwelling off Peru (and California)- cold nutrient-rich waters
  • Warm water to west- cooler water to east- Peru high productivity
  • Storms concentrated in the west over warm water
  • In normal, non-El Nino conditions, the trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at Indonesia than at Ecuador.
  • The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels.
  • This cold water is nutrient-rich, supporting high levels of primary productivity, diverse marine ecosystems, and major fisheries.
  • Rainfall is found in rising air over the warmest water, and the east Pacific is relatively dry.

 

  • SO (Southern Oscillation)- decrease in the pressure gradient across the southern equatorial Pacific
  • Trades weaken
  • Countercurrent strengthens- warm water across the equatorial region
  • Decreased upwelling-warm low nutrient waters off Peru
  • Storm pattern shifts toward the east
  • During El Nino, the trade winds relax in the central and western Pacific leading to a depression of the thermocline in the eastern Pacific, and an elevation of the thermocline in the west.
  • The observations at 110W show, for example, that during 1982-1983, the 17-degree isotherm dropped to about 150m depth.
  • This reduced the efficiency of upwelling to cool the suface and cut off the supply of nutrient rich thermocline water to the euphotic zone.
  • The result was a rise in sea surface temperature and a drastic decline in primary productivity, the latter of which adversely affected higher trophic levels of the food chain, including commercial fisheries in this region.
  • Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia.
  • The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in regions far removed from the tropical Pacific.

An animation of sea surface temperature (SST) anomaly conditions during the current El Niño are available courtesy of NOAA's Climate Diagnostics Center.

The ENSO is a combination of interrelated oceanic and atmospheric processes that occur every two to seven years. The Southern Oscillation refers to the flip-flop of atmospheric pressure between the eastern and western halves of the Equatorial Pacific. It is closely related to El Nino events. Although the Southern Oscillation and the El Nino are closely related, an El Nino may occur independently or at the same time as a Southern Oscillation. When the two coincide, however, the result is an extreme global atmospheric and oceanographic event.

Under normal conditions, the atmosphere of the Eastern South Pacific is dominated by an eastern center of high pressure, while a zone of lower pressure prevails to the west. The resulting pressure difference causes the trade winds to blow east to west. The Southern Oscillation Index (SOI) is calculated by subtracting the sea-level pressure (SLP) in the west from that in the east. During an El Nino event, the SOI will have a negative value. When the SOI is negative, the trade winds may relax or sometimes even reverse. This is what happens during an El Nino. Since ocean currents are greatly influenced by the winds blowing above them, this ease of the trade winds also affects surface ocean currents, particularly the Peruvian Current.

The Peruvian Current is normally a cold current that moves northward along the coast of South America that causes an upwelling of cold, nutrient and oxygen-rich water that is conducive to dense concentrations of marine life. During an El Nino, however, the relaxation of the trade winds may allow warm water to appear at the surface of South America. This warm water not only kills off marine life, but also affects the atmosphere directly above it causing convection, which can cause intense rainfall in a region that is normally dry.

In addition to the ENSO effects along the coast of South America, there are many other global impacts. These climatic aberrations are called teleconnections because statistical correlations have been found between these atypical weather events and the ENSO. Among these teleconnections are droughts in Central America, Philippines, Southern India, Indonesia, Africa, and Australia. Large scale brush fires and forest fires- some on the order of millions of acres- are associated during these drought periods in Australia and Kalimantan (Borneo). Flooding is more prevalent in the United States, Cuba, Northern Peru, Southern Brazil, Northern Argentina, Eastern Paraguay, Bolivia, and Western Europe. These teleconnections statistically correlated with the ENSO have extensive social impacts that have been known to leave people homeless and poverty stricken, crops failed, and national economies disrupted.

Closer to home, the ENSO teleconnections found in the US are more clearly seen in the winter. Intense rainfall has been recorded in the Southern and Western US. Winters in the northern US may be unusually mild during El Nino years. In addition, the occurrence of U.S. landfalling hurricanes is reduced during El Nino years.

Although not fully understood in the scientific community, the El Nino is an extremely important event. Research is being done to further knowledge about this phenomenon so that forecasts may be made on El Ninos and the numerous teleconnections associated with it.

Much of the information contained here was derived from a National Oceanic and Atmospheric Administration (NOAA) report "El Nino and Climate Prediction" from Reports to the Nation On Our Changing Planet (Spring l994, NO. 3), written by John M. Wallace at the University of Washington and Shawna Vogel, Science Writer.

 

For further El Nino information, visit the following informative websites:

What is an El Nino? (from NOAA)
The 1990-1995 El Nino-Southern Oscillation event: Longest on record. Kevin E. Trenberth and Timothy J. Hoar. Geophysical Research Letters, Vol. 23, No. 1, pp 57-60. January 1, 1996.
NOAA/PMEL TOGA-TAO Home Page: TOGA-TAO Observing Array in the Tropical Pacific.
Houghton, J.T., G.J. Jenkins, J.J. Ephraums, eds, 1990: 1990 Intergovernment Panel on Climate Change, Cambridge University Press, 226-229.
Latest ENSO advisory
The Record Setting 1990-95 El Nino: Harbinger of a Changing Climate? UCAR News Release, 5 January 1996.
ENSO Teleconnections
NOAA home page

     

References

    Allan, R., 1996: El Nino, Southern Oscillations and Climate Variability. CSIRO Publication.
    Mayewski, P. A., M. S. Twickler, S. I. Whitlow, L. D. Meeker, Q. Yang, J. Thomas, K. Kruetz, P. M. Grootes, D. L. Morse, D. J. Steig, E. D. Waddington, E. S. Saltzman, P.-Y. Whung, and K. C. Taylor, 1996: Climate change during the last deglaciation in Antarctica. Science, 14 Nov., Vol. 272.
    Leetmaa, Ants, 1990: The interplay of El Nino and La Nina. Oceanus Vol.32: 30-34.