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Interannual Variability

Table of Contents

  •  A.  Statement of Topic
  •  B.  Summary of Topic
  •  C.  Research Setting (Spatial Region and Temporal Period)
  •  D.  Data Description
  •  E.  Stepwise Investigational Instructions
  •  F.  Presentation of Data Analyses
  •  G.  Interpretation of Data Anlyses
  •  H.  Dicussion and Statement of Conclusion
  A.  Statement of Topic

This module initiates with an examination of the variability of the Peru Upwelling Zone, and expands to an examination of data covering the southeastern Pacific Ocean basin.A comparison of the El Nino and La Nino characteristics will be presented.

  B.  Summary of Topic

El Nino and La Nina tend to develop during the period April - June, and they tend to reach their maximum strength during December - February (spring and winter in the Northern Hemisphere, respectively).Both phenomena typically persist for 9-12 month (though occasionally persisting for up to two years) and recur every 2-7 years.

There has been a somewhat confusing range of uses for the terms El Nino, La Nina, and El Nino -Southern Oscillation (ENSO) by both the scientific community and the general public. El Nino is characterized by a large scale weakening of the trade winds and warming of the surface layers in the eastern and central equatorial Pacific Ocean.  La Nina is the opposite phase;  it is characterized by a cooling of the surface layers in the eastern and centeral equatorial Pacific Ocean.  The term ENSO is used to describe the full range of variability observed the Southern Oscillation Index (SOI), including both El Nino and La Nina events. As a result to these events, Australia experiences a characteristic cycle of droughts and floods, all caused by the El Nino/La Nina cycle.  Note that while it is common for the La Nina phase to follow the El Nino phase, this does not always happen.

C.  Research Setting

" El Nino" is named after a Peruvian tradition, where the warming of the waters off Peru is said to occur near the birthday of "The Boy" (El Nino), or the Christ child. The cooling of the eastern Pacific waters was at first called "Anti - El Nino", until it was realized that this literally translated as "Anti- Christ". To avoid this, it was renamed "La Nina" (The Girl).

ENSO is the result of a cyclic warming and cooling of the surface ocean of the centeral and eastern Pacific. This region of the ocean is normally colder than its equatorial location would suggest, mainly due to the influence of northeasterly trade winds.  An El Nino event occurs when the influence of weakening trade winds causes the surface of the eastern and central Pacific to warm. The observable results from El Nino are heavy rainfall in South America, but severe droughts in eastern Australia. The more intense the El Nino, the more intense and extensive the Australian droughts. El Nino is characterized by unusually warm ocean temperatures in the Equatorial Pacific.

The La Nina event is the opposite of the El Nino event. The La Nina event occurs when the injection of cold water becomes more intense than usual, causing the surface of the eastern Pacific to cool. The results from La Nina are droughts in South America and heavy rainfalls, even floods, in eastern Australia. La Nina is characterized by unusually cold ocean temperatures in the Equatorial Pacific.

  D.   Data Description

For this analysis, SeaWiFS monthly global 9km products (chlorophyll a concentration) and MODIS SST data, both of which are available in Giovanni, were utilized. 

  E.  Stepwise Investigational Instructions

  Chlorophyll in the equatorial Pacific during El Nino     

Ocean Color for El Nino    

   Chlorophyll in the equatorial Pacific during La Nina

     La Nina, May-July 1998, chlorophyll, equatorial upwelling zone

For each image shown above, the methods were the same.  After going to Giovanni, the exact coordinates for the location were entered.

The exact coordinates for this study region: 

  • 20.0 N       North Latitude
  • 20.0 S       South Latitude
  • 180.0 W    West Longitude
  • 70.0 W      East Longitude.  

  An area plot of the time averaged parameter was first selected as the plot type for each of the images.  The desired months and years were selected before clicking the "Generate Plot" button.

F.  Presentation of Data Analyses

  El Nino:  effects on chlorophyll concentration

   The normal occurrence of elevated chlorophyll concentrations in the Equatorial Upwelling Zone is suppressed during an El Nino, and the size and extent of the Peru Upwelling Zone and phytoplankton productivity near the Galapagos Islands is reduced.

Ocean Color for El Nina


  La Nina:  effects on chlorophyll concentration

  During La Nina, the strength of the Equatorial Upwelling Zone increases, and phytoplankton productivity near the Galapagos Islands and in the Peru Upwelling Zone is significantly enhanced.    During spring 1998, the transition from El Nino to La Nina conditions took less than six weeks.

La Nina, chlorophyll, May-July 1998, equatorial upwelling zone

The 2002-2003 El Nino and La Nina event:

     The first image is a Hovmoller longitude vs. time plot of sea surface temperature.  The 2003 El Nino was a weak event, followed by an even weaker La Nina phase.   However, the extension of warm waters into the eastern Pacific (east of 110 West) between February and April 2003 can be clearly seen, and the re-establishment of colder waters in the eastern Pacific (particularly off the coast of Peru) can be seen from July to November 2003.

El Nino, La Nina, 2002-2003, sea surface temperature, longitude versus time

Here is an area plot of sea surface temperatures for 2003, at the peak of the El Nino:

El Nino, March 2003, sea surface temperature

This is an area plot of the same area for July 2003, when weak La Nina conditions were present:

La Nina, July 2003, sea surface temperature

G.  Interpretation of Data Analyses

  There are two main effects visible in the equatorial and eastern Pacific Ocean in the chlorophyll concentration data.   The first effect is the suppression of the Equatorial Upwelling Zone, which is the most extensive upwelling zone in the world.    The warm El Nino waters cause a deepening of the thermocline, which strongly weakens the strength of the upwelling.   Because the nutrient-rich waters in the deeper ocean do not reach the surface, the productivity of the phytoplankton is severely affected, resulting in much lower chlorophyll concentration.    The second effect is the resurgence of the Equatorial Upwelling Zone when La Nina takes place, where the upwelling zone is even stronger than "normal", resulting in dramatically increased phtoplankton productivity and chlorophyll concentration.

  The SST data plots illustrate the main surface effect of an El Nino -- warmer waters in the eastern Equatorial Pacific Ocean.   Also note the extremely high temperatures off the coast of Panama in March 2003.   In July 2003, note how the cooler waters extend westward to about 110 West Longitude, and the coastal water ttemperatures off of Peru and Ecuador are back in the normal range of 16-18 degrees C.

H.  Discussion and Statement of Conclusions

For further research on El Nino and La Nina, the anomaly analysis tool for SeaWiFS ocean color data can be used to compare the normal values for chlorophyll to the minima which occur during El Nino and the maxima that occur during La Nina. 


SeaWiFS and Global Warming

El Nino, La Nina & PDO



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Last updated: Apr 08, 2016 11:00 AM ET