Suggestions for using the PSD's interactive webpages to investigate El Niņo:

The following is an example of PSd's webpages can be used to help teach the meteorological concept of El Niņo. It is suggested that you open another browser window to follow the examples suggested below. Background information on El Niņo can be found at the PSD El Niņo Web Page. Users are encouraged to experiment with other options.

The Southern Oscillation


Web Analysis Tools for this section

In the 1930's, Gilbert Walker documented the out-of-phase relationship between sea level pressures(SLP's) in the western tropical Pacific and the southeastern tropical Pacific. He called this pattern the "Southern Oscillation" (SO) (see History of the Southern Osciallation Index for a more detailed history). This sea-saw pattern of pressures was associated with different rainfall and windfield patterns over the Pacific and over other parts of the globe. The out-of-phase relationship of the SOI can be seen by plotting the time series of sea level pressure over the different parts of the Pacific:

The SOI index is defined by convention as the Tahiti pressures minus the Darwin pressures and can be viewed with the parameters below. Note that it is noisy as small scale storms and circulations are included.

You can use the page to calculate time correlations [correlation parameters] of the different timeseries. For example, using monthly values, the correlation coefficient of the SOI to the Tahiti SLP is about .5 for all months.

  1. Looking at the Tahiti and Darwin timeseries, are there years when the sea-saw is more or less pronounced?
  2. What can you say about the seasonality of the relationship (hint: look at correlations by season)?
Learn More
  1. Wikipedia Southern Oscialltion Index
  2. Correlations
  3. Gilbert Walker

Spatial Pattern of the SOI

  1. GCOS Pressure Datasets Composite
  2. NCEP composite
  3. 20th Century Reanalysis Composites

The SOI is just a single number. The typical large scale Sea Level Pressure pattern can be seen by looking at a map of the pressure and pressure anomalies for an average of years with high SOI values. Average (or compositing tends to smooth out some of the noise from small scale circulations. High SOI years are associated higher than normal pressures over Tahiti and lower over Darwin.

  1. Produce a similar plot to the high SOI years but for the opposite phase of the SOI. How does that compare? Is the map exactly opposite?
  2. How much does the pattern differ for different (but very high or very low SOI years). What does that say about using a single index to descibe the SLP?
  3. Do the patterns differ depending on which SLP dataset is used? (hnit, use the different composite pages
Learn More
  1. 20thC Reanalysis
  2. NCEP Reanalysis

SOI, SST's El Nino/La Nina

  1. GCOS Pressure Datasets Composite
  2. NCEP composite
  3. 20th Century Reanalysis Composites
  4. Correlations

The connection between this see-saw or "Southern Oscillation" pattern and a similar reversal of typical Sea Surface Temperatures wasn't noticed until the 1960's (by Jacob Bjerknes). He noticed that when the SOI was high, SST temperatures in the eastern tropical pacific ('map') were lower. High SST's in the eastern tropical pacific tended to occur irregularly ever 2-7 years. El Niņo was the term used to refers to the quasi-periodic increase in ocean temperatures in the eastern tropical Pacific and the resulting circulation changes. El Niņo is traditionally defined by the ocean temperatures in the eastern Pacific with El Niņo referring to warmer than normal temperatures and La Niņa colder. SST patterens.

Note the much warmer than usual temperatures over the eastern tropical Pacific.
Learn More
  1. Jacob Bjerknes
  2. El Nino term

ENSO Timeseries: Current and Historical Monitoring

  1. climateindices
  2. SST correlations
Researchers have used different timeseries to monitor El Nino. Some of these are temperature averages over a region (Nino 3, Nino 3.4, Nino4 and Nino 1.2, ONI). There are other indices that use a combination of variables in order to try to capture more fully the variability of El Nino. For exmaple, thge MEI uses C.Try plotting the various indices and comparing them. Also, correlate each with SST.
  1. Do the indices give the same years that are worm (or cold)? How do they compare for all months? For different months (say summer vs winter)
Learn More
  1. ONI
  2. MEI

Subtopics to Add

Associated Tropical Circulation Patterns

Web Analysis Tools
crossections, composite, correaltion: Walker Circulation
The relationship between the SLP and the SST's is, of course, not a coincidence. Changes in the circulation are directly related to changes in the ocean temperatures. Typically, the circulation in the tropics is called a 'Walker Circulation' and is characterized by east to west surface winds, rising over the western tropics and sinking over the east. Plotting a crossection of zonal winds for non-e years shows this pattern. Likewise, plotting omega shows the upward motion in the west and upwards ij the east.
  • u zonal cross section (no n el nino)
  • omega- non el nino
  • u zonal: el nino
  • omega : el ino
Easterly surface winds in the tropical pacific are associated with a phonemonen called 'upwelling' which brings colder, nutrient rich water up to the surface.. That contrasts with westerly winds which are associated with downwelling and increased temperatures. Plotting the corelation of Nino 3.4 temperatures and surface winds shows this well.
  • correlation plot
Learn More
  1. Gilbert Walker
  2. upwelling

Relationship to Midlatitudes

composites/correlations, risk, composite enso signal page. enso differences, usstation

Seasonal risk

climate risk vs composites

Relaionship to MJO's

Daily Timeseries, daily composites


composites vs correlaton

Ocean patterns with depth

search and plot

Relationship to other patterns like the PNA, PDO

climate indices (long, short), correlation

Hurricanes, Tornados

daily timeseries, composites





What kind of circulation would you expect to be associated with these temperatures (hint; hotter air goes up)? How would you expect the difference in temperatures to affect rainfall (primarily convection)? How does this SST pattern compare to other El Niņo years (hint, look at a time series of SST averaged over the eastern tropical Pacific)? How does it compare to La Nina years (colder than normal in the eastern Pacific)? How does it compare to the SLP time sereis? Local wind field plot Dec-Feb [plot parameters] Local wind field anomaly plot Dec 1982-Feb 1983 [plot parameters]
Note that the winds go to westerly over the equatorial region during an El Niņo. Is that what you would expect? How would the change in wind speed affect temperature near the surface? What about change in direction?

Other climate variables and their association with the SOI

The rainfall patterns that are associated with this pattern can be seen by comparing an the average rainfall pattern with the pattern from Dec-Jeb 1983. Precipitation over globe during Dec-Feb [plot parameters] Precipitation anomalies Dec-Feb 1982-83over globe [plot parameters]

When the pressure is higher than normal over the western tropical Pacific, the convection that is normally there shifts eastward and as a result there is less rainfall than normal there but an increased amount east of the dateline.

  1. Produce a precipitation plot for the opposite phase of the SOI. How does that compare?
  2. Is the precipitation pattern similar for other seasons?