El Niño is a climatic phenomenon occurring in the Pacific Ocean every two to ten years. During an El Niño event, a few months before Christmas anomalous warm water accumulates off the coast of Peru. The El Niño event that occurred in 1997 was a good example of where satellite altimetry made a major contribution to monitoring such phenomenon.

Data used

No particular reprocessing of altimeter data is necessary in this case, so it is possible to use ready-made maps of Sea Level Anomalies. As combined data offer the best quality, and delayed-time the optimal orbit, we have chosen the merged DT-MSLA dataset, up-to-date (‘Upd’) data for better quality for a given date, and reference (‘Ref’) data for long temporal studies (see the second part: ‘Ocean planetary waves’).

Methodology

Temporal extraction

Download MSLAs from 1992 to 2005. This could take some time (!), if necessary you can reduce the data period to 1996-2000. The main advantage of selecting such a long time period is to put the El Niño event into context.

Geographic extraction

Data selection concerns the Pacific Ocean, and specifically the following coordinates:
30°S-30°N,170°W-120°W (fig 1).

Temporal series of MSLA

  • Compute the geographic average for SLAs corresponding to the area defined by (5°S-5°N, 170°W-120°W). The value obtained, for a given time, represents the mean sea level anomaly over the equator in the Pacific Ocean. This averaged SLA shall now be referred to as M.
  • Plot the curve M=f(t) in order to show the temporal variations in M all along the chosen period.

The diagram shows periodic oscillations in M and illustrates the annual variability in sea surface heights. Moreover M’s value significantly increased in 1997 to over 25 cm: the signature of an occurring El Niño. Then, from summer 1998, sea surface height oscillations began again.

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fig 1: Area for data extractionnino_199210-200401_sm
fig 2: Temporal series of MSLAs in the El Niño area

 

1997 El Niño

Focusing on 1997 enables us to consider specifically the month of November as an indicator of El Niño’s intensity. Using only the fourth MSLA datasets available for November 1997, a new monthly mean map can be plotted (fig 3, simply re-enlarge the selected area to 30°S/30°N). This gives an overview of El Niño’s distribution throughout the Equatorial Pacific Ocean during the month of November.

november_mean_sm
fig 3: Monthly Mean of Maps of Sea Level Anomalies, El Niño area, during November 1997.

It is now acknowledged that an El Niño event is caused by significant changes in wind stress; it thus provides a good example of existing interactions between ocean and atmosphere. However, sea surface slope changes, in terms of space and time scales, involving planetary waves (ie those with long wavelengths, that can travel thousands of kilometres). These are known as Kelvin waves and Rossby waves.

Kelvin waves

Kelvin waves propagate eastwards in response to wind stress.
In the same way as for November, plot SLA maps for December and January. The resulting diagrams show the situation at successive monthly intervals: the maximum SLA values (i.e Kelvin waves) progress towards the South American coast and divide into two poleward Kelvin waves.

Rossby waves

One part of the Kelvin wave is deflected westwards: this is a Rossby wave, which propagates across the rise in thermocline at lower speeds (by a factor of approximately 3) than Kelvin waves. Space and time features of planetary waves suggest another approach to their study, as plotting maps for each month, would be ineffective and tedious. Other diagrams, such as Hovmöller diagrams, can be useful.

Hovmoller diagram

In a Hovmöller diagram, SLA variations are plotted for time and longitude at a fixed latitude, which highlights the role of waves.
To plot such a diagram on the area (5°N-5°S, 135°E-75°W), select your time period (in the example the datasets go from the beginning of 1996 to the end of 1999), then compute the averaged SLA for latitude. Then plot this mean SLA for longitude and for the whole time period.

On this diagram, SLA appears to be streaked with colours: these straight lines represent ocean waves. The red stretch shows the El Niño event, where maximum values reached 40 centimetres around 125°W at the end of 1997.

Planetary waves transport heat and energy across the oceans, and satellite altimetry allows us to detect them because of the variations in sea level they generate.

november_mean_smdecember_mean_smjanuary_mean_smhovmol_sm

fig 4: SLA Hovmöller diagram