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.
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’).
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.
Data selection concerns the Pacific Ocean, and specifically the following coordinates:
Temporal series of MSLA
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.
fig 1: Area for data extraction
fig 2: Temporal series of MSLAs in the El Niño area
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 propagate eastwards in response to wind stress.
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.
In a Hovmöller diagram, SLA variations are plotted for time and longitude at a fixed latitude, which highlights the role of waves.
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.
fig 4: SLA Hovmöller diagram