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 Hovmoller diagrams, can be useful.
Hovmoller diagram
In a Hovmoller 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, 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.
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