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Ocean Surface Topography from Space
Caption for SSH-SST Global Animation

Because the anomalies are relative to an annual sine fit, the animation we are looking at presents the combination of 2 dominant types of anomalies happening in the Indian Ocean.

  1. the semi-annual component
  2. the strong interannual events of 1997-98-99.

If one concentrates on sea level anomalies (SLA), one mostly sees the anomalies propagating eastward along the equator pretty fast, Kelvin waves cross the basin from Africa to Indonesia in ~40 days. Rossby waves on both sides of the equator make ~120 days to cross the basin from East to West. A pair of Rossby waves propagating westward on both sides of the equator can be seen in late 1997 with the sudden sea level rise along Java after the strong easterlies blew in the equatorial central Indian ocean. Such equatorial waves are also visible twice per year during normal years, they are due to westerlies/easterlies along the equator that reverse with the NE monsoon in winter and the SW monsoon in summer and are associated with Wyrtki jets along the equator (surface currents maxima to the East in April and November).

Other SLA anomalies further away from the equator in the South Indian Ocean or Bay of Bengal or Arabian sea are locally pumped up by wind curl anomalies + connected to remote variations via oceanic waves can also be seen slowly propagating westward.

If one concentrates on sea surface temperature anomalies (SSTA), one mostly sees zonal fronts of anomalies propagating meridionnally twice per year from 20S to 20N. A direct explanation comes from the following: solar radiation has one maximum per year at the solstice of Capricorn in summer and one at the solstice of Cancer in winter, so it has 2 maxima per year at the equator during the equinoxes. The semi-annual signal is thus very strong in air-sea radiative flux, it is also strong in wind, precipitation and latent heat. These processes have a strong impact on SSTA. By contrast with the ocean dynamics processes which are linear to a great extent, these hydrodynamic processes are highly non linear, they depend on thresholds. SSTA is not the result of the ocean dynamics only, it is not either the result of the radiative forcing only, it is the result of the coupling between the ocean and the atmosphere. So, one can see both meridional and zonal propagating SSTA features in this animation.

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