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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.
- the semi-annual component
- 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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