Geoscience Reference
In-Depth Information
2.4.2.
Ocean-atmosphere in the tropics
In tropical regions, the ocean's dynamics respond rapidly to
disturbances since the equatorial ocean, because of its sharp
thermocline close to the surface and the cancellation of the Coriolis
parameter, reacts as a wave guide [GIL 82]. The phenomena that keep
the ocean in equilibrium with the atmosphere are not only advective
but propagative and are measured in weeks or months. With this
rapidity of reaction, the coupled evolution between the ocean and the
atmosphere is thus easy: the ocean moves into a new state under the
influence of the atmosphere, and this new ocean state causes the
atmosphere to evolve. The two fluids evolve in concert.
If we seek to characterize the recurring patterns of variability in
temperature at the surface of the oceans, once the seasonal variability
has been filtered, a dominant mode is revealed, centered in the
equatorial Pacific Ocean, with a broad spatial and temporal signature:
it is El Niño. This mode is characterized by a pronounced warm
anomaly in the central and eastern Pacific, which starts at the end of
spring and continues and intensifies until the end of the following
winter, until it disappears rapidly in spring. This anomaly in surface
temperature is accompanied by a large modification in the oceanic and
atmospheric circulations.
Figure 2.20.
Schematic representation of the ocean-atmosphere
circulation in the Pacific. In a) its normal state and in b) its El Niño state
