Advanced ENSO Theory: The Delayed Oscillator
Kelvin and Rossby Waves
Thus, an eastward wind-stress forcing produces equator-ward mass transport
in both hemispheres, acting to increase locally the depth of the warm water
layer near the equator, and decrease it locally farther poleward in either
hemisphere. The mass surplus near the equator then begins to disperse eastward
as a so-called (downwelling) Kelvin wave, and the mass-deficit areas begin
to propagate westward as so-called (upwelling) Rossby waves (upwelling
and downwelling refer to the wave tendencies either to shallow or deepen
the warm water layer). Kelvin and Rossby waves have different propagation
speeds (and directions) because of their different latitudinal structure,
once again owing to the important effect of the Coriolis force, which is
strongly latitude-dependent.
In the eastern and central equatorial Pacific, wave signals in general
have important impacts on the Sea Surface Temperature (SST) because, due
to the presence of the climatological westward blowing trade winds, there
is mean poleward surface flow in either hemisphere, and mean upwelling
at the equator, and thus a mechanism to translate subsurface anomalies
to the surface.
The Kelvin wave and Rossby wave signals generated by this wind stress
are shown in the upper left panel of Figure 4. The Kelvin wave with positive
ocean surface height anomalies is represented by region shaded in red and
gold. The Rossby wave, with negative surface height anomalies, is shown
by the region shaded in blue and green.
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Figure 4. Ocean surface height anomaly
(in cm)
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The different latitudinal structures of the Kelvin and Rossby waves
are evident in the cross sections shown in Figure 5a and 5b. The Kelvin
wave surface height anomaly is qualitatively similar to the imposed surface
stress anomaly, with maximum amplitude on the equator and reduced amplitude
off the equator, although the ocean wave structure decays much more rapidly
with latitude. The Rossby wave structure is nearly symmetric about the equator,
featuring a relative minimum height anomaly along the equator and areas
of largest height anomaly at approximately 4 degrees latitude in either
hemisphere.
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Figure 5a. Ocean surface height anomaly
along 140 West
Figure 5b. Ocean surface height anomaly along 180 East
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