Advanced ENSO Theory: The Delayed Oscillator
Perturbing the Ocean
The motionless state is perturbed by turning on an isolated patch of westerly
(eastward) wind stress near the equator in the central part of the ocean
basin. This wind patch is shown in Figure 2. The wind forcing is turned on
instantaneously and is kept constant for 30 days.
Figure 2. Zonal wind-stress anomaly
A more detailed look at the wind stress anomaly is shown in Figure
3. Specifically, this figure shows a cross section along 175° West through
the wind stress anomaly. The anomaly is nearly Gaussian in the meridional
(latitude) direction with maximum amplitude on the equator.
Figure 3a. Zonal wind-stress anomaly along
Figure 3b. Wind-stress curl anomaly along
The response of such a wind-stress forcing on the ocean is strongly influenced
by the so-called Coriolis force, deriving ultimately from the earth's rotation.
Away from the equator, the near-surface ocean (above the thermocline) exhibits
a balance between the wind stress forcing and the Coriolis force, which results
in net transport (mass flow) to the right of the direction of wind stress
in the Northern Hemisphere, and to the left of the wind-stress forcing in
the Southern Hemisphere.