Dynamics of ENSO
281
Stronger trade winds lead to a deeper thermocline in the west and a shallower one
in the east and vice versa.
12.2.3. Feedbacks
In the previous sections, the elementary physical processes have been discussed
and there is enough background now to discuss feedbacks in the coupled system. There are three important feedbacks, which are called the thermocline, the
upwelling and the zonal advection feedback. The first two are associated with
changes in the vertical heat transport modelled by (12.6), and the third is associated with horizontal heat transport. The first two feedbacks are described in this
section in their most elementary form and the zonal advection feedback will be
Ex. 12.3
the topic of an exercise at the end of this chapter.
This thermocline feedback is best explained by looking at a sloping thermocline
in a constant upwelling ocean (with constant w ∗ =¯ w ∗ ) as sketched in Fig. 12.7.
Assume that a positive SST perturbation ˜
T is present in the eastern part of the
basin (Fig. 12.7). This leads to a perturbation in the low level zonal wind which
is westerly with a maximum located west of the maximum of the SST anomaly
according to the response shown in Fig. 12.4. Since the background trade winds
are weakened locally, the slope of the thermocline decreases and it becomes more
flat. In this case, the colder water will be closer to the surface in the west but
it will be farther down in the east. In other words, in the east the thermocline
is deeper and hence the subsurface temperature is higher. Hence, the subsurface
temperature effectively increases at the level of upwelling, giving a positive heat
flux perturbation at the bottom of the mixed layer according to (12.4) and (12.6),
i.e.,
∂ ˜
T ∗
∂t ∗
≈−¯ w ∗
˜
T ∗ − ˜
T s∗
H u
.
(12.11)
As ¯
w ∗ is positive, the first term in the right hand side represent the local damping
of SST anomalies. However, when ˜
T s∗ > 0 then the second term on the right
hand side is positive and the original disturbance may be amplified.
As another prototype situation, consider that the thermocline (and hence the
subsurface temperature ¯
T s∗ ) is fixed with a certain slope related to the background
winds, the latter similar to the previous case. Again a positive SST anomaly
is present in the east which generates the same changes in the wind as before
(Fig. 12.7). However, now the changes only influence the upwelling ( ˜
w ∗ ), mainly
through the Ekman layer dynamics. Weaker easterly winds imply less upwelling
and hence less colder water enters the mixed layer. This can also be seen from
(12.6), i.e.,
∂ ˜
T ∗
∂t ∗
≈−˜ w ∗
¯
T ∗ − ¯
T s∗
H u
.
(12.12)
281
Stronger trade winds lead to a deeper thermocline in the west and a shallower one
in the east and vice versa.
12.2.3. Feedbacks
In the previous sections, the elementary physical processes have been discussed
and there is enough background now to discuss feedbacks in the coupled system. There are three important feedbacks, which are called the thermocline, the
upwelling and the zonal advection feedback. The first two are associated with
changes in the vertical heat transport modelled by (12.6), and the third is associated with horizontal heat transport. The first two feedbacks are described in this
section in their most elementary form and the zonal advection feedback will be
Ex. 12.3
the topic of an exercise at the end of this chapter.
This thermocline feedback is best explained by looking at a sloping thermocline
in a constant upwelling ocean (with constant w ∗ =¯ w ∗ ) as sketched in Fig. 12.7.
Assume that a positive SST perturbation ˜
T is present in the eastern part of the
basin (Fig. 12.7). This leads to a perturbation in the low level zonal wind which
is westerly with a maximum located west of the maximum of the SST anomaly
according to the response shown in Fig. 12.4. Since the background trade winds
are weakened locally, the slope of the thermocline decreases and it becomes more
flat. In this case, the colder water will be closer to the surface in the west but
it will be farther down in the east. In other words, in the east the thermocline
is deeper and hence the subsurface temperature is higher. Hence, the subsurface
temperature effectively increases at the level of upwelling, giving a positive heat
flux perturbation at the bottom of the mixed layer according to (12.4) and (12.6),
i.e.,
∂ ˜
T ∗
∂t ∗
≈−¯ w ∗
˜
T ∗ − ˜
T s∗
H u
.
(12.11)
As ¯
w ∗ is positive, the first term in the right hand side represent the local damping
of SST anomalies. However, when ˜
T s∗ > 0 then the second term on the right
hand side is positive and the original disturbance may be amplified.
As another prototype situation, consider that the thermocline (and hence the
subsurface temperature ¯
T s∗ ) is fixed with a certain slope related to the background
winds, the latter similar to the previous case. Again a positive SST anomaly
is present in the east which generates the same changes in the wind as before
(Fig. 12.7). However, now the changes only influence the upwelling ( ˜
w ∗ ), mainly
through the Ekman layer dynamics. Weaker easterly winds imply less upwelling
and hence less colder water enters the mixed layer. This can also be seen from
(12.6), i.e.,
∂ ˜
T ∗
∂t ∗
≈−˜ w ∗
¯
T ∗ − ¯
T s∗
H u
.
(12.12)