349
There is no guarantee that the three criteria always give the same ordering,
but a combination of the three allows a comprehensive stability analysis.
3.3 Thermohaline feedbacks
A negative (positive) feedback is present when a perturbation weakens
(enhances) itself through the changes it causes. To discuss the feedbacks
of the present model, it is useful to linearise the oceanic equations as well
as the atmospheric transports. One obtains for a thermally direct mean
state (ij> 0)
T'
S'
q'
H~(T') - 2q't - 2ijT' - ... ,
H~(T') - 2q'S - 2ijS' - ... ,
k(aT' - (3S') ,
with the surface flux anomalies given by
H~(T')
H~(T') ~ 2m''/l S OT'.
Dew
( 40)
(41)
(42)
(43)
(44)
Feedback #0: Mean Flow The mean circulation tends to eliminate all
anomalous temperature and salinity gradients. This feedback is almost
but not entirely trivial. It is always present, always negative (for example,
positive T' leads to negative T'), and represented by the respective terms
#3 on the right hand sides of the linearised heat and salt equations, (40)
and (41).
The other feedbacks all involve anomalous circulation.
Feedback #1: Ocean Heat Transport Weaker overturning circulation,
q' < 0 ::::}- T' > 0 ::::}- Larger meridional temperature gradient ::::}- Stronger
overturning.
This is a negative feedback, purely oceanic, and represented by term 2
in the heat conservation equation (40). It is suppressed by prescribing sea
surface temperature and conceptually assumes fixed surface heat fluxes.
Feedback #2: Salinity Transport Weaker overturning circulation, q' <
o : : : : }- S' > 0 ::::}- Larger salinity gradient ::::}- Even weaker overturning.