350
This is a positive feedback and purely oceanic; it is represented by term
2 in the salt conservation equation (41) and is suppressed if sea surface
salinity is prescribed as effectively done in GeMs with restoring surface
salinity boundary condition. It is the fundamental feedback responsible
for the existence of multiple equilibria of the thermohaline circulation under mixed boundary conditions. Stommel (1961) did not, however, discuss this process explicitly, which was apparently first described by Walin
(1985). Marotzke (1990) analyzed how the interaction between feedbacks
#0 and #2 leads to Walin's (1985) analytical stability criterion for the
2-box model with prescribed temperature and freshwater flux. Notice that
without feedback #0, there would be no stabilising process at all under
mixed boundary conditions. Notice, also, that feedback #2 conceptually
assumes fixed P-E.
We now turn to the coupled feedbacks, which involve changes in the
meridional transports in the atmosphere and hence changes in the surface
heat and freshwater fluxes. All these changes are driven by SST changes;
therefore their connection with anomalies in the circulation comes through
the first three stages of feedback #l.
Feedback #3: Atmospheric Heat Transport Weaker overturning circulation, q' < 0 =? 7" > 0 =? Larger meridional temperature contrast =?
Increased atmospheric heat transport =? Smaller meridional temperature
contrast =? Even weaker overturning circulation.
This is a positive, coupled atmosphere-ocean feedback. It involves the
fundamental negative feedback in the atmosphere between the meridional
temperature gradient and eddy activity and hence heat transport, which
tends to wipe out anomalous temperature gradients and in its linearised
form acts as a Newtonian damping. It is counterintuitive that this feedback, which in its purely atmospheric part is a paradigm of a stabilising
effect, destabilises the thermohaline circulation. The apparent paradox is
resolved if one considers feedbacks #1 and #3 in conjunction: The change
in atmospheric heat transport is induced by the SST change, which is instrumental in setting up the oceanic negative feedback. As the anomalous
atmospheric heat transports limits the change in SST, it weakens the negative oceanic feedback - and hence constitutes a positive feedback.
Notice that changes in longwave radiation act in the same sense as
changes in dynamical atmospheric transports: A greater atmospheric temperature gradient means lower (higher) temperature at high (low) latitudes,
hence smaller (greater) longwave loss at high (low) latitudes, which reduces
This is a positive feedback and purely oceanic; it is represented by term
2 in the salt conservation equation (41) and is suppressed if sea surface
salinity is prescribed as effectively done in GeMs with restoring surface
salinity boundary condition. It is the fundamental feedback responsible
for the existence of multiple equilibria of the thermohaline circulation under mixed boundary conditions. Stommel (1961) did not, however, discuss this process explicitly, which was apparently first described by Walin
(1985). Marotzke (1990) analyzed how the interaction between feedbacks
#0 and #2 leads to Walin's (1985) analytical stability criterion for the
2-box model with prescribed temperature and freshwater flux. Notice that
without feedback #0, there would be no stabilising process at all under
mixed boundary conditions. Notice, also, that feedback #2 conceptually
assumes fixed P-E.
We now turn to the coupled feedbacks, which involve changes in the
meridional transports in the atmosphere and hence changes in the surface
heat and freshwater fluxes. All these changes are driven by SST changes;
therefore their connection with anomalies in the circulation comes through
the first three stages of feedback #l.
Feedback #3: Atmospheric Heat Transport Weaker overturning circulation, q' < 0 =? 7" > 0 =? Larger meridional temperature contrast =?
Increased atmospheric heat transport =? Smaller meridional temperature
contrast =? Even weaker overturning circulation.
This is a positive, coupled atmosphere-ocean feedback. It involves the
fundamental negative feedback in the atmosphere between the meridional
temperature gradient and eddy activity and hence heat transport, which
tends to wipe out anomalous temperature gradients and in its linearised
form acts as a Newtonian damping. It is counterintuitive that this feedback, which in its purely atmospheric part is a paradigm of a stabilising
effect, destabilises the thermohaline circulation. The apparent paradox is
resolved if one considers feedbacks #1 and #3 in conjunction: The change
in atmospheric heat transport is induced by the SST change, which is instrumental in setting up the oceanic negative feedback. As the anomalous
atmospheric heat transports limits the change in SST, it weakens the negative oceanic feedback - and hence constitutes a positive feedback.
Notice that changes in longwave radiation act in the same sense as
changes in dynamical atmospheric transports: A greater atmospheric temperature gradient means lower (higher) temperature at high (low) latitudes,
hence smaller (greater) longwave loss at high (low) latitudes, which reduces