380
DYNAMICAL OCEANOGRAPHY
heat flux, freshwater flux and wind stress, the resulting circulation has a very
weak meridional overturning in the Atlantic. Hence, ocean modelers have used
the observed surface salinity field to force the ocean models. Once equilibrium
solutions are obtained, the freshwater flux is derived by computation and used in
further investigations on sensitivity.
In typical studies of the stability of the thermohaline circulation, the freshwater
forcing is changed by adding slowly varying perturbations at different locations.
In this way the sensitivity of the overturning flow to the freshwater forcing can be
determined. When the perturbation is added in the northern North Atlantic (with
an inflow of typically 0.05 Sv per 1,000 years) a response as shown in Fig. 16.4
is obtained. On the vertical axis, for example, the amount of North Atlantic Deep
Water is plotted (or the strength of the zonally integrated flow, the meridional overturning circulation) while the horizontal axis shows the strength of the anomalous
freshwater forcing. With increasing freshwater forcing, the strength of the overγ p
ψ
(Sv)
(Sv)
L +
L _
collapse
recovery
Figure 16.4. Typical hysteresis behavior associated with the changes in the strength of the Atlantic meridional overturning when the strength of the freshwater flux γp is changed slowly in time
(first an increase followed by a decrease).
turning circulation decreases; at some point L + the overturning collapses. When
the freshwater input is reversed, hysteresis occurs as the transition to the original
state occurs at a smaller value of γ p (at L − ) than the reverse transition.
This result indicates that there is more than one equilibrium circulation pattern
for a given set of forcing conditions and that rapid transitions between them may
occur on relatively short time scales. This is an alarming result when we consider
the anticipated changes in the buoyancy forcing of the ocean due to the increase
in greenhouse gases. In most climate models, precipitation increases over higher
DYNAMICAL OCEANOGRAPHY
heat flux, freshwater flux and wind stress, the resulting circulation has a very
weak meridional overturning in the Atlantic. Hence, ocean modelers have used
the observed surface salinity field to force the ocean models. Once equilibrium
solutions are obtained, the freshwater flux is derived by computation and used in
further investigations on sensitivity.
In typical studies of the stability of the thermohaline circulation, the freshwater
forcing is changed by adding slowly varying perturbations at different locations.
In this way the sensitivity of the overturning flow to the freshwater forcing can be
determined. When the perturbation is added in the northern North Atlantic (with
an inflow of typically 0.05 Sv per 1,000 years) a response as shown in Fig. 16.4
is obtained. On the vertical axis, for example, the amount of North Atlantic Deep
Water is plotted (or the strength of the zonally integrated flow, the meridional overturning circulation) while the horizontal axis shows the strength of the anomalous
freshwater forcing. With increasing freshwater forcing, the strength of the overγ p
ψ
(Sv)
(Sv)
L +
L _
collapse
recovery
Figure 16.4. Typical hysteresis behavior associated with the changes in the strength of the Atlantic meridional overturning when the strength of the freshwater flux γp is changed slowly in time
(first an increase followed by a decrease).
turning circulation decreases; at some point L + the overturning collapses. When
the freshwater input is reversed, hysteresis occurs as the transition to the original
state occurs at a smaller value of γ p (at L − ) than the reverse transition.
This result indicates that there is more than one equilibrium circulation pattern
for a given set of forcing conditions and that rapid transitions between them may
occur on relatively short time scales. This is an alarming result when we consider
the anticipated changes in the buoyancy forcing of the ocean due to the increase
in greenhouse gases. In most climate models, precipitation increases over higher