262
Theory of the Ventilated Thermocline
A particularly intriguing result of Huang's calculation is his estimate of the
total amount of fluid which is ventilated through the mixed layer compared to
the circulation captured by deep recirculating fluid in the gyre. Of the roughly
42 sverdrups circulating in Huang's model beneath the mixed layer, nearly 25
Sverdrups enter the thermocline through the mixed layer. Another 11
Sverdrups enter across the zero Ekman pumping line and about 7 Sverdrups
recirculates in the deep layers of the gyre without ventilation. If the cross-gyre
flow is considered as a kind of ventilation of the gyre, this would leave only 7 I
42 or 16% of the circulation remaining as an unventilated, recirculating
contribution to the flow. It is probably more appropriate physically to count
this 11 sverdrups as recirculation within the gyre which is really defined by the
Sverdrup transport streamfunction. That is, it is probably more realistic to take
the zero of the Sverdrup transport function as the defining boundary of the
gyre rather than the Ekman pumping velocity itself. If this is done, the
partition between the ventilated and unventilated portion of the gyre transport
is more evenly balanced, i.e., it is 25 (ventilated) Sverdrups vs. 18 (recirculating)
Sverdrups so that the ventilated portion of the thermocline flow is about 60%
of the total. The sloping mixed layer depth has altered the prediction given by
the Rhines ratio ( 4.1 0.1) and emphasizes the important roles both subduction
and recirculation play in determining the dynamic structure of the oceanic
thermocline.
4.12 Observations and Numerical Models
Observations
The unified theory containing the processes of ventilation and recirculation is
capable of making detailed predictions of the structure of the steady, stratified,
wind-driven circulation in the oceanic interior. The physics of the model is,
however, incomplete. The theory as it has been presented ignores any feedback
from the western boundary current on the interior. It also neglects the effect of
eddy mixing in the interior and the effects of cross-isopycnal motion in the
interior. In this sense, as rich as the theory is conceptually, it should be regarded
as a type of null hypothesis. That is, the question we can pose is to what extent
this adiabatic theory, based on simple, steady, geostrophic, Sverdrup dynamics,
explains the observed circulation and density structure of the oceanic interior in
spite of the idealizations of the theory. How well does it do?
One of the first difficulties which we face is the fact that the theory also
completely neglects the time dependence of the fields of motion. There is a
strong seasonal signal in the surface density field as the surface heating and
cooling of the ocean by the atmosphere takes place. To a large extent the effect
of this variable surface heating is limited to the mixed layer, which in response
deepens in the winter and shallows in the summer (see Woods and Barkmann
Theory of the Ventilated Thermocline
A particularly intriguing result of Huang's calculation is his estimate of the
total amount of fluid which is ventilated through the mixed layer compared to
the circulation captured by deep recirculating fluid in the gyre. Of the roughly
42 sverdrups circulating in Huang's model beneath the mixed layer, nearly 25
Sverdrups enter the thermocline through the mixed layer. Another 11
Sverdrups enter across the zero Ekman pumping line and about 7 Sverdrups
recirculates in the deep layers of the gyre without ventilation. If the cross-gyre
flow is considered as a kind of ventilation of the gyre, this would leave only 7 I
42 or 16% of the circulation remaining as an unventilated, recirculating
contribution to the flow. It is probably more appropriate physically to count
this 11 sverdrups as recirculation within the gyre which is really defined by the
Sverdrup transport streamfunction. That is, it is probably more realistic to take
the zero of the Sverdrup transport function as the defining boundary of the
gyre rather than the Ekman pumping velocity itself. If this is done, the
partition between the ventilated and unventilated portion of the gyre transport
is more evenly balanced, i.e., it is 25 (ventilated) Sverdrups vs. 18 (recirculating)
Sverdrups so that the ventilated portion of the thermocline flow is about 60%
of the total. The sloping mixed layer depth has altered the prediction given by
the Rhines ratio ( 4.1 0.1) and emphasizes the important roles both subduction
and recirculation play in determining the dynamic structure of the oceanic
thermocline.
4.12 Observations and Numerical Models
Observations
The unified theory containing the processes of ventilation and recirculation is
capable of making detailed predictions of the structure of the steady, stratified,
wind-driven circulation in the oceanic interior. The physics of the model is,
however, incomplete. The theory as it has been presented ignores any feedback
from the western boundary current on the interior. It also neglects the effect of
eddy mixing in the interior and the effects of cross-isopycnal motion in the
interior. In this sense, as rich as the theory is conceptually, it should be regarded
as a type of null hypothesis. That is, the question we can pose is to what extent
this adiabatic theory, based on simple, steady, geostrophic, Sverdrup dynamics,
explains the observed circulation and density structure of the oceanic interior in
spite of the idealizations of the theory. How well does it do?
One of the first difficulties which we face is the fact that the theory also
completely neglects the time dependence of the fields of motion. There is a
strong seasonal signal in the surface density field as the surface heating and
cooling of the ocean by the atmosphere takes place. To a large extent the effect
of this variable surface heating is limited to the mixed layer, which in response
deepens in the winter and shallows in the summer (see Woods and Barkmann