4 Theory of the Ventilated Thermocline
4.1 Introduction
One of the fundamental problems in physical oceanography is explaining the
observed density structure of the ocean. The density field varies significantly in
all three spatial directions, and perhaps its most striking feature is the
limitation of the largest density variations to the upper 1-2 km of the ocean.
The density increases with depth almost everywhere in the ocean, but the major
part of this increase occurs in the upper kilometer. Figure 4.1.1 from the
Levitus Atlas (1982) shows basin-averaged density profiles for the major oceans
in which this feature is manifest. The density increases rapidly to its abyssal
value in a rather short distance compared to the total depth of the ocean. In
Section 3.1 we described the link between the motion field and the field of
density that this implies. The two are intimately coupled through the
advection/diffusion equation for density and the thermal wind relation for
the velocity. We also noted in Chapter 3 that the explanation of the density
structure requires a dynamical explanation of the circulation.
The density field varies strongly in the horizontal direction. Figure 4.1.2
shows the zonally averaged density field for the Pacific Ocean, also from the
Levitus Atlas, and its qualitative resemblance to the corresponding figure for
the Atlantic, (Fig. 3.1.1) suggests that the problem of understanding the density
field is of a general dynamical nature not connected with any peculiar feature
of one or another of the major ocean basins. The density field in fact varies so
strongly that the overall horizontal variation in density is as large as the overall
vertical variation in density in each of the gyres. This implies that an
explanation of the observed density field requires, at a minimum, a relaxation
of the quasi-geostrophic approximation in which the horizontal density
variation is supposed small with respect to the vertical variation. Isopycnal
surfaces experience an 0( 1) change in depth over the horizontal extent of the
gyre (see Fig. 4.1.3; Lozier et al. 1996). At the same time a complete theory for
the density field should dynamically predict the vertical as well as the
horizontal variation of density and the associated velocity field. Moreover, we
anticipate that the horizontal and vertical structures are linked through the
motion field. This is the problem of the thermocline. It is nonlinear and
difficult.
4.1 Introduction
One of the fundamental problems in physical oceanography is explaining the
observed density structure of the ocean. The density field varies significantly in
all three spatial directions, and perhaps its most striking feature is the
limitation of the largest density variations to the upper 1-2 km of the ocean.
The density increases with depth almost everywhere in the ocean, but the major
part of this increase occurs in the upper kilometer. Figure 4.1.1 from the
Levitus Atlas (1982) shows basin-averaged density profiles for the major oceans
in which this feature is manifest. The density increases rapidly to its abyssal
value in a rather short distance compared to the total depth of the ocean. In
Section 3.1 we described the link between the motion field and the field of
density that this implies. The two are intimately coupled through the
advection/diffusion equation for density and the thermal wind relation for
the velocity. We also noted in Chapter 3 that the explanation of the density
structure requires a dynamical explanation of the circulation.
The density field varies strongly in the horizontal direction. Figure 4.1.2
shows the zonally averaged density field for the Pacific Ocean, also from the
Levitus Atlas, and its qualitative resemblance to the corresponding figure for
the Atlantic, (Fig. 3.1.1) suggests that the problem of understanding the density
field is of a general dynamical nature not connected with any peculiar feature
of one or another of the major ocean basins. The density field in fact varies so
strongly that the overall horizontal variation in density is as large as the overall
vertical variation in density in each of the gyres. This implies that an
explanation of the observed density field requires, at a minimum, a relaxation
of the quasi-geostrophic approximation in which the horizontal density
variation is supposed small with respect to the vertical variation. Isopycnal
surfaces experience an 0( 1) change in depth over the horizontal extent of the
gyre (see Fig. 4.1.3; Lozier et al. 1996). At the same time a complete theory for
the density field should dynamically predict the vertical as well as the
horizontal variation of density and the associated velocity field. Moreover, we
anticipate that the horizontal and vertical structures are linked through the
motion field. This is the problem of the thermocline. It is nonlinear and
difficult.