164
Vertical Structure: Baroclinic Quasi-Geostrophic Models
whether regions of uniform potential vorticity can be observed. The elegant
character of the Rhines and Young theory has inspired several careful
investigations of the planetary potential vorticity on the large scale. Principal
among these are McDowell et al. (1982), Holland et al. (1984), Keffer (1985),
and Talley (1985, 1988).
The results are quite striking. Each density surface can be characterized by
whether or not its outcrop line lies in the subtropical gyre in late winter, which
is the season when the mixed layer is able to affect the potential vorticity
distribution of the permanent thermocline. During the course of the year, as
warming of the surface takes place in summer, the surface of the ocean is
increasingly covered by warm water, and the outcrop line of colder layers
occurs at higher latitudes. When the cooling season starts, the outcrop line of
each density surface moves southward, exposing colder water. It is, however,
the position of the outcrop line in late winter which is decisive for the
ventilation of the time-averaged thermocline. The reason for the choice of this
season is presented in more detail in the next chapter, where the physics of the
ventilation of the thermocline is discussed. For now we simply accept that
season as the appropriate one for deciding whether or not a particular density
surface is substantially exposed to Ekman pumping and hence to sources of
potential vorticity.
We should expect homogenization to occur only on density surfaces which
are unventilated by the mixed layer. That is, they should be deep enough that
the positions where the density surface intersects the sea surface, the outcrop
line, occur too far north to occur in a region of downward Ekman pumping.
Figure 3.11.3 (McDowell et al. 1982) shows the potential vorticity on
selected density surfaces in the North Atlantic. Panel a shows q on a fairly
shallow density surface with a mean density between (Je = 26.3 and 26.5. [The
density at a particular depth is a function of temperature, pressure, and
salinity. The potential density (Je is the density in cgs units after a correction is
made to eliminate the effect of the adiabatic increase in temperature that the
fluid element suffers as a consequence of the increase in pressure with depth. A
more complete discussion can be found in many texts, for example, Pickard
and Emery (1982)]. This surface ranges from about 100m in the eastern part of
the subtropical gyre to about 400 m in the west, just outside of the western
boundary current. Its winter time outcrop line trends northwest to southeast
probably as a result of the movement of the clockwise circulation of the fluid.
The winter time outcrop line lies well within the subtropical gyre, and the
potential vorticity on this surface is nonuniform and clearly not homogenized
as it is subject to communication with the surface Ekman layer. A more recent
analysis of the potential vorticity on essentially the same surface has been
carried out by Lozier et al. (1996) and is shown in Fig. 3.11.4. This figure shows
the depth of the density surface at (Je = 26.5 and the potential vorticity on that
surface. The shape of the gyre in the potential vorticity field is quite apparent.
The potential vorticity contours are bent around in the shape of the gyre, but
Vertical Structure: Baroclinic Quasi-Geostrophic Models
whether regions of uniform potential vorticity can be observed. The elegant
character of the Rhines and Young theory has inspired several careful
investigations of the planetary potential vorticity on the large scale. Principal
among these are McDowell et al. (1982), Holland et al. (1984), Keffer (1985),
and Talley (1985, 1988).
The results are quite striking. Each density surface can be characterized by
whether or not its outcrop line lies in the subtropical gyre in late winter, which
is the season when the mixed layer is able to affect the potential vorticity
distribution of the permanent thermocline. During the course of the year, as
warming of the surface takes place in summer, the surface of the ocean is
increasingly covered by warm water, and the outcrop line of colder layers
occurs at higher latitudes. When the cooling season starts, the outcrop line of
each density surface moves southward, exposing colder water. It is, however,
the position of the outcrop line in late winter which is decisive for the
ventilation of the time-averaged thermocline. The reason for the choice of this
season is presented in more detail in the next chapter, where the physics of the
ventilation of the thermocline is discussed. For now we simply accept that
season as the appropriate one for deciding whether or not a particular density
surface is substantially exposed to Ekman pumping and hence to sources of
potential vorticity.
We should expect homogenization to occur only on density surfaces which
are unventilated by the mixed layer. That is, they should be deep enough that
the positions where the density surface intersects the sea surface, the outcrop
line, occur too far north to occur in a region of downward Ekman pumping.
Figure 3.11.3 (McDowell et al. 1982) shows the potential vorticity on
selected density surfaces in the North Atlantic. Panel a shows q on a fairly
shallow density surface with a mean density between (Je = 26.3 and 26.5. [The
density at a particular depth is a function of temperature, pressure, and
salinity. The potential density (Je is the density in cgs units after a correction is
made to eliminate the effect of the adiabatic increase in temperature that the
fluid element suffers as a consequence of the increase in pressure with depth. A
more complete discussion can be found in many texts, for example, Pickard
and Emery (1982)]. This surface ranges from about 100m in the eastern part of
the subtropical gyre to about 400 m in the west, just outside of the western
boundary current. Its winter time outcrop line trends northwest to southeast
probably as a result of the movement of the clockwise circulation of the fluid.
The winter time outcrop line lies well within the subtropical gyre, and the
potential vorticity on this surface is nonuniform and clearly not homogenized
as it is subject to communication with the surface Ekman layer. A more recent
analysis of the potential vorticity on essentially the same surface has been
carried out by Lozier et al. (1996) and is shown in Fig. 3.11.4. This figure shows
the depth of the density surface at (Je = 26.5 and the potential vorticity on that
surface. The shape of the gyre in the potential vorticity field is quite apparent.
The potential vorticity contours are bent around in the shape of the gyre, but