328
DYNAMICAL OCEANOGRAPHY
The absence of a continental barrier near the latitude of Drake Passage has
a large influence on the ocean circulation in the Southern Ocean. Between
45 ◦ S and 65 ◦ S there is a strong zonal current, known as the Antarctic Circumpolar Current (ACC), which connects the three ocean basins. The
ACC plays an important role in the transport of heat, salt and other quantities between the ocean basins and hence has an important role in the
climate system. In this chapter some characteristics of the ACC, as determined from the (limited) observations, are presented in section 14.1. One
of the central motivations of the material in this chapter is the physical
processes which control the volume transport through Drake Passage. In
section 14.2 we consider barotropic wind-driven flows in a zonal channel
discuss the role of bottom topography (and the so-called ‘form stress’)
in these flows. The theory of stratified flows is presented in section 14.3
where we discuss the JEBAR effect. At the end of this section we touch
on the role of instabilities and the resulting eddies.
Note: All equations in this chapter are dimensional and we will therefore omit
the star subscript.
14.1. Observations
The annual average wind-stress forcing (τ φ ,τ θ ) from Trenberth et al. (1989)
over the globe was plotted in Fig. 2.1. The zonally averaged profile of the zonal
wind stress τ φ over the Southern Hemisphere is shown in Fig. 14.1. The zonally
averaged value of τ φ is positive between 65 ◦ S and 30 ◦ S and negative elsewhere.
As a consequence, the meridional Ekman transport M θ
E , determined in section
13.2 as
M
θ
E = −
τ φ
2Ωρ 0 sin θ
,
(14.1)
is northward between 65 ◦ S and 30 ◦ S (note that sin θ<0 in the Southern Hemisphere) and southward south of 65 ◦ S and north of 30 ◦ S.
A sketch of the flow patterns in the Southern Ocean is given in Fig. 14.2a
together with the annual mean temperature and salinity in Fig. 14.2b and c, respectively. The ACC is the dominant flow around Antarctica, with gyres – the
Weddell Gyre and the Ross Gyre – near the boundaries of this continent. The
ACC is composed of different zonal jets, each associated with a strong gradient in
density, a so-called front. Two of these fronts are indicated in Fig. 14.2, e.g., the
Polar Front and the Subantarctic Front. Signatures of the fronts are visible in the
plots of sea surface temperature and sea surface salinity (Fig. 14.2b,c).
The fronts are also visible in the profiles of temperature and salinity
(Fig. 14.3a,b), as measured along the WOCE 140 ◦ E meridional section (between
DYNAMICAL OCEANOGRAPHY
The absence of a continental barrier near the latitude of Drake Passage has
a large influence on the ocean circulation in the Southern Ocean. Between
45 ◦ S and 65 ◦ S there is a strong zonal current, known as the Antarctic Circumpolar Current (ACC), which connects the three ocean basins. The
ACC plays an important role in the transport of heat, salt and other quantities between the ocean basins and hence has an important role in the
climate system. In this chapter some characteristics of the ACC, as determined from the (limited) observations, are presented in section 14.1. One
of the central motivations of the material in this chapter is the physical
processes which control the volume transport through Drake Passage. In
section 14.2 we consider barotropic wind-driven flows in a zonal channel
discuss the role of bottom topography (and the so-called ‘form stress’)
in these flows. The theory of stratified flows is presented in section 14.3
where we discuss the JEBAR effect. At the end of this section we touch
on the role of instabilities and the resulting eddies.
Note: All equations in this chapter are dimensional and we will therefore omit
the star subscript.
14.1. Observations
The annual average wind-stress forcing (τ φ ,τ θ ) from Trenberth et al. (1989)
over the globe was plotted in Fig. 2.1. The zonally averaged profile of the zonal
wind stress τ φ over the Southern Hemisphere is shown in Fig. 14.1. The zonally
averaged value of τ φ is positive between 65 ◦ S and 30 ◦ S and negative elsewhere.
As a consequence, the meridional Ekman transport M θ
E , determined in section
13.2 as
M
θ
E = −
τ φ
2Ωρ 0 sin θ
,
(14.1)
is northward between 65 ◦ S and 30 ◦ S (note that sin θ<0 in the Southern Hemisphere) and southward south of 65 ◦ S and north of 30 ◦ S.
A sketch of the flow patterns in the Southern Ocean is given in Fig. 14.2a
together with the annual mean temperature and salinity in Fig. 14.2b and c, respectively. The ACC is the dominant flow around Antarctica, with gyres – the
Weddell Gyre and the Ross Gyre – near the boundaries of this continent. The
ACC is composed of different zonal jets, each associated with a strong gradient in
density, a so-called front. Two of these fronts are indicated in Fig. 14.2, e.g., the
Polar Front and the Subantarctic Front. Signatures of the fronts are visible in the
plots of sea surface temperature and sea surface salinity (Fig. 14.2b,c).
The fronts are also visible in the profiles of temperature and salinity
(Fig. 14.3a,b), as measured along the WOCE 140 ◦ E meridional section (between