Geoscience Reference
In-Depth Information
Many large-scale characteristics of ocean dynamics
resemble features of the atmosphere. These include:
the general circulation, major oceanic gyres (similar to
atmospheric subtropical high-pressure cells), major
jet-like streams such as sections of the Gulf Stream (see
Figure 7.29), large-scale areas of subsidence and uplift,
the stabilizing layer of the permanent thermocline,
boundary layer effects, frontal discontinuities created
by temperature and density contrasts, and water mass
('mode water') regions.
Mesoscale characteristics that have atmospheric
analogues are oceanic cyclonic and anticyclonic eddies,
current meanders, cast-off ring vortices, jet filaments,
and circulations produced by irregularities in the north
equatorial current.
(2) Macroscale
Figure 7.27
The Ekman ocean current pattern in the northern
hemisphere. Compare Figure 6.5.
Source
: Bearman (1989). Copyright © Butterworth-Heinemann,
Oxford.
The most obvious feature of the surface oceanic circu-
lation is the control exercised over it by the low-level
planetary wind circulation, especially by the subtropical
oceanic high-pressure cells and the westerlies. The
oceanic circulation also displays seasonal reversals of
flow in the monsoonal regions of the northern Indian
Ocean, off East Africa and off northern Australia (see
Figure 7.29). As water moves meridionally, the con-
servation of angular momentum implies changes in
relative vorticity (see pp. 119 and 140), with poleward-
moving currents acquiring anticyclonic vorticity and
equatorward-moving currents acquiring cyclonic
vorticity.
The more or less symmetrical atmospheric sub-
tropical high-pressure cells produce oceanic gyres with
centres displaced towards the west sides of the oceans
in the northern hemisphere. The gyres in the southern
hemisphere are more symmetrically located than those
in the northern, due possibly to their connection with
the powerful west wind drift. This results, for example,
in the Brazil current being not much stronger than the
Benguela current. The most powerful southern hemi-
sphere current, the Agulhas, possesses nothing like the
jet-like character of its northern counterparts.
Equatorward of the subtropical high-pressure cells,
the persistent trade winds generate the broad north
and south equatorial currents (see Figure 7.29). On the
western sides of the oceans, most of this water swings
poleward with the airflow and thereafter comes increas-
ingly under the influence of the Coriolis deflection and
of the anticyclonic vorticity effect. However, some
the thickness of the wind-driven Ekman layer is about
100 to 200 m. North (south) of 30°N, the westerly (east-
erly) winds create a southward (northward) transport of
water in the Ekman layer giving rise to a convergence
and sinking of water around 30°N, referred to as
Ekman
pumping
.
b Horizontal
(1) General
Comparisons can be made between the structure and
dynamics of the oceans and the atmosphere in respect
of their behaviour above the permanent thermocline
and below the tropopause - their two most significant
stabilizing boundaries. Within these two zones, fluid-
like circulations are maintained by meridional thermal
energy gradients, dominantly directed poleward (Figure
7. 28), and acted upon by the Coriolis force. Prior to the
1970s oceanography was studied in a coarsely averaged
spatial-temporal framework similar to that applied in
classical climatology. Now, however, its similarities
with modern meteorology are apparent. The major
differences in behaviour between the oceans and the
atmosphere derive from the greater density and viscosity
of ocean waters and the much greater frictional con-
straints placed on their global movement.
