Geoscience Reference
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and equatorward-moving currents acquiring
cyclonic vorticity.
The more or less symmetrical atmospheric
subtropical high pressure cells produce oceanic
gyres with centers 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
Hemisphere 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 increasingly comes under
the influence of the Coriolis deflection and of the
anticyclonic vorticity effect. However, some water
tends to pile up near the equator on the western
sides of oceans, partly because here the Ekman
effect is virtually absent, with little poleward
deflection and no reverse current at depth. To
this is added some of the water that is displaced
northward into the equatorial zone by the
especially active subtropical high pressure
circulations of the Southern Hemisphere. This
accumulated water flows back eastward down
the hydraulic gradient as compensating narrow
surface equatorial counter-currents, unimpeded
by the weak surface winds. Near the equator in the
Pacific Ocean, upwelling raises the thermocline to
only 50-100m depth, and within this layer there
exist thin, jet-like Equatorial Undercurrents
flowing eastward (under hydraulic gradients) at a
speed of 1 to 1.5m s
-1
.
As the circulations swing poleward around the
western margins of the oceanic subtropical high
pressure cells, there is the tendency for water to
pile up against the continents, giving, for example,
an appreciably higher sea level in the Gulf of
Mexico than along the Atlantic coast of the United
States. The accumulated water cannot escape by
sinking because of its relatively high temperature
and resulting vertical stability. Consequently, it
continues poleward driven by the dominant
surface airflow, augmented by the geostrophic
force acting at right angles to the ocean surface
slope. Through this movement, the current gains
anticyclonic vorticity, reinforcing the similar
tendency imparted by the winds, leading to
relatively narrow currents of high velocity (for
example, the Kuroshio, Brazil, Mozambique-
Agulhas and, to a lesser degree, the East Australian
Current). In the North Atlantic, the configuration
of the Caribbean Sea and Gulf of Mexico especially
favors this pile-up of water, which is released
poleward through the Florida Straits as the
particularly narrow and fast Gulf Stream. These
poleward currents are opposed both by their
friction with the nearby continental margins and
by energy losses due to turbulent diffusion, such
as those accompanying the formation and cutting
off of meanders in the Gulf Stream. These
poleward western boundary currents (e.g. the
Gulf Stream and the Kuroshio Current) are
approximately 100km wide and reach surface
velocities greater than 2m s
-1
. This contrasts
with the slower, wider and more diffuse eastern
boundary currents such as the Canary and
California (approximately 1000km wide with
surface velocities generally less than 0.25m s
-1
).
The northward-flowing Gulf Stream causes a heat
flux of 1.2
10
15
W, 75 percent of which is lost to
the atmosphere and 25 percent in heating the
Greenland-Norwegian seas area. On the pole-
ward sides of the subtropical high pressure cells,
westerly currents dominate, and where they are
unimpeded by land masses in the Southern
Hemisphere they form the broad and swift West
Wind Drift. This strong current, driven by
unimpeded winds, occurs within the zone 50 to
65
×
S and is associated with a southward-sloping
ocean surface generating a geostrophic force,
which intensifies the flow. Within the West Wind
Drift, the action of the Coriolis force produces a
°
