Geoscience Reference
In-Depth Information
Figure 7.28
Mean annual meridional heat transport (10
15
W) in the Pacific, Atlantic and Indian Oceans, respectively (delineated by
the dashed lines). The latitudes of maximum transport are indicated.
Source
: Hastenrath (1980), by permission of the American Meteorological Society.
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 accumu-
lated 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 to 100 m depth, and within
this layer there exist thin, jet-like equatorial under-
currents flowing eastwards (under hydraulic gradients)
at a speed of 1 to 1.5 m 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 appre-
ciably higher sea-level in the Gulf of Mexico than
along the Atlantic coast of the United States. The accu-
mulated 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 favours this pile-up of water, which is
released poleward through the Florida Straits as the
narrow and fast Gulf Stream (Figure 7.30). These pole-
ward 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
