Geoscience Reference
In-Depth Information
0
x
τ
x
1
2
τ
#
S
V
=−
z dz
=−
,
(8.10)
2
ρ
f
ρ
f
W
W
h
and
v
t
k
#
τ
1
v
t
t
S
y
x
V
=
`
τ
i
τ
j
j
=−
.
(8.11)
S
S
ρ
f
ρ
f
W
W
The units of v are m 2 /s, so multiplying V times a horizontal length scale,
L , yields a volume transport (m 3 /s) that measures the number of cubic meters
of water transported each second by the Ekman transport . This flux is of-
ten expressed in sverdrup (Sv) units, where 1 Sv  10 6 m 3 /s. Since the Ekman
transport is proportional to the curl of the surface wind stress (Eq. 8.11), it
is perpendicular the surface wind. In the Northern Hemisphere, the Ekman
transport is to the right of the wind direction ( Fig 8.2) , and in the Southern
Hemisphere it is to the left.
Recall from Figure 2.22 that ocean surface currents are organized into
gyres, or anticyclonic circuits in each ocean basin, with westerly flow in middle
latitudes, easterly flow in the tropics, cool (equatorward) eastern boundary
currents, and warm (poleward) western boundary currents. Because the Ek-
man transport is perpendicular to the surface winds, water within the Ekman
layer moves toward the center of the ocean basins, creating hills of water in
the subtropics. For example, the Sargasso Sea in the tropical North Atlantic
is some 1.5 m higher in the center than near the coasts. This redistribution of
mass within the ocean basin sets up pressure gradients forces. A parcel of water
traveling, for example, northward in the Gulf Stream experiences a westward
pressure gradient force, away from the mass accumulation in the center of the
North Atlantic basin, and an eastward Coriolis force.
The mounding of water in the ocean basins does not occur exactly in the
center of the basins but, rather, in the western portion of the basin due to
the eastward rotation of the earth. As a result, zonal pressure gradients in the
western ocean basins are greater than those in the eastern basin. This western
boundary intensification, produces western boundary currents that are stron-
ger, narrower, and deeper than eastern boundary currents. For example, in the
Atlantic, peak Gulf Stream velocities exceed 2 m/s, while the Canary Current
flow tops out at about 0.3 m/s. The Canary Current is characteristically 1000
km wide and 500 m deep, compared with the Gulf Stream, which is only 100-
200 km wide but up to 2000 m deep.
This geostrophic argument provides a irst-order understanding of western
boundary intensification, but it does not completely explain observed western
current velocities. The role of coastal friction in the balance of forces is also
important.
Because Coriolis forces play a prominent role in the Ekman dynamics, the
governing equations break down close to the equator. Note, for example, that
Eqs. 8.6 and 8.7 yield infinite current velocities for f   0. On the equator, cur-
rents are accelerated by pressure gradient forces that are directed eastward
down the geopotential height gradients formed by the massing of water in
the western parts of the ocean basin by the North and South Equatorial sur-
face currents. The result is eastward-lowing equatorial countercurrents , which
bring water mass back to the eastern ocean basins below the surface (~100 m)
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