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Western half of northern hemisphere circulating gyre
Eastern half of northern hemisphere circulating gyre
z
p
-ve
z
r
-ve
z
r
-ve
z
r
z
p
-ve
-ve
z
p
+ve
z
r
-ve
z
p
+ve
W-side story: f increases N and so
z
p
more negative N
z
r
from wind stress is negative
Overall on this westward leg a net decrease of relative
vorticity (-
z
p
-
z
r
< 0)
E-side story:
f
decreases S and so
z
p
more positive S
z
r
from wind stress is still negative
Overall on this eastward leg a net balance of relative
vorticity (+
z
p
-
z
r
~ 0)
Overall, across the whole circuit (west and east combined) there is net loss of vorticity. This is not allowed
because the total vorticity must be kept constant. Extra relative vorticity must be generated by either
pronounced western lateral boundary shear or by western bottom shear, or a combination of both.
The eastern flow needs no such enhancement and is thus weaker and more spatially uniform.
West
East
ζ
f
+ve
Fig. 6.28
Sketches to show that conservation of vorticity requires western boundary currents to be stronger than eastern ones.
p
is planetary
vorticity (or
f
),
r
is relative vorticity due to wind shear, and
f
is relative vorticity due to lateral friction.
measurements and bottom scour features indicate that
strong vortex motions are sometimes able to propagate tur-
bulent energy all the way (i.e.
Front: this is known to shift zonally by large amounts
depending upon the amount of cold but buoyant freshwater
issuing out of the Arctic from ice melting.
4 km) down to the ocean
floor, where they cause unsteadiness in the deep thermoha-
line current flow (see Section 6.4.5; so-called
deep-sea
storms
), enhanced resuspension of bottom sediment, and
nutrient mixing. Also, the currents are unsteady with time,
both on the longer time scale, for example, major erosive
events on the Blake Plateau have been attributed to Gulf
Stream flow during glacial epochs when the current was
thought to be at its strongest, and on a subyearly basis as
spectacular eddy motions, meanders and cutoffs of cooler
waters form
cold-core mesoscale eddies
(Figs 6.26 and 6.27).
Notions that the Gulf Stream circulation might “fail” due to
global warming and a shutoff in the deep circulation (see
below) are erroneous: in the words of one oceanographer,
“As long as the wind blows and the Earth turns then the
surface current will exist.” The one thing that will change is
the junction between the warm surface current and the cold
southerly flows from the Arctic Ocean along the Polar
6.4.4 Internal waves and overturning:
“Mixing with latitude”
Internal waves (Section 4.10) of much longer period than
normal wind-driven surface waves have recently been dis-
covered to be a major source of turbulent mixing in the
deep oceans. The internal wave field arises due to wave-like
disturbances of the density stratification that occurs at var-
ious depths, but particularly within the deep-ocean water
column. The disturbances or forcing occurs due to:
1
Internal tides
formed when the main ocean tidal currents
flow over rough sea-floor topography and act upon the inter-
nal stratification to form tidal period internal waves.
2
A response of the stratification to
inertial surface waves
piled up by wind shear during storms, the internal waves
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