Environmental Engineering Reference
In-Depth Information
Turbulent surface-upper air exchange in the atmosphere contrasts with surface-deep
water current dis connection in the oceans due to the thermocline, except in certain
locations set out below.
WIND-DRIVEN (SURFACE) CIRCULATION
Wind imparts a frictional force or wind stress on the ocean, proportional to the square of
the wind speed, which creates a film of surface waves over a more persistent, slower
current. Moving at 3-5 per cent of the wind speed, the current extends to 50-100 m
below the surface. Once out of the immediate friction zone, successively deeper layers
are deflected by the Coriolis force. Warm equatorial waters are driven west across the
oceans by atmospheric trade-wind convergence and are deflected poleward by the
opposing continental shoreline. The currents accelerate to conserve angular momentum
and develop a westerly component in consort with mid-latitude atmospheric westerlies -
transporting warm water to high latitudes. Cooled there by glacier melt and long-wave
radiation loss, the returning cold currents complete each gyre equatorward along western
continental shorelines. Minor gyres are driven, like a series of gearwheels, within and
between the principal currents and coastlines (Figure 11.8).
The importance of tectonically driven ocean geometry now becomes apparent. It
supports two hemispherical gyres in the Atlantic and Pacific Oceans and two major
northern hemisphere warm mid-latitude currents - the Gulf Stream or North Atlantic
Drift and the Kuroshio or North Pacific Drift . Atlantic circulation developed only when
the ocean became wide enough (probably a minimum of 1,500 km) in the Cenozoic. The
formation of the Panama isthmus in the late Pliocene (3 Ma ago) shut off its westerly
equatorial current and strengthened the Gulf Stream. The Indian Ocean is restricted to a
single gyre and is seasonally more varied by atmospheric monsoon circulation. The polar
oceans afford fascinating contrasts between the landlocked Arctic Ocean and circumpolar
Antarctic Ocean. The Antarctic circum - polar current in the southern ocean attenuates
the individual warmth and vigour of the southern hemisphere mid-latitude warm westerly
Agulhas, Brazil and Australian currents at the Antarctic convergence (Figure 11.8). The
Arctic Ocean is fed by Gulf Stream influx, which circulates beneath polar sea ice and
exits via the Denmark, Davis and Bering Straits.
Principal gyres located at 30° N-S, associated with atmospheric subtropical
divergence, aid circulation by pushing water into their cores owing to the Coriolis force
(see Chapter 6). This builds very shallow domes 1-2·5 m high in each gyre, enough to
add a significant gravity component to circulation as water flows out of the dome.
Although currents are shallow and velocities low, their persistence transfers very large
quantities of water and heat over time. The Gulf Stream transports a maximum 150 × 10 6
m 3 sec −1 at over 1·5 m sec −1 past Boston and the Kuroshio transports 46 × 10 6 m 3 sec −1 at
up to 1·7 m sec −1 past Japan. There is a significant anomaly in the main equatorial gyres.
Water build-up against the western coast in both oceans creates a rise in sea surface not
entirely dissipated by poleward flow. An equatorial counter-current flows back
eastward, between the westward limbs of each hemispheric gyre, opposed by only slight
winds, between 3° N and 10° N (the Doldrum belt) at the thermal equator.
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