Geoscience Reference
In-Depth Information
phytoplankton) to brown (muddy or polluted waters).
The
photic zone
contains sufficient light for photosyn-
thesis by
phytoplankton
- the oceans' primary producers
- and also depends on water turbidity, controlled by
suspended sediments. The zone varies in depth from
10 m to 200 m, representing differences between turbid
muddy estuaries and the clearest sunlit water. Below this,
marine life depends on chemosynthesis and scavenging of
organic rain-out.
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 shore-
lines. Minor gyres are driven, like a series of gearwheels,
within and between the principal currents and coastlines
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 circumpolar
current
in the southern ocean attenuates the individual
warmth and vigour of the southern hemisphere mid-
latitude warm westerly Agulhas, Brazil and Australian
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
Fluid properties shared by atmosphere and ocean permit
the development of thermally driven near-surface
circulatory systems, comprising ribbons of air or water
currents moving around seasonally mobile cells (atmos-
phere) or
gyres
(ocean). The apparently stable character
of the oceans is indicative of thorough surface mixing.
Ocean motion occurs in two other, non-circulatory forms.
Shallow, transient
wave
motion is generated by air flow
at the ocean-atmosphere boundary layer and may
superficially mimic larger current systems.
Tides
form an
oscillatory response to gravitational mass attractions
between Earth, sun and moon.
Ocean-atmosphere coupling of water, heat and
momentum transfers is also influenced by Earth's
rotation. Thus we may model similar simple overturning
cells of tropically heated water moving poleward, cooling
and
downwelling
(subsiding) to form lower return
currents which
upwell
(ascend) at the equator. The
Coriolis force (see
Chapter 6)
draws water to the right of
its path in the northern hemisphere and to the left south
of the equator. As in the atmosphere, there are areas of
divergence and convergence. Broad similarities end there.
Oceans are capable of sustaining internal heat and motion
for far longer, by virtue of their higher specific heat
capacity and slower-moving mass, but basin geometry
restricts circulation more than continental relief
influences the atmosphere. 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.
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
Wind-driven (surface) circulation
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
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