The Record 2009 - Keble College - University of Oxford

The College at Large
Oceans, overflows and climate
Sonya Legg, BA, (Ph.D.,
London)
When I tell people I’m an oceanographer, they usually imagine
this means I spend my days diving with dolphins. The reality
of my working life is somewhat more mundane — sitting in front
of a computer screen, talking in front of a room full of scientists
or students, or if I’m very lucky, heading off somewhere as
exotic as Washington DC (a short train ride from New Jersey
where I live) for a funding meeting. Oceanography encompasses
many disciplines and my particular niche within physical
oceanography (the study of the circulation, currents, heat and
salt) is turbulent mixing and the role it plays in climate. And my
tool of choice is computer simulation, which means I study the
ocean from my office computer rather than in the ocean itself.
So what does ocean turbulence have to do with climate? Well,
the main role of the ocean in the climate system is through heat
storage and transport of heat from Equatorial to Polar regions.
Most people have heard something of the Gulf Stream, the
great ocean current that carries warm surface water northward
in the North Atlantic Ocean. Without it Western European
winters would be considerably colder. Fewer people, however
(outside my profession), have heard of its deep counterpart,
the Deep Western Boundary Current, which carries the
return flow of cold waters southward. Yet this current plays an
equally important role in our climate as the return loop of what
combined is known as the Atlantic Meridional Overturning
Circulation (shown in the schematic). This circulation may
fluctuate on time-scales of decades to centuries, leading to
decadal variations in the climate of the Atlantic region.
The cold deep current is fed by water coming through the
straits which connect the Greenland-Iceland-Norwegian Sea
with the North Atlantic, cold water which results from intense
cooling of the surface water by the bitter subpolar winds.
These straits are narrow (when compared to the size of the
basin they join) — between 10 and 100km wide — but play a
disproportionately large role in determining the character of
the deep current. The cold water moves through the straits and
accelerates like a rollercoaster down the slope, sinking below
warmer water. The regions of descending cold water, like
under-sea waterfalls, are known as overflows, and it is here that
turbulence comes into play. More turbulent mixing means a less
dense, less cold current with greater volume, less mixing means
a denser, colder current of smaller volume. The more mixing,
the more water goes south in the deep current, and the more
warm water has to be pulled north to replace it in the surface
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