The Record 2009 - Keble College - University of Oxford

Keble College: The Record 2009
currents. The climate of Western Europe therefore depends
indirectly on the mixing in these narrow straits.
Since the real ocean contains overflows, and overflows are
a crucial part of the Atlantic circulation, we obviously want
the climate models which are being used for predictions of
climate change to include overflows too. However, overflows
are very difficult to represent in large-scale computer models
of the ocean, of the sort which are used (when coupled with
atmospheric and ice models) to make climate predictions.
These models have grids with points only every 100km or
so; the overflow straits are about this size or smaller, and the
turbulence happens on even smaller scales. Climate models
without proper overflows get the deep western boundary
current all wrong, and so aren’t credible for understanding
climate variations on the timescales (decades to centuries) when
the ocean’s deep circulation comes into play.
The Atlantic Meridional Overturning
Circulation, after Rahmstorf, Nature
1997
For the past few years I have led a team of researchers from
several different academic and research institutions spread
over the USA, in an effort to improve the way in which
climate models represent these overflows. The study of ocean
circulation uses many different techniques — old fashioned
laboratory experiments of the fundamental fluid dynamics,
observations of the real ocean from ships and moored
instruments, computer simulations of processes on many
different spatial scales. Often, because of the different resources
needed (e.g. ships versus computers) specialists in different
techniques are located in different institutions — our team was
put together to cross those disciplinary barriers and get us all
working together on a common problem. All these different
ways of looking at the problem are essential — for example only
field observations can tell us what the real ocean is doing, but
we can’t do experiments (i.e. changing the size of the straits,
the density of the current) on the natural world, so that’s
where the laboratory comes in. Some quantities are difficult to
measure in the laboratory, and so computer models are useful
there. But computer models don’t capture all the turbulence
of the real ocean… and so on… After five years, we’re pleased
that we have been able to combine new understanding
from recent field campaigns and theoretical analysis of the
rotating turbulent fluid dynamics with expertise in numerical
algorithms to produce several improvements to our climate
model representation of overflows. These new models will be
used in the next generation of climate simulations which will
contribute to the next assessment of climate change by the
Intergovernmental Panel for Climate Change.
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