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The oceans move about one-third of the planet's surface heat from the
equator to its poles
and scientists have long been eager to understand how they do it.
One tool some oceanographers use is the computer model,
a simulation that they program to replicate as faithfully as possible
how water moves in the oceans. It's no easy task.
The problem is that
the important scales in the oceans are both very large and very small
so you have to be able to have something very large
but with fine details and that is very difficult to do
because it takes a lot of memory and computer time
at least from the modeling perspective also from them the understanding point of
view it's difficult to
understand both the very large scale and the small-scale somehow those
two extremes interact.
But Scripps oceanographers Paola Cessi and Christopher Wolfe
have taken advantage of the power of supercomputers to create one of the most
sophisticated models
ever of the Atlantic Ocean. The model recreates the motions of swirling ocean
currents called eddies.
Eddies are tens to hundreds of kilometers in diameter,
small in relation to the entire ocean but important drivers of ocean
circulation.
Cessi and Wolfe's model has produced one of the most
accurate and dynamic simulations of the ocean ever.
That allows us to observe
and model these swirls but even though
they are small they transport heat eventually
from one hemisphere to the other and
so that is I think our contribution and I don't think anyone has done
a calculation with such high resolution and for
an extended period of time. Cessi and Wolfe
use the model to show that climate change could make the oceans move in different
ways.
They discovered that polar ice cap melting could create a seesaw
effect, as the vast pools of freshwater deposited from ice sheets in Antarctica
and Greenland
change how salty Atlantic sea water flows within a major circulation
component called Meriodional Overturning Circulation
and how the ocean transport heat one
or the other could come to dominate and control the speed of Atlantic
circulation.
The thing that controls the strength of the overturning circulation
is essentially the range of densities
that are shared between the northern North Atlantic
and the Southern Ocean so basically you have this window
right now in the Southern Ocean where there's a patch of surface water
which is got the same density roughly as
the water that is sinking in the Northern Hemisphere and that gives a
away to connect those two points through
circulation which goes to the deep ocean. Now
adding buoyant water by melting, melting ice in the Northern Hemisphere the
Southern Hemisphere
has the effect of changing the size of those windows. If you
add fresh water to the Northern Hemisphere, the water in the Northern
Hemisphere
can become lighter than any water found in
in the Antarctic Circumpolar Current so in that case you would remove that
window entirely by closing it off in the north
or you can just make it shrink and in that case
you would expect the overturning circulation to slowdown. On the other hand,
if you add fresh water to the Southern Hemisphere
you're making that water lighter and
thus increasing the window that the
Southern Ocean shares with the Northern Hemisphere and that would cause the
overturning circulation to speed up
Cessi and Wolfe's model also confirms
a recent revision of long standing theories of ocean circulation
that many researchers have come to embrace. In the Atlantic,
they now believe that water travels the entire length of the ocean basin
from Greenland to Antarctica. The realism of the model
also suggests that the changes brought about by melting ice caps
could happen within a few generations rather than over millennia.
The overturning circulation carries a lot of heat
northward in the North Atlantic and is basically
is responsible in part for the temperate climate in the Northern Hemisphere.
If you slow that circulation down, you expect the amount of heat transport
to go down and so you expect the countries especially surrounding the North
Atlantic
to become colder and that could have dramatic negative effects
just a few degrees could change the way that crops are grown or
livestock is grown and
and where people live. Conversely, if you
speed up the overturning circulation you could make the Northern Hemisphere
warmer
which might seem like a good thing but then you have the possibility that you
could have
more droughts, the deserts could expand.
Instead of thinking of things changing in thousands of years we're actually
thinking of things changing on decade's
which is something which is very important. You know you don't wanna see
a major change in the climate in your lifetime, you would never be able to adjust
it and so these are things that we're trying to understand from a conceptual
level and then move into
prediction and so that we can provide information for
policymakers and the public in general to decide how to
react to the possibility of climate change. The two created their simulation
logging millions of computer hours on supercomputers in various locations
around the country
but there is still much more computation to be done. Cessi and Wolfe would like to
be able to simulate a full revolution of the Atlantic circulation cycle
and add still more realism to their model. In a sense, we've added a great deal of
uncertainty
to the problem but what we've done is taken false certainty and replaced it with
with accurate uncertainty and then the whole
the whole project going forward over the next few years
is to create more actual certainty. We are trying to
increase some of the geometric realism
in our model and make adding coastlines and
about topography mountains but I think we're still limited by
computational power to do it in a single
ocean basin, the Atlantic. Ideally you would want to have a global
ocean but our resolution but I don't think that
the computer power yet is there and probably won't be there for
another ten to twenty years but who knows
maybe someone will come up with a brilliant idea
to make faster chips that use less energy
and we will be able to do it sooner.
This has been a presentation of
Scripps Institution of Oceanography at UC San Diego.