I'll start with a definition again, though defining the Southern Ocean
is tricky. This seems to be one of those "ask any two oceanographers
and you'll get three answers" situations. Some people argue that it is
the ocean south of 30, 40, or 45 S. Captain DeSouza says it is the
ocean south of 60 S, and he has the weight of the British Admiralty
behind him so his answer is probably the best. However, defining the
Southern Ocean using lines of latitude is a bit like trying to figure
out exactly which year "the 80s" started... where there's a correct
answer, but most people will just know it when they see it. Well,
you'll definitely know the Southern Ocean when you see it. It is windy,
rough, cold, rainy, and fast moving. Perpetual trains of large waves
march from horizon to horizon. The wind and water moves from east to
west generally, though the Southern Ocean has an extraordinarily intense
eddy field which can turn the currents about any which way on shorter
timescales. In the southernmost reaches you'll find calmer and even
colder waters riddled with Antarctic icebergs and sea ice.
So why is the Southern Ocean this way? There are three unusual things
about the Southern Ocean that make it the way it is, the most important
of which is probably that it is the only place in the world where you
can draw a line around the Earth without ever touching land, aside from
the smaller Arctic Ocean. This allows waves, winds, and currents to
build up momentum without ever being redirected by landmasses. Second,
there is a strong westerly (coming from the West) wind belt that blows
over the Southern Ocean associated with the atmospheric Antarctic Polar
Vortex. Finally, the Southern Ocean is far enough south that parts of
it don't see sunlight at all for portions of the year, and, even during
summer, the sun is never directly overhead. This makes it cold.
Winds do something peculiar to the ocean which is central to the
importance of the Southern Ocean for global climate: the winds make the
seawater flow to the right of the direction that they blow in the
Northern Hemisphere, and to the left in the Southern. A student named
Vagn Ekman--who was trying to figure for why icebergs drifted to the
right of the direction of winds--first explained that this is due to the
interplay between the Coriolis force and friction between layers of
water. The Coriolis force is the tendency of objects to drift away from
the direction they are heading while moving across the spinning
spherical Earth. It is strongest near the poles, so it has an
especially large influence on water flow in the high latitude Southern
Ocean. Ekman realized that if the winds blow on the surface waters, and
the surface waters drag the waters below them, and the water below those
waters drags on the water deeper still, and each of these layers flows
in a direction that is a little off from the layer above due to
Coriolis, then the direction water flows will spiral down away from the
wind direction with depth. The speed of the flow decreases with depth,
and the result is that the net flow is to the right of the winds (in the
Northern Hemisphere). There are strong winds blowing from East to West
over the Southern Ocean, and, due to Coriolis, these winds will move the
water to the left of the direction of flow, or to the North. As you
might expect, the winds move water North fastest where the winds are
fastest. This means that in the center of the wind belt you have water
moving northward faster than you do on the slower flanks of the wind
belt. As a result, you have water piling up in the northern parts of
the wind belt and spreading out in the southern parts. Gravity will
only tolerate so much build-up of fluid water, so water tends to sink
where it is piles up in the North, and deeper water upwells from below
to replace surface water where it spreads out. The low temperatures of
the Southern Ocean also help to keep the surface water dense, which
helps the sinking water sink.
Okay, so water upwells and sinks there. Who cares? Well,
oceanographers get very excited about exchanging water between the
surface ocean and deeper depths because this is how the real power of
the ocean is brought to bear on Earth's climate. The ocean is massive.
It holds a tremendous amount of heat, gases, and nutrients. However,
most of the mass (and gases and nutrients) are trapped by strong density
gradients at depths deeper than the Washington Monument is tall. This
means only a small fraction of the miles-deep ocean is exchanging with
the atmosphere on short timescales. It also means that the places where
you can bring water into or out of the deep ocean are really special...
and there aren't too many of them. The North Atlantic, the
Mediterranean, and the Sea of Okhotsk are a few of the places outside of
the Southern Ocean where intermediate and deeper waters are formed.
Upwelling also happens along coasts and at the equator, but nowhere are
deep waters brought to the surface as broadly or dramatically as in the
Southern Ocean. The Southern Ocean also forms the coldest, densest, and
deepest waters on Earth down around the coast of Antarctica.
To put some numbers on the importance of the Southern Ocean for
climate, a modeling study done by my advisor, Jorge Sarmiento, found
that if you prevented nutrients from upwelling in the Southern Ocean,
then the biological productivity of the rest of the world (i.e. the
ocean outside of the Southern Ocean) would decrease by 75%. I've
attached a figure to this effect. Also, about 60% of the ocean heat
uptake (of which the ocean accounts of 93% of the planetary total) and
about half of the uptake of the carbon emitted by mankind (of which the
ocean absorbs about 25%) happens in the Southern Ocean.
The other reason to pay attention to the Southern Ocean is that there
are a number of climate changes occurring there. The westerly winds
that drive the circulation patterns have gotten stronger, and modeling
studies have linked this to climate warming and ozone depletion over
Antarctica as well as to one of the decade-long climate oscillations
mentioned in a past blog post. There is a lot of fascinating research
going on into exactly how the circulation patterns in the Southern Ocean
might change in response to the wind increases, much of the research
having to do with figuring out how much the extra energy from the winds
will ramp up the currents we've discussed here and how much will just go
into making more eddies. We already noted that the Southern Ocean is
taking up large amounts of heat and carbon, and it follows that the
water here is getting warmer and more acidic. There are also
interesting questions regarding how the freshwater distribution will
change down there. Glacial ice and sea ice have the potential to change
with warming, and we have already witnessed the abrupt collapse of the
Larsen B ice sheet off of the Antarctic Peninsula, which is one of the
fastest warming places in the world. We noted earlier that the wet
areas of the world will likely get wetter, and I can personally attest
to just how rainy the Southern Ocean is already.
We are currently at 42 S, so we are well on our way to the placid and
warmer waters of the Southern Pacific Subtropical Gyre. For those of us
who left the Northern Hemisphere's winter, our "Spring thaw" will take
place over two weeks with Easter appropriately punctuating the midpoint
of our journey North. We've had relatively smooth sailing and calm
weather for the last week. This has given us a much needed respite to
get station work done and deal with the wire problems Lynne noted in the
official cruise report. Switching to the outdoor winch created a large
number of additional and unexpected challenges for the technical and
engineering teams. We had several long days and nights of working
around the issues before we got things fully sorted out, but the
expertise and dedication of our crew prevailed yet again. I'm convinced
that our people could land the RV Palmer on the Moon given enough time.
I'm humbled to be working with them.
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