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Seawater Property Changes in the Southern Ocean

Juan, i must confess it's been awhile since I logged in to see where you were. You HAVE been busy! Te felecito for the outstanding job you're doing in explaining what can be difficult concepts regarding oceanography. I have two questions for you: First, I understand that Antarctica has a greater heat loss than the Arctic, making it colder (due to albedo). However some of that is due to the fact that the Arctic has more clouds. Why does the Arctic have more clouds? Second, How does Polar winter affect the thermohaline circulation, or does it? I loved the aurora pictures. I saw my first aurora in Barrow Alaska in January; thanks to you, I've seen one in the Antarctic now as well! Adelante! Lollie

Sat, 03/26/2011 - 22:59

Juan Botella

Hola Lollie, thanks a lot for writing, and for sending great questions. We are all
busy down here; I am amazed by how much everyone has worked during this
cruise. We are all on a roll.
Regarding your question about the difference in heat loss by both polar
regions that difference seems to be tied to the the land distribution.
Antarctica is a continent surrounded by oceans and the Arctic is an ocean
surrounded by continents. This means that the Arctic has a bigger source of
moisture, which can form clouds. I am told by a couple of our scientists
that there is a lot of fog in the Arctic. Water vapor is the main green
house gas, as it slows down the release of energy from earth back to space.
So the Arctic has a comforter made out of water vapor.
Antarctica has less moisture because there is no liquid water on the
continent. This also generates a high pressure system above the land which
dries the air even more. The lack of moisture on the air allows for a
faster release of energy back to space. There are plenty of clouds around
Antarctica once we move away from the continent and back to the ocean,
because there is a moisture source. I can count with the fingers of one
hand the number of clear nights. A bit disappointing, since we have not
been able to watch more of the Auroras that you like; not even stars.
I will have to be very careful with the thermohaline circulation question.
Perhaps I was misleading when I wrote about the effects of density on the
oceanic circulation. The ocean circulation is produced by the winds (wind
driven circulation) and to a lower degree to tides. The density differences
produced by changes in salinity and temperature are not enough to power the
ocean circulation, but they do impart on large quantities of water their
properties. The term has been used for describing so many phenomena that
some scientists are using the term Meridional Overturning Circulation
instead. There is an article in the Journal Science by Carl Wunsch that
basically provides this argument (Science, Vol 298 8 November 2002 p.
1179-1180). I say this because Hollywood made a movie a few years ago in
which the stoppage of the thermohaline circulation was blamed for a new
glaciation period ('The day after tomorrow'). This is not the case.
Having said that, the Polar winters are very important to what I called the
thermohaline circulation before. Deep water masses are formed on the poles
by becoming more dense than their surroundings. This happens on the
continental shelves of certain Polar areas as well as in open areas of the
Greenland Sea and Labrador Sea . These waters loose heat during the Polar
winter. Salts cannot dissolve in ice, so as water freezes on the shelves in
winter, it releases its salts to the surrounding water. This water becomes
very salty and cold, which makes it denser. The water sinks to the bottom
of the polar ocean and flows into the other basins. Water at the surface
moves to the area where water sank. We would not have this feature of the
circulation if deep water formation on the polar winters were stopped.
Even without Polar winters, the GulfStream that brings warmer waters to
higher latitudes would be there because this is a wind driven feature.
Please let me know if I confused you more.
Saludos,
Juan.

Sun, 03/27/2011 - 15:29

Juan Botella

Hi Lollie, I should have waited to send the e-mail until the second watch was
awake. I got some more information from one of the scientists aboard that
are in line with what I sent, but clarify a couple of things.
For the question about cloud cover, Alex corroborated that over there is a
lot of cloud cover over the Southern Ocean. He sent me a climatological map
that shows that over the ocean, the Southern end pole is cloudier. The
climatology does not show the cloud coverage over the continent, which as
you said, it is less than the Arctic.
Based on some climatologies, I'd say the Southern Ocean (south of 50°S)
cloud coverage is significantly more persistent all year-round than in the
Arctic Ocean, and probably more extreme as well (> 80%).
About my explanation for the second question, Alex wants me to be more
precise in the term denser rather than deeper. Here is the comment he sent
to me.
More precisely you need to say "dense", rather than deep, since these
waters are found over shallow marginal shelves in the Arctic and Antarctic
regions. Something like "Extremely dense surface waters are produced in
winter at certain regions of the polar continental margins, when extremely
cold seawater forms sea ice at the sea surface and releases brine to the
remaining seawater underneath. By increasing its salt content the remaining
near freezing surface water becomes heavier and sinks (convects) to
equilibrium levels, thus replenishing the polar shelves with dense Shelf
Water., i.e. simply surface water that underwent winter buoyancy loss
during sea-ice production. Shelf Water inshore of the shelf break is much
denser than any water found farther offshore in the oceanic domain, thus
upon crossing the regional shelf break Shelf Water is able to displace all
other waters and continues to sink to great depths down the continental
slope. Entrainment of relatively warmer ambient waters augment the volume
of these newly formed Polar bottom waters, that continue to flow
equatorward along a system of deep western boundary currents replenishing
the world ocean abyssal layer."
Hope that helps,
Juan

Sun, 03/27/2011 - 16:15

Lollie Garay

Thanks Juan, No I'm not totally confused :) I'm glad you clarified the point about wind driven circulation.So, in your Mar 9th post(water masses) you said that using the properties of surface water can help identify the source of a particular water mass. Does mixing (through circulation) change those properties? and what about biology? I may be behind on this...excuse me if you have already discussed this. I'm trying to catch up on your journals :)
Also, I know there have been many studies profiling the seawater in the Southern Ocean. Your journal indicates the one you used for data is 19 years old. Are there no others in this specific area more recent??
Fun question: What is the current position of the Sun in the sky? How much daylight do you have each day?
A stab at the question from March 9: would you have to know the speed limit or geography??
Hasta luego, Lollie

Tue, 03/29/2011 - 14:29

Juan Botella

Hi Lollie, Thanks again for following the posts. In general water masses below the
surface waters go through little mixing because the water flow is not very
fast. There are some areas in which mixing does occur, like on the
Antarctic Circumpolar Current, where the Lower Deep Circumpolar Water
(LCDW) that we see here is produced by mixing North Atlantic Deep Water
NADW) that was produced mostly in the Greenland, Labrador and Norway seas
with some outflow from the Mediterranean waters, and intermediate Antarctic
water. biological activity is also negligible in deeper waters since there
is no sun for phytoplankton to live.
There have been other small studies in the area, but none trying to repeat
the very large survey effort that happened in the 90's, called WOCE. During
WOCE, many countries set the task of surveying all of the oceans. They
agreed on certain transects they would sample and took many years to
complete it. A decade later they began to think on repeating some of those
transects in another multinational program called CLIVAR. Economically it
was impossible to repeat all WOCE. Scientists saw the results of WOCE and
decided which transects would provide more information, so CLIVAR is a
subset of WOCE.
The sun question is hard to answer. It has been pretty cloudy here, so I
could cheat and look on Stellarium, but I am sure that is not what you are
asking for. I think we still have close to 12 hours of sun a week after the
equinox.
And you are absolutely right about the question from March 9. I can now
write an entry describing how scientist figure out the speed. We know the
distance the water masses travel, but we need the time it takes them to
travel. How can we time water masses? To be read soon on the journal...
Thanks again for keeping in touch,
Juan.