|
By Peter Rejcek
Most people who cross Drake Passage by ship are eager
for the two-day journey to be over as quickly as possible.
The ocean passage splitting the tips of South America
and the Antarctic Peninsula is infamously rough, turning
even hardened seafarers green around the gills.
But a team of oceanographers will spend more than three
weeks in the Drake in November and December to learn more
about the world’s largest ocean current, the Antarctic
Circumpolar Current (ACC) . Drake Passage, the chokepoint
where the current narrows, is the ideal place to study
the ACC, according to Teresa Chereskin , a principal investigator
for the project and chief scientist for the cruise aboard
the RVIB Nathaniel B. Palmer .
“We’re there long enough so that we see all
kinds of weather,” said Chereskin, a researcher with
Scripps Institution of Oceanography at the University
of California in San Diego . “Last year, we were
really blessed in the first few days because it was ‘Drake
Lake,’ and you don’t get that very much.”
A year ago at about the same time, Chereskin and her
team, including principal investigators Kathleen Donohue
and Randolph Watts , both with the Graduate School of
Oceanography at the University of Rhode Island (URI) ,
deployed an array of instruments in the passage to take
data on a number of ocean properties — from acoustic
travel times in the water column, which can be related
to density and temperature, to ocean current and pressure.
This year they will collect their first data via telemetry
from the instruments.
.“Our observations will help us understand why the
current is there, what forces it, and what controls its
variability,” explained Donohue, URI associate research
professor.
The ACC is a set of sharp density fronts and their associated
currents that spirals around Antarctica from west to east,
linking the world’s three major bodies of water —
the Atlantic, Pacific and Indian oceans. “In that
sense, you can think of it as a major conduit between
the [ocean] basins. That’s one important reason to
study it,” Donohue noted.
The ACC formed about 41 million years ago when Drake
Passage between South America and Antarctica opened. Some
scientists believe the birth of the ACC eventually led
to the formation of major ice sheets on the world’s
southernmost continent, though the theory is still widely
debated. [See related story: Open case]
However, oceans and climate are indisputably linked.
For example, the ocean current that brushes across Western
Europe helps keep temperatures more temperate there than
its high latitude would otherwise suggest. Similarly,
the ACC helps insulate Antarctica.
But climate change has come to Antarctica, particularly
West Antarctica, where glaciers are moving faster and
discharging more ice into the ocean and ice shelves are
collapsing. The changes are also whipping up the west
winds that drive the ACC around the continent.
“We anticipate that the Antarctic Circumpolar Current
may be highly sensitive to climate change,” Donohue
said. “It is driven by winds, and since the winds
appear to be changing as a result of climate change, the
current may change as well.”
Instruments to monitor ocean passage for five-year
project
But to observe those changes — and provide data to
model ocean processes for predictive purposes — the
oceanographers need some basic information about the ACC.
Last year aboard the Palmer, the ship’s crew and
the scientists deployed 38 instruments called current
and pressure measuring inverted echo sounders (CPIES)
in a line across the passage, as well as in a more densely
packed array to map circulation and eddy patterns.
“In the array, we think we have the mapping skill
to do two-dimensional maps, analogous to synoptic weather
maps,” Chereskin said.
The devices use sound to determine water temperature.
Sound travels faster in warmer water than colder water,
so by sending a short sound through the water and listening
for how long it travels, the researchers can infer the
average temperature of the water.
The inverted echo sounder and pressure sensor part of
the instrument is anchored to the bottom of the ocean
by a round steel frame and two 50-pound weights. Inside
a white plastic case is a glass sphere with all the electronics,
a computer and batteries. Tethered about 50 meters above
that is another device that measures the water current
directly.
Chereskin explained that the devices were strategically
placed based on Doppler sonar data gleaned from the ARSV
Laurence M. Gould , which travels across the Drake numerous
times each year to transport people and material between
Punta Arenas, Chile, and the U.S. Antarctic Program’s
Palmer Station .
“We have a decade-long time series of ocean currents
that are just taken from the Gould when it transits across
Drake Passage to supply Palmer Station,” she said.
“The idea of having the line is to give us good enough
resolution that we can resolve the transfer [of properties]
in the individual jets that comprise the ACC. It’s
not just a single broad current.”
In Drake Passage, the scientists will actually study
three distinct jets or fronts of the ACC. The Subantarctic
Front is the strongest of the trio and meanders farthest
north as it hugs the Patagonian continental shelf. The
middle one is the Polar Front and the most southerly is
the Southern ACC Front.
The oceanographers are also interested in how properties
such as heat get transferred across the ACC from north
to south and vice versa. “Is the current a barrier
or a blender?” Chereskin said.
The scientists have five years to answer that and many
other questions for the International Polar Year project.
International partners include the French and the British.
This is the second year of the National Science Foundation
-funded experiment.
“This is going to be the first time to revisit the
instruments,” Chereskin said. “We’re bringing
some spares in case any need to be replaced, and we’re
getting the first year of data via telemetry. … It’s
a hard place to keep equipment going.”
The instruments are fairly easy to deploy, Chereskin
said, even in rough weather. But to calibrate the instruments,
the ship must first conduct a conductivity, temperature
and depth (CTD) measurement for each site. That requires
steadier seas.
“We were really lucky last year. The weather held
out really well for us … it does change really fast
in Drake Passage,” Donohue said. “It was a great
experience for us, and the captain and crew of the Palmer,
as well as Raytheon technical team, were crucial to the
success of the deployment cruise.”
The team will retrieve the instruments during the last
year of the experiment. “The other thing we’re
hoping to do at the end of this experiment is to recommend
some strategies for the long-term monitoring of the current,”
Donohue added.
-
Antarctic
Sun
|
