The Low Zone - A Science Focus! Inquiry Study
The image below was sent to us by Matt Reuer of Princeton
University as part of an inquiry about the use of SeaWiFS data in
a particular software package. When we looked at this image, we
were immediately intrigued by the sinuous area of low productivity
that appears north of the Antarctic continent [well, everything is
north of the Antarctic continent] between 60 and 90 east latitude.
We called this area "The Low Zone". (Note that the false-color palette
used to generate the image is different than the standard palette
used by the SeaWiFS Project.)
Spatial excerpt from a austral polar projection of
SeaWiFS chlorophyll data for December 2002.
When we then examined SeaWiFS monthly Level 3 images for each of
the months of December since the beginning of the mission, we realized
that this area of low productivity is a definite feature of the
circulation of the Southern Ocean. This initial examination undermined
some of our more fanciful hypotheses, such as the possibility that
this area was the track of a large tabular iceberg. Below are extracts
from the SeaWiFS monthly December images for this area from December
1997 to December 2003. The images are shown half-size; click on each
to see the full-size version.
December 1997
December 1998
December 1999
December 2000
December 2001
December 2002
December 2003
On re-examination, it appears that this feature is due to the
circulation of the Antarctic Circumpolar Current (ACC) around the relatively
shallow Kerguelen Plateau. The island of Kerguelen is the small
and isolated black spot that appears on the far western side of each
image, about halfway between the top and bottom. The island is
definitely associated with nearbly elevated chlorophyll concentrations.
The Inquiry Study: What we would like to do now (March
2004) is to solicit explanations of the cause of the "Low Zone" from
the oceanographic community. This Web page will be released at the
end of February 2004, just before the author goes on a two-week hiatus.
During that time, members of the oceanographic community are invited to
send explanations (and any accompanying imagery or graphics) describing
as simply and completely as possible what is happening here. As part of
the explanation, it would be interesting to know:
- what are the basics of the interaction between physical oceanography (ocean circulation) and biological activity in this region?
- is there a specific reason that "The Low Zone" appears particularly well-defined in December 2002, and is significantly less visible in
other years, such as December 2000?
- why are the chlorophyll concentrations so much higher and the productive region larger in December 2003?
- are there any other areas in the world ocean where a similar phenomenon occurs, or is this area unique?
March update:
Contributions to "The Low Zone" were gratefully received from
Michel Rixen (NATO/SACLANT Undersea Research Centre), Johann
Lutjeharms (Department of Oceanography, University of Cape Town),
and Raymond Pollard (Southampton Oceanography Centre).
Rixen indicated that the "Low Zone" appears to be a frontal
instability that is generated by horizontal pressure gradients within
the ACC system. Instabilities generate mesoscale fronts that provide
nutrients to the surface, and which therefore generate increased
primary production. This mechanism is likely responsible for the
generation of the persistent "downstream" elevated region of
productivity, i.e., the region to the east of Kerguelen that extends
to an area south of western Australia.
Lutjeharms comments on the one of the primary features of the
biological system in the Southern Ocean, which Pollard expanded upon.
Lutjeharms and colleagues have been conducting research in the Prince
Edward Islands, which are located to the west of the Kerguelen and
Crozet archipelagoes. Productivity here, as well as in the oceanic
waters near Kerguelen, is limited by the availability of iron. This
makes the general Southern Ocean a "high nutrient low chlorophyll
(HNLC)" region, as noted by Pollard. Lutjeharms suggests, noting
that more research is required, that the Prince Edward Islands and
Kerguelen may generate "island effect" productivity by providing a
source of iron to the adjacent waters. Lutjeharms notes that one
possible source of iron is the fresh water runoff, which passes
through the droppings of millions of seabirds, thus possibly supplying
iron and other nutrients in a low-salinity buoyant plume of water
that would stay near the surface and enhance productivity.
Pollard provided additional information on the characteristic HNLC nature of
Southern Ocean waters. The "Low Zone" is not unique; other areas (such as the
Bellingshausen Sea) are also characterized by low productivity. Thus, the "Low
Zone" is an area where the low productivity waters around Antarctica are being
transported northeastward in the ACC system, and the low chlorophyll
concentration contrasts markedly with the higher productivity waters north of
them. Therefore, like Lutjeharms, Pollard indicates that the focus of research
is on the generation of the higher productivity regions near the islands.
Pollard's preferred explanation, which will be investigated by two
upcoming research expeditions (see below) is that the iron is derived
from the seafloor, brought to the surface in shallow waters due to
turbulent mixing processes in the water column. (Interestingly, this
is the same explanation for the extended plume of productivity near
the Galapagos archipelago in the tropical Pacific Ocean.) The "Low
Zone" is an oceanic region where the local bathymetry - otherwise known
as the depth of the water - is too deep to bring iron to the surface,
according to Pollard.
Now a bit more information on the currents around Kerguelen:
NOAA Technical Memorandum NMFS-AFSC-6 describes a survey of
cetacean habitat in the Southern Ocean. The southeast edge of the
Kerguelen Plateau was a popular area for both sperm and minke
whales, and 50% of humpback whale sightings were also associated
with the Kerguelen Plateau. The underlying physical oceanography
which makes this region popular with cetaceans is due to the
interaction of the Southern Front of the ACC and the southern
water mass boundary [Southern Boundary] of the ACC. (The Southern
Front is at the surface, while the southern water mass boundary is
subsurface.) In this region, the Southern Boundary moves to
higher latitudes, and causes warm, nutrient rich waters of the
Upper Circumpolar Deep Water (UCDW) to reach the surface mixed
layer, partly due to interaction with the Kerguelen Plateau. The
result is increased productivity by phytoplankton (diatoms), which
are consumed by krill (euphausid zooplankton), which are eaten
by whales -- and thus the Kerguelen Plateau is where the whales are.
New links:
We aren't done yet; we would appreciate additional information and
assessment of these ideas. Please submit contributions to the email
address
ocean@daac.gsfc.nasa.gov. To aid processing
of the contributions, please put "The Low Zone" in the Subject: line.
We look forward to contributions from anyone who wishes to participate
in our "inquiry study".
Kerguelen Island
Kerguelen Island is one of the last great relatively unexplored locations
in the world. This is due partly to its isolation, as it can only be
reached by ship. It is also due to the harsh weather that it is
characteristic of the region. There is a small French research station
on the island, but the island is mainly home to penguins, seals, and
elephant seals. (There is even an elephant seal crossing sign at the
research station.) The author of this article was very
fortunate to visit the research station on Kerguelen
Island during a research and supply cruise in February 1985.
See the Links section below for a few links to Web sites about Kerguelen.
Kerguelen is still an active volcanic province; expeditions to the interior
have discovered fumaroles and hot springs.
Acknowledgements
Acknowledgements to all contributors will be placed here!
Links
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