Old, New, Borrowed, and Blue

by David Malmquist

The second cruise of the 2004-5 IVARS season was originally scheduled for the U.S. Coast Guard Cutter Polar Star, but that vessel damaged its propeller during the first IVARS cruise in December. We were thus re-routed to join a previously scheduled research leg aboard the R/V Nathaniel B. Palmer. One good thing about our borrowed time aboard the Palmer is that it's given us the opportunity to interact with scientists from other institutions working on separate projects. We've gotten to know a great group of people from the Scripps Institute of Oceanography, Texas A&M University, Cal Tech, Oregon State University, the University of Southern Mississippi, and the University of Wisconsin. We've helped each other collect samples and analyze data, and in the process learned a great deal about each other's research. The non-IVARS projects include a mooring study designed to investigate the movement of Antarctic Bottom Water, a seafloor-mapping study to help reconstruct the tectonic history of the Southern Ocean, and an acoustic study of blue whale vocalization. Dispatches over the next few days will describe these projects.

AnSlope

Physical processes in the Southern Ocean around Antarctica play a key role in driving the global system of deep-ocean currents. Particularly important are the processes that help form and transport Antarctic Bottom Water (AABW). This body of cold, salty water forms along the Antarctic continental shelf and subsequently cascades down the continental slope. AABW interacts with other water masses to help drive the thermohaline component of the global ocean circulation. The global ocean circulation in turn plays a significant role in worldwide climate and predicted climate change.

The AnSlope project, headed by Drs. Arnold Gordon of Lamont-Doherty and Alex Orsi of Texas A&M, is one of two on-going studies to better understand how shelf water moves down the continental slope, thus contributing to Antarctic Bottom Water. (The other study is the ARCHES project in the Weddell Sea.) AnSlope researchers deployed a dozen moorings off Cape Adare in April 2003. They retrieved those moorings last year, consolidated their instruments onto six moorings, and re-deployed those for the final year of the project. Current Meter

Three members of the AnSlope team are aboard the Palmer. Christina Stover is one of Dr. Orsi's graduate students at Texas A&M. She's here to begin downloading and analyzing AnSlope data for her dissertation. Jay Simpkins and Kathryn Brooksforce are mooring technicians affiliated with Oregon State University. They have more than 50 years combined experience in deploying and retrieving ocean equipment and are here to help collect the remaining six AnSlope moorings.

The water along Antarctica's continental shelf is cold, salty, and thus very dense. It's chilled by its exposure to the area's frigid air and constant winds, and acquires its elevated salinity through the formation of sea ice, most notably in the Ross and Weddell seas. Ice formation drives salt into the underlying seawater, producing a brine. This process is promoted by the katabatic winds that flow off the Antarctic ice cap, consistently pushing already formed sea ice offshore and subjecting new leads of open water to freezing conditions.

Because it's more dense than its surroundings, Antarctic shelf water tends to flow down the continental slope into neighboring basins. The goal of the AnSlope project is to measure the rate of this flow, determine its chemical and physical properties, and find any bathymetric features that might channel or disperse it.

The six moorings we're here to recover were deployed in an area previously identified as a likely spot for down-slope flow—the Drygalski Trough of the western Ross Sea. This is a submarine canyon that runs from southwest to northeast across the continental slope just off of Cape Adare. The trough lies just seaward of the Drygalski Ice Tongue, a floating projection of the David Glacier.

Each of the moorings holds anywhere from 1 to 5 current meters and mini-CTDs, based on water depth. One of the moorings also employs an ADCP (Acoustic Doppler Current Profiler). The current meters look and function like underwater wind gauges, with a broad tail to align the meter with the current and a rotating turbine to measure the current's velocity. The mini-CTDs collect data on salinity, temperature, and depth, just like the larger model we deployed at the IVARS stations.

The ADCP mimics the sonar system used by marine mammals. The unit emits sound pulses that reflect off of waterborne particles, and then listens for the resulting echoes. An on board computer translates the returned signals into a two-dimensional representation of current speed and direction throughout the water column.

AnSlope researchers have so far identified two modes by which shelf water descends to the deep ocean. In one mode, a distinct current cascades rapidly down the Drygalski Trough, leaving a strong chemical and physical imprint on Antarctic Bottom Water. A second current flows more slowly and mixes with water from the Antarctic Circumpolar Current.

Antarctic Bottom Water is the largest contributor to the global pool of oxygen- and nutrient-rich bottom waters, which later rise to ventilate and nourish the world ocean. (The second largest contributor is the North Atlantic Deep Water, which forms by cooling and sinking of the Gulf Stream near Iceland). Some oceanographic models predict significant changes in ocean circulation as global climate warms. Because Antarctic Bottom Water plays such a crucial role in that circulation, a better understanding of its formation will help refine global models.