AUV seeks clues to global climate change in Labrador Sea


Most New Englanders booking a winter cruise opt for tropical destinations like the Caribbean. But this February, Jim Bellingham and his traveling companions headed north instead, to the icy, gale-whipped reaches of the Labrador Sea.

The principal research engineer with MIT Sea Grant's Autonomous Underwater Vehicles (AUV) Laboratory chose the Labrador Sea precisely for its violent seasonal offering -- and the clues they offer toward a clearer understanding of global climate change.

Along with regions in Antarctica and the Mediterranean, the Labrador Sea is the site of deep ocean convection, or the mixing of near-surface water into the ocean's depths. During the summer months, a warm cap of water (5���������C or 6���������C) forms on the surface. In winter, cold Arctic winds chill that relatively salty surface water, making it heavier and causing it to mix with warmer waters below. As winter progresses, that mixed layer of water extends deeper toward the ocean floor.

This vertical mixing of the Labrador Sea -- a critical part of the thermohaline cycle -- delivers cold water to the deep ocean as part of a larger circulation process transferring heat from warm tropical climes northward. As such, the currents are known to play a critical role in regulating climate throughout the world. In fact, many scientists believe that changes in the thermohaline cycle caused a little historical weather event a few years back known as the Ice Age.

Unfortunately, observing the processes by which deep waters are formed is difficult. As a result, many questions remain unanswered about this phenomenon -- which, in turn, leaves gaps in models that predict global climate events.

Up until recently, researchers had to rely on traditional oceanographic tools that generally allowed them to study various processes with vertical measurements. In that method, Dr. Bellingham said, "you lower something down and then you pick it up. What you get is a sense of how things change as a function of depth."

However, what researchers don't get is a sense of horizontal variability. Through a five-year project called the Autonomous Ocean Sampling Network (AOSN), researchers from MIT Sea Grant are using durable, low-cost, easily maneuverable AUVs to navigate and collect data about deep ocean convection over large areas and under harsh conditions. Supported by MIT Sea Grant with major funding from the Office of Naval Research, AOSN is a collaborative effort involving scientists from the Woods Hole Oceanographic Institution (WHOI), the University of Washington, and Chapel Hill, NC-based Electronic Data Consultants (EDC).

In the latest portion of this project, Dr. Bellingham and colleagues left Woods Hole in late January aboard the research vessel R/V Knorr -- the same ship that discovered the Titanic off of the Grand Banks in 1985. Working between major winter storms and 30-foot seas, the researchers successfully deployed their elaborate mooring system, complete with a docking station where the AUV could recharge itself and download data.

The goal of the mission was to leave the AUV and the mooring at sea to collect data while the researchers returned home. From the warmth of their offices, researchers would communicate with the vehicle via a two-way satellite link. The idea, said Dr. Bellingham, is that scientists will be able to "configure surveys responding to specific convection episodes, allowing a real-time reactive presence in a remote, hostile environment."

From hurricanes to deep-ocean convection, most of the ocean's high-energy events are episodic in nature. With several AUVs roving an area and sending data back to shore, scientists would be poised to respond to such events. One application might be to monitor an open-ocean aquaculture site for cold currents of water that can stream in and kill fish. A network of AUVs could also provide information useful for fisheries management, for military purposes, and for studying pollution transport.

During their time at sea, the research team ran numerous successful missions with its latest version of MIT Sea Grant's AUV, Odyssey IIB. "We were measuring simple things such as temperature and salinity, but doing a very accurate job of it," Dr. Bellingham said.

The WHOI team members included Hanumant Singh, Dana Yoerger and Martin Bowen, who worked on the AUV's docking system; Keith von der Heydt, who concentrated on satellite and radio communications; John Kemp, an expert in putting complex moorings into place; and Mark Johnson, who focused on acoustic communications. Mike Feezor (EDC), Albert Bradley (WHOI) and James Bales (MIT Sea Grant) worked on battery charging and power and data transfer; Robert Grieve (MIT Sea Grant) handled vehicle operations; and Brad Moran (MIT Sea Grant) handled vehicle software.

MISSION LANDMARKS

The team chalked up a number of firsts, especially in the realm of communications. Position and scientific measurements were transmitted acoustically by the AUV to the dock, which recorded the data and relayed them by radio to the ship, where scientists monitored the mission's progress. Back on shore, satellite transmissions from the mooring in the Labrador Sea appeared as e-mail on colleagues' computers. Satellites relayed e-mail from scientists back to the mooring.

During the mission, the AUV demonstrated the first successful homing on a deep (500-meter) dock. In addition, vehicle measurements of the spatial variability and dynamics of the mixed layer highlighted a much more active process than is imaged from shipboard sensors.

From an operational perspective, the growing maturity of the systems was evident as well, with more than 20 vehicle missions run from the Knorr in winds up to 35 knots. Despite this progress, the system was not ready to be left behind in the Labrador Sea unattended. As a result, the researchers decided to pull up the mooring and bring the AUV home with them -- a tough decision, says Bellingham, because it was "one of the first times I wanted to come back with fewer vehicles."

Back from the Labrador Sea, the team continues to work on docking. With growing confidence in their techniques, they plan more scientific operations with AUVs for the future, including joint operations with the Harvard-led Littoral Ocean Observation and Predictions System Project in Massachusetts Bay. Even as these systems are perfected, the team is creating systems for the next generation of vehicles. A Bellingham-led project for monitoring the Arctic Ocean with long-range AUVs is being kicked off this month at MIT.

Bellingham noted that there is "no one silver bullet for making measurements in the ocean." Instead, he described a developing arsenal that includes many AUVs with different energy, payload, propulsion and endurance systems. What the various systems share is the ability to operate for extended periods without a person nearby.

"Our need for getting data from the ocean is rising exponentially for all kinds of monitoring reasons, while the dollars that are going into those systems are plummeting," he said. As a result, the wave of the future will, by necessity, include more and more of these types of economical and robust systems.

A version of this article appeared in MIT Tech Talk on May 13, 1998.


Topics: Earth and atmospheric sciences, Energy, Environment, Environment and energy

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