Gliders

This month we will be talking about one of the instrument systems that oceanographers use to gather critical data about the ocean’s interior: gliders. A glider is an unmanned underwater autonomous vehicle that uses buoyancy to move through the water column. Gliders are about two meters (or six feet) long and are usually painted bright yellow. At Texas A&M University, researchers use a glider manufactured by Teledyne Webb Research known as Slocum, named after Joshua Slocum, the first man to single-handedly sail around the world. The Texas A&M fleet includes four gliders. Two gliders are shelf gliders, capable of diving to 200 meters, and are named Reveille and Howdy. The other 2 are deep gliders, diving to a depth of up to 1,000 meters, named Sverdrup and Stommel.

The gliders move using the physics of buoyancy. A pump within the glider moves back forth, changing the position of a diaphragm within the glider, which alters the glider’s volume and, consequently, the glider’s buoyancy. This causes the glider to sink and rise through the water, while wings on either side propel the glider forward.

Gliders can be deployed in coastal or offshore waters, and the duration of a specific mission depends on the instruments the glider is carrying and the goal of scientists deploying the glider. Alkaline batteries can power a glider for up to a month and gliders with lithium batteries can operate for as long as three months. Gliders measure several oceanic parameters as they move through the water column, giving scientists a spatial or temporal representation of the path they travel. If encountered, gliders should never be touched or tampered with by civilians while deployed. Gliders may cause damage or be damaged by collisions with other vessels.

Gliders are an incredible tool that oceanographers use to study the ocean’s interior. These autonomous underwater vehicles are an integral part of oceanographic research, and grant scientists insight into processes and changes occurring in the ocean. How does the process of using a glider work? It all begins with a need. Scientists identify the information they need to answer questions about the ocean, such as: What are dissolved oxygen concentrations like in a certain area? What is the temperature of the deep ocean? And a variety of other fields of interest relating to fisheries, climate, biology, chemistry, and ocean physics.

Scientists may reach out to state and federal agencies for funding for glider missions, explaining what gliders are, how they operate, and how the information they gather will benefit not only scientists but society at large. Researchers then design mission requirements for a glider deployment. The first and perhaps most important part of a glider deployment is ballasting, which adjusts the weight and balance of a glider to ensure the glider ascends and descends properly. Gliders are ballasted according to the density of the water they will operate in, which is most strongly affected by salinity. If ballasted incorrectly, a glider may not be able to sink, or sink too quickly and be unable to surface.

The next pre-deployment test is the H-moment test, or testing of the glider’s stability in water. After this, the glider is loaded on to a boat and deployed in the ocean. From there it will be operated remotely by glider pilots on land who can communicate with the glider each time it surfaces via satellite networks. When a glider’s mission is over, it is recovered from the water by boat by tracking its location.

Gliders are incredibly powerful and versatile instrument systems that allow oceanographers to probe the ocean remotely for long periods of time and over great distances. Sometimes, as with many scientific instruments, there are problems scientists must solve in the middle of a mission or experiment. Gliders move across the open ocean, meaning they are subject to a harsh and unpredictable environment with many potential dangers.

Gliders move passively, meaning they do not usually use any motorized power to propel themselves through the water. When the direction ocean currents are flowing changes, a glider must move with it, and large changes can potentially throw a glider far off its original intended course. This may lead to gliders washing ashore in unexpected locations. To prevent this, gliders may be outfitted with propellers that use battery power to help them escape from a strong current. Since battery life often determines the length of a glider mission, using the propeller is a last resort.

Correct ballasting of a glider is critically important to its ability to function. Ballast measurements before deployment are set to the salinity that is expected at the mission sites. However, if that salinity somehow changes, perhaps due to freshwater runoff from a river, the glider may have difficulty coming to the surface or sinking. Another problem gliders face is ocean animals. Remoras are fishes that have modified fins that operate like a suction cup, allowing them to attach to large cruising animals in the ocean like sharks. Remoras can attach to gliders, as well, affecting the weight and making them unable to move correctly. Nylon mesh sleeves are often fitted over gliders to prevent remoras from attaching.

Gliders are instrument systems capable of covering incredible distances in the ocean and taking measurements as they travel. What kinds of things do they measure? Scientists have many questions about the composition of the ocean, and how that relates to oceanic and biological phenomena. As such, the gliders are formatted with a suite of instruments to take different kinds of measurements based on a scientist’s questions and needs. These measurements include data regarding temperature, salinity, dissolved oxygen concentrations, chlorophyll concentrations, or dissolved organic and inorganic carbon.

Gliders can also be outfitted to gather data on nutrients, hydrocarbons (for oil and gas products), carbon dioxide, and even sound. This information helps scientists make connections between what is going on in the oceans, or how the oceans are changing over time or responding to pollution. Deploying gliders regularly each year allows scientists to build a special dataset called a timeseries which allows scientists to monitor oceanic conditions and track responses to climate change, pollution, or conservation efforts. Scientists at Texas A&M University use this data to understand oceanic phenomena such as hurricane intensification, oil spill movement, harmful algal blooms, hypoxia (or low oxygen concentrations, usually in specific areas), and other processes that may be relevant to society and industries. By studying these events and the factors that drive them, oceanographers can begin to make predictions and recommendations about how best to conserve, protect, and utilize oceanic resources.