4.9: Ocean Moorings

Ocean moorings provide a fixed platform to which various oceanographic instruments can be attached to acquire measurements of ocean properties for long periods of time. They excel in sampling temporal scales at an unprecedented resolution—minutes to weeks—and they can be left unattended for months to years at a time.

The foundation of an ocean mooring involves a piece of machinery all too familiar to us. Next time you’re stuck at a railroad crossing, check out the wheels on the train. At several hundred pounds each, they make great anchors. And indeed, that’s what oceanographers use them for. (And just to clear this up, there’s absolutely no truth to the rumor that oceanographers steal wheels from parked trains at night.) Train wheels provide the foundation for a steel cable to which floats (often hollow glass balls) and various oceanographic instruments can be attached. The cable may stretch vertically all the way from the seafloor to the surface, or it may rise from the seafloor to a particular depth of interest. Meteorological instruments attached to the surface floats collect weather data. Below the surface, oceanographic instruments may be attached at various intervals to collect depth-specific information.

While several types of instruments may operate simultaneously at fixed depths, profiler moorings allow instruments to travel up and down a cable and sample the water column in a manner similar to an instrument on a cable on a ship. In either case—fixed or profiling—when an oceanographer wants to retrieve the instruments, they send a secretly coded acoustic signal to the mooring that activates a release mechanism, unhooking the cable from the railroad wheel and permitting the instruments to float to the surface. The wheel remains on the seafloor, forever a part of Davy Jones’s locker.

Moorings have provided some of the longest continuous observations of the ocean on record. Off the coast of Bermuda in the Sargasso Sea (or the Bermuda Triangle, as some people call it), oceanographers with the Ocean Flux Program (OFP) have maintained a mooring since 1978. The OFP mooring hosts sediment traps, devices that collect and store sinking particles. Its data—more than 40 years’ worth—offers invaluable insights on the ocean’s recent past. These data have been especially useful for assessing the effects of climate change in the ocean (e.g., Conte and Weber 2014; Cael et al. 2021).

Because ocean moorings can provide critical meteorological information, the US established in 1967 the National Data Buoy Center (NDBC). The NDBC oversees the operations and data quality control of hundreds of moored buoys deployed in US coastal and oceanic waters across the Atlantic and Pacific. It also provides quality control and dissemination of data from hundreds of partner stations. Its website provides near-real-time data from stations in the Atlantic, Pacific, and Indian Oceans. The NDBC serves a wide range of scientific, military, governmental, private, and civilian needs. Fishers, sailors, and boaters, among others, go there for up-to-date information about the waters in which they travel.

Despite their many advantages, the harsh conditions to which moorings are subjected can cause breakdowns, corrosion, and, on occasion, total loss. Moorings attract fish that bite and fray the cables. Fish attract fishers, who accidentally snag moorings when trawling or intentionally latch onto moorings and steal their parts. A cable snaps or the anchor accidentally releases, and the mooring drifts away, never to be seen again. On some occasions, the release fails to respond to its acoustic signal, in which case the crew must drag an anchor through the water hoping to catch the subsurface cable and bring the mooring to the surface. Rescue missions to find moorings take up costly ship time and fray the nerves of the oceanographers responsible for maintaining them. Nonetheless, the benefits they serve, such as El Niño prediction, tsunami warning, storm warning, and weather forecasting, more than outweigh their costs. We would be blind to what’s happening beneath the ocean’s surface without them.