D2 In Situ Profiles - Floats and Anchors

Topic

In situ profiles provide direct measurements of temperature, salinity, and other properties throughout the entire water column, free from the limitations of satellite remote sensing, which can only observe the ocean surface. The backbone of this in situ observation system is a global network of nearly four thousand autonomous drifting floats; these cover the oceans by profiling temperature and salinity from the surface down to a depth of two thousand meters—repeating the profile every ten days—with a precision of just two-thousandths of a degree for temperature and one-hundredth of a salinity unit. An extension of this network extends coverage to a depth of six thousand meters, making this collective array of autonomous floats the most comprehensive in situ ocean observation system in existence.

Each of these floats operates via controlled buoyancy: an internal bladder changes volume, altering the float's density relative to the surrounding water and thereby controlling whether it rises or sinks. The full cycle begins when the float descends to its drift depth—near one thousand meters—where it drifts for ten days; it then descends further to two thousand meters before ascending back to the surface, measuring temperature and salinity throughout the ascent. Upon reaching the surface, the float transmits its data via satellite before sinking again to repeat the cycle.

Unlike these floats, which control only their own density to move vertically, ocean gliders are autonomous platforms that combine small buoyancy changes with fixed wings; consequently, rather than moving in a straight line up or down, they follow a sawtooth trajectory while moving laterally. That combination of buoyancy and wings allows them to cover between thirty and fifty kilometers per day, and they can maintain that pace for months on end.

Sensor-equipped moorings, unlike floats and gliders, do not move at all: they are structures anchored to the ocean floor, featuring their own floats that keep the line under vertical tension, and they carry sensors for temperature, salinity, currents, and other variables at fixed depths. It is precisely this immobility that makes sensor-equipped moorings indispensable for building continuous, long-term time series—such as those used to measure the weakening of the Atlantic Meridional Overturning Circulation or to closely monitor the moored buoy systems that track El Niño and La Niña in the equatorial Pacific.

Finally, surface drifter buoys are neither anchored nor submerged; they drift freely with the ocean's surface current, measuring sea surface temperature, atmospheric pressure, and their own drift speed—a speed that serves as a direct proxy for the surface current carrying them. This allows these buoys to map the planet's surface currents in their entirety using minimal instrumentation. Thus, from floats moving up and down the water column, gliders zigzagging through the water, and moorings monitoring a single point for years, to buoys carried by the current, *in situ* profiling completes—point by point and depth by depth—the picture that satellite remote sensing can only view from the surface.

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