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Modern Drift Bottles Measure Ocean Circulation |
| Physical Oceanography |
In recent decades we have learned that, like the atmosphere, the ocean's patterns of motion include time variations of all sizes (the analogs of wind gusts to weather systems) and a general circulation (similar to, but more complex than, the atmospheric patterns of westerlies, trade winds, etc.). The mean motion is called the ocean general circulation (OGC). The OGC dominates ocean transport of things like heat and salt that affect the global climate and for the last 150 years oceanographers have attempted to measure and theoretically predict it. Remarkably, the data used for measurement have been mainly of seawater temperature and salinity (hydrography), not direct velocity measurements. Hydrography directly measures the shear of horizontal velocity between different depths but inference of absolute velocity is indirect.
Recently, satellite navigation and communication have made it feasible to directly measure the OGC over wide areas. This is done using neutrally buoyant floats (developed at SIO) that cycle up to 150 times between the surface, where they are located and transmit data, and the deep ocean, where they follow the ocean flow. Since only the positions of diving and surfacing are known, these floats give information rather like drift bottles with electronic messages. While surfacing they also measure profiles of temperature and salinity. The figure shows the distribution of measurements in the South Pacific. Each arrow represents the net displacement over 25 days of floats drifting at 900 m depth. The spaces between arrows are the surface motion. An analysis of the OGC from these data appeared in the Journal of Geophysical Research.
Floats of this sort are now gathering data in the Indian Ocean, Subpolar North Atlantic, Southern Ocean and tropical Pacific and this data is available for student research. These autonomous floats are good platforms for a range of physical and biological measurements and one student is now analyzing results from the Labrador Sea where the vertical velocity caused by ocean deep convection was measured. An offshoot of the float development is an underwater glider, now under development, that can autonomously travel 2000 km or maintain station while profiling to 1000 m. This could be the platform for a student initiated measurement program.
Scripps faculty using floats include:
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Physical Oceanography at Scripps