Floats and drifters
by Elizabeth Simons
Drifters float and floats sink. This is a pretty apt line to describe the difference between these two instruments used to autonomously measure aspects of the world's oceans. Both drifters and floats are used to estimate velocity in the oceans as well as temperature and usually salinity (a catch-all term for the amount of 'stuff' in the water).
Drifters are surface dwelling instruments that are unmoored, also known as drifting buoys, and today's models are considered high tech versions of a message in a bottle. These drifters consist of a surface buoy, where the battery, electronics, and communications are housed, and a drogue, which 'anchors' the buoy to ocean currents in the surface layers. Drifters have been an integral part of oceanographic research since the inception of oceanography and today play a role in the global coverage and understanding of upper ocean dynamics. The Global Drifter Program (GDP) is a joint venture to maintain a global 5 x 5 array in order to provide high-density observations of currents in the mixed layer, sea surface temperature, and in some cases, salinity, atmospheric pressure, and winds. Drifters are also released in higher density in areas of interest by smaller projects; there have been drifter releases in the Arctic, Gulf of Mexico, Mediterranean, and countless other places around the globe. For more information on drifters, the GDP, and to see live positions of drifters currently in the oceans go to: http://www.aoml.noaa.gov/phod/dac/gdp_drifter.php
Floats are similar to drifters, in that they are autonomous, unmoored instruments meant to measure currents, temperature, and salinity. Floats differ however, in that they profile the deeper waters of the ocean. Most floats are set to drift at a nominal pressure between 500 and 2000m deep. In the case of the Argo program, floats drift at 1000m for roughly 9 days before descending another 1000m reaching 2000m deep where the float then starts profiling the water column vertically for temperature and salinity as it comes back to the surface where it then relays its data to satellites overhead. This cycle repeats until the float battery runs out of power, anywhere from 3 -7 years. The Argo program is a global array of floats that measure the upper 2000m of the ocean. Since standard measurements on ships is time consuming and can be quite expensive, the advent of profiling floats have helped further the understanding of what goes on under the surface layer of the ocean.
Refinements to the floats are constantly being made, sensors for other water properties (pH, alkalinity, oxygen, carbon, etc.) are being added, and deeper profiling floats are currently being tested. For more information about the different types of floats currently in use as well as about the Argo program go to: www.argo.ucsd.edu