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A brief history of float developments
W. John Gould - WOCE IPO

(Note - The references cited here are far from a comprehensive list and only to identify new developments or activities).
The concept of a neutrally buoyant float to measure subsurface ocean currents developed simultaneously and independently in the mid-1950s by Henry Stommel (Stommel, 1955) in the USA and by John Swallow (Swallow,1955) in the UK. The first floats were built and tested by Swallow and consisted of pressure cases made of aluminium scaffold tube that were less compressible than sea water and that were tracked by obtaining bearings from an attendant ship on the floats' free-running 10kHz sound source.

These floats were used extensively to investigate features of the deep ocean circulation and revealed for the first time the mesoscale currents that are now known to populate the open ocean. The floats remained virtually unchanged until two developments were made in the late 1960s/early 1970s for the Mid-Ocean Dynamics Experiment (MODE) (MODE Group, 1978). A transponding version of the float (still tracked from a ship but operating at 3 - 4kHz) was used to extend detection ranges up to ca 50km. (Swallow et al 1974). A low frequency version (~ 200 Hz) was developed to remove the need for a tracking ship. These floats transmitted via the SOFAR channel (using Stommel's initial idea) to shore-based US-Navy listening stations in the Western Atlantic (Rossby and Webb, 1971). The geographical constraints imposed by the use of these listening stations was later relaxed by the development of moored Autonomous Listening Stations (ALS) (Bradley, 1978) . SOFAR floats were used primarily to explore the circulation of the western N Atlantic (Owens, 1991).

In the 1980s further developments were made. The sound sources were moved from the floats to moorings which transmitted to acoustic receivers on the floats. The signal arrival times used for float tracking were downloaded to the Argos satellite system when the float surfaced at the end of its submerged trajectory. These were known as RAFOS floats (the inverse of SOFAR) (Rossby, Dorson and Fontaine, 1986). Floats were also built that could follow density rather than pressure surfaces by the addition of a compressee (Rossby, Levine and Connors, 1985).

Among derivatives of the SOFAR and RAFOS floats were floats that could measure vertical water velocities (Voorhis 1970), could cycle between temperature surfaces (Price 1996) and measure electromagnetic fields (Sanford et al 1995). Extensive use of both SOFAR and RAFOS floats was made primarily by research groups in the USA, France (Ollitrault,1994) and Germany (Zenk et al 1992).

In the late 1980s plans for the World Ocean Circulation Experiment (WOCE) required global coverage with subsurface floats. This precluded acoustic tracking and resulted in the development of the Autonomous Lagrangian Circulation Explorer (ALACE), (Davis et al 1996). The ALACE is a neutrally buoyant float that surfaces at regular intervals by inflating an external bladder (and changing its density), transmits temperature/pressure data and is positioned by the Argos satellite system and then returns to its operating depth. A similar buoyancy-change mechanism is used in the ALFOS floats (of which the French Marvor is one, (Ollitrault et al 1994) in which the float is tracked acoustically between surfacings just as is the RAFOS float.

An ingenious companion of the ALACE is the SLOCUM (Curtin et al 1993) that derives the energy needed for buoyancy change from the temperature difference between the ocean surface and the ambient temperature at its operating depth. A prototype has been built and deployed in the Sargasso Sea.

During the 1990s the ALACE floats started to carry CTD sensors and transmit profiles of T and S each time they surfaced (Sherman 1993). Such floats are the mechanism by which the global array of 3000 floats (the so-called Argo array) will provide upper ocean T and S information from the global into the 21st century (Argo Science Team, 1998).

Remaining challenges in the early 21st century are

  • to develop improved salinity sensors (or at least the means of determining calibrations shifts),
  • using low earth orbit satellites to provide greater data bandwidth (leading to more detailed profile information and a shorter time at the ocean surface) and two way communication with floats,
  • improving float lifetimes and reliability,
  • developing other autonomous sensors that could, for instance, make biogeochemical measurements.

It is interesting to note that an observing system in many ways similar to Argo was suggested by Henry Stommel in 1989. He envisaged that in 2021 there would be a World Observing System (WOOS), in which a fleet of 1000 neutrally buoyant floats/gliders (called SLOCUMS) would deliver 3000 CTD profiles per day in real time (Stommel 1989). It looks as if we are rapidly converging onto something close to Stommel's vision.

A review of float developments and use is given by Davis and Zenk (2001) and of other techniques for deep ocean current measurement by Gould (2001).

Here is John Swallow assembling a float on RRS Discovery II, watched intently by two matelots and the ship's cat. Click image for enlargement.

Prototype SOFAR float in Woods Hole in the late 1960s. (Photo courtesy of Tom Rossby). The sphere housing the electronics and battery has a diameter of 1m, with the transducer hanging below. See Rossby & Webb 1970. Click image for enlargement.

An autonomous listening station being deployed from RRS Discovery in the mid 1980's. The ALS consists of a 10m hydrophone unit (furthest from the ship) and recording unit (cylinder above the hydrophone). The ALS was deployed at a depth of approximately 1000m on a mooring. Click image for enlargement.


References

Argo Science Team, 1998: On the design and Implementation of Argo. An initial plan for a Global Array of Profiling Floats. ICPO Report No 21, Godae Report No 5. Bureau of Meteorology, Melbourne Australia, 32pp.

Bradley, A., 1978: Autonomous Listening Stations. Polymode News (4) Unpublished Manuscript.

Curtin, T. B., J. G. Bellingham, J. Catipovic and D. Webb, 1993: Autonomous oceanographic sampling networks. Oceanography, 6(3), 86-94.

Davis, R. E., P. D. Killworth and J. R. Blundell, 1996: Comparison of autonomous Lagrangian circulation explorer and fine resolution Antarctic model results in the South Atlantic. Journal of Geophysical Research, 101(C1), 855-884.

Davis, R. E. and W. Zenk, 2001: Subsurface Lagrangian Observations during the 1990s. Chapter 3.2 in Ocean Circulation and Climate - Observing and Modelling the Global Ocean. Academic Press International Geophysics Series Vol 77. Gerold Siedler, John Church and John Gould, Eds. 715pp and plates.

Gould W. J 2001: Direct measurements of subsurface ocean current: a success story. Chapter 10 in Understanding the Oceans. UCL Press London and New York. Margaret Deacon, Tony Rice and Colin Summerhayes. Eds 300pp.

MODE Group, The, 1978: The Mid-Ocean Dynamics Experiment. Deep-Sea Res., 25, 859-910.

Ollitrault, M., 1994: The Topogulf Experiment Lagrangian Data. Repères Océan No 7. Editions de l'IFREMER, Centre de Brest, 622pp.

Ollitrault, M., G. Loaëc and C. Dumortier, 1994: MARVOR: a multi-cycle RAFOS float. Sea Technology, 35(2), 39-40, 43-44.

Owens, W. B., 1991: A statistical description of the mean circulation and eddy variability in the northwestern Atlantic using SOFAR floats. Progress in Oceanography, 28(3), 257-303.

Price, J. F., 1996: Bobber floats measure currents' vertical component in the Subduction Experiment. Oceanus, 39(1), p.26.

Rossby, T. and D. Webb, 1970: Observing abyssal motions by tracking Swallow Floats in the SOFAR Channel. Deep-sea Research, 17, 359 - 365.

Rossby, T. and D. Webb, 1971: The four month drift of a Swallow float. Deep-sea Research, 18, 1035-1039.

Rossby, T., D. Dorson and J. Fontaine 1986: The RAFOS system. Journal of Atmospheric and Oceanic Technology, 3(4), 672-679.

Rossby, H. T., E. R. Levine and D. N. Connors 1985: The isopycnal swallow float - a simple device for tracking water parcels in the ocean. In Essays in Oceanography: a tribute to John Swallow. Progress in Oceanography, 14, 511-525.

Sanford, T. B., R. G. Drever and J. H. Dunlap, 1995: Barotropic ocean velocity observations from an electric field float, a modified RAFOS float. pp.163-168 in, Proceedings of the IEEE Fifth Working Conference on Current Measurement, February 7-9, 1995, St. Petersburg, Florida. (ed.S.P.Anderson et al). Taunton, MA: William S. Sullwold Publishing. 270pp. & appendices.

Sherman, J., 1993: Profiling ALACE instruments. La Jolla, CA: Scripps Institution of Oceanography. 28pp. (Unpublished manuscript)

Stommel, H., 1955: Direct measurement of subsurface currents. Deep-sea Research, 2(4), 284-285

Stommel. H., 1989: The SLOCUM Mission. Oceanography - the official journal of the Oceanography Society, Vol 2 No.1

Swallow, J. C., 1955: A neutral-buoyancy float for measuring deep currents. Deep-sea Research, 3(1), 93-104

Swallow, J. C., B. S. McCartney and N. W. Millard, 1974: The Minimode float tracking system. Deep-sea Research, 21, 573-595.

Voorhis, A. D., 1970: Measuring vertical vorticity in the sea. Deep-sea Research, 17(6), 1019-1023.

Zenk, W., K. S. Tokos and O. Boebel, 1992: New observations of eddy movement south of the Tejo Plateau. Geophysical Research Letters, 19(24), 2389-2392.

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