Reaching a global scale

In the late 1980s, interest in the role of ocean circulation in the regulation of earth’s climate was at the forefront of oceanography. The result was the international World Ocean Circulation Experiment (WOCE), which in addition to satellite derived data and direct observations required global coverage of the World Ocean with subsurface floats.

ALACE floats

Acoustically tracking floats lacked the global scope needed to meet these new demands. Thus, Russ Davis and Doug Webb responded by developing the Autonomous Lagrangian Circulation Explorer (ALACE; Davis et al., 1992) . The pioneering feature of the ALACE float was that the otherwise neutrally buoyant float was able to adjust its buoyancy through inflating and deflating an external bladder. This allowed the float to repeatedly surface (typically every 10 days) and then return to depth and drift with subsurface ocean currents. When at the surface the float was able to be tracked by the Argos satellite system and transmit back data. The cycle would repeat for as long as the float’s batteries lasted, which could be several years.

The first ALACE floats to contribute to WOCE were deployed in the Drake Passage in 1990 and were set to drift at 1000 m depth. In total 1110 ALACE floats were deployed during WOCE. During the 1990s, the ALACE floats started to carry CTD sensors (P-ALACE) and transmit temperature and salinity depth profiles each time they surfaced (Sherman 1993).

The ALACE design had some limitations, however. While the ALACE float is able to adjust its buoyancy to surface in an hour or two at a vertical velocity of 10-20 cm/s or sink at a rate of about 5 cm/s, it cannot profile downwards from its drift depth. The float’s energy efficiency was compromised by the high pressure pump used with the external bladder to alter buoyancy and the internal oil bladder (the reservoir used to fill the external bladder) ruptured at high accelerations and thus the float could not be deployed from aircraft or from ships moving at speed.

MARVOR and PROVOR floats

The French MARVOR float was first deployed in 1994. A similar external bladder mechanism to that of the ALACE float enables the MARVOR to surface at regular intervals. Unlike the ALACE floats, however, MARVOR floats are tracked acoustically in the same way as RAFOS floats except that, rather than the data being obtained at the end of the float's mission, MARVOR floats transmit their recorded signal arrival times each time they surface (Ollitrault et al 1994). A profiling version of the MARVOR float was later developed, named PROVOR.

 

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