ARGO Floats

In 2007 and 2008, Cape Farewell deployed two Argo Floats 'Arty-Bob' and 'Diskovery Bob', on behalf of ARGO UK, as part of an international programme involving 30 countries, to understand the world's oceans and thermohaline circulation. This programme has (as of 18 January 2011) a global array of 3256 active free-drifting battery-powered autonomous floats that measure the temperature and salinity of the upper 2000m of the world's oceans. Each float is programmed to sink to a specific depth to target and drift with a specific ocean current. In the case of the Cape Farewell floats, these currents were the West Greenland Current and the gyre in the Greenland Sea.

Every 10 days, each float rises to the surface over the course of approximately 6 hours, measuring the temperature and salinity profile of the water column as it goes. On reaching the surface, the float transmits its exact location and all the acquired temperature and salinity data, before sinking back to its drifting depth to once more follow the currents. The floats generally perform this cycle ~150 times before becoming inactive.

The first float deployed (Arty-Bob) was an APEX model, measuring 130cm in height, with a diameter of 17cm. The second float (Diskovery Bob) was a Provor model, measuring 170cm in height and 17cm diameter, having a maximum lifespan of 170 cycles. The profiles gathered by both these floats will help scientists understand the complexities of ocean circulation. Arty Bob managed to explore the Greenland Sea (Figure 1) for well over a year (520 days) with 52 cycles before deactivating.

However, “Diskovery Bob” is still active, although now working as a surface drifter, having transmitted 84 cycles to date (Figure 2; January 18th 2011).

Care must be taken when viewing the current results as it is necessary to build a dataset capable of resolving global changes in the time series. At present, the global dataset is prone to seasonal and inter-annual variability causing biasing on the statistical analyses. However, some basic trends have been interpreted:

• A warming of the upper ocean (0-2000m) of 0.06°C since the 1960's (Levitus et al, 2005).
• An increase in salinity in the evaporative mid-latitudes and a freshening in the high latitudes, implying an increase in the global hydrological cycle of several percent (e.g. Hosoda et al, 2009).

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