Antarctic voyage: Argo floats

By Guest Author 11/02/2013

Written by Phil Sutton and Helen Bostock of NIWA

Date: 9/2/2013
58.324713°S, 147.986503°E
Weather: Cloudy
Sea State: Calm – 1-2 m swell

Deploying an Argo float (TAN1302 voyage) – it is in a cardboard box to protect it before it is deployed, which is then launched off the back of the ship, and once the cardboard box falls off the float starts its cycle. [Helen Bostock]

Over the next few days we will be deploying a number of autonomous Argo floats.

Argo is an international programme that deploys profiling floats in all of the world’s oceans. Currently there are 3625 floats globally, deployed by 27 countries. This sounds like a lot, but the oceans are vast and there are still areas that haven’t got good coverage.

A typical float cycle consists of the float sinking to 1000m, where it drifts with the ocean currents for 10 days. The float then sinks deeper, to 2000m, before rising to the surface. The float measures the temperature and salinity of the water as it rises, and when it reaches the surface the data is transmitted to a satellite. From there the data is transferred to data facilities, where it is available in near-real time.

The batteries on the Argo floats last a couple of years and continuously collect data. They are so easy to deploy that they can be launched from any “ship of opportunity”. They provide far more data, and are far cheaper, than going on a voyage and doing lots of CTD casts.  Some of the Argo floats on this voyage, however, are initially deployed at the same site as a CTD station, so that they can use the temperature, salinity and oxygen data to check the calibration of the sensors.

The resulting detailed knowledge of the temperature and salinity of the ocean in all seasons – even winter, when there aren’t many voyages as conditions are too rough and unpleasant – over the last 10 years has been critical for climate research. The high heat capacity of water means that 90% of the global heat energy changes in the past 50 years have been absorbed by the ocean.

The floats deployed near Antarctica have ice-detection capabilities to avoid damaging themselves if they try to surface under sea ice. Basically, it the water is too cold, the floats stop rising and sink back down to the 1000m parking depth. They store the profile data and try to rise again in a further 10 days.

Two of the floats being deployed on this voyage incorporate some new sensors, which measure nutrient nitrate, oxygen, fluorescence (a measure of how much chlorophyll or phytoplankton are in the water) and backscatter (how many particles are in the water).

The Southern Ocean is one of the most difficult areas to keep populated with Argo floats because of the limited number of ships that go there. This voyage is a great opportunity to deploy floats this far south, and learn more about this remote region of the world’s oceans.