Going, going, gone…

By Robert King

Picture credit: Emma Slater

In the early hours of this morning, we dropped some expensive kit overboard. Not to worry: we meant to. This event was the deployment of two Argo floats. These are autonomous ocean profilers which have been deployed all over the world’s oceans as part of an international effort to measure below the surface of the global ocean.

The photograph below shows one of the two floats being lowered carefully into the water and then floating away as we steam towards our next CTD station. This float should now spend the next 3 years or so (more if we’re lucky) making measurements of temperature and salinity as it travels where ever the currents take it.

Much like the CTD measurements we have been making, Argo floats are used to measure the temperature and salinity of the ocean sub-surface. They spend most of their time waiting at a depth of around 1000 m, then every 10 days they first descend to 2000 m before measuring temperature and salinity on their way back toward the surface. Once at the surface, they send data back via satellite and drop back down again to 1000 m.

By ‘parking’ at 1000 m, the Argo float can avoid the stronger surface currents and so remain closer to where it was deployed. This allows the global array of Argo floats to better sample the whole ocean. If they were allowed to drift freely at the surface, we would likely end up with uneven coverage.

Today there are around 3900 Argo floats distributed over all the Earth’s oceans. The observations from this array are used by scientists in many countries. For instance, at the Met Office we ‘assimilate’ the observations of temperature and salinity into our operational ocean forecasting models. These in turn are used in a number of applications, including ship routing, offshore operations, search and rescue, and fisheries management. They are also used in climate research to monitor large-scale changes in the ocean.

Unlike the CTD measurements described by Liz yesterday, we won’t be pulling the Argo floats back on-board. This plunge into the ocean wakes them up as they start their missions.

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