by Penny Holliday
Today we waved goodbye and good luck to the fourth of our eight Argo floats that we are putting into the ocean on this cruise. This is the first one that I’ve seen go in the water this trip; the others have all been at night while I’ve been tucked up in bed. Argo floats are fantastic devices that have completely changed the way we observe the temperature and salinity of the global ocean. For five years each one drifts around with the ocean currents and every 10 days takes a profile of temperature and salinity from 2000m deep to the surface. At the surface they transmit their position and data back to base via satellites, before sinking back to the deep for another 10 days. They move up and down in the water by changing their buoyancy (their density relative to the water they sit in); within them they have a tiny tank of oil and by pushing the oil in and out of an external bladder they change their volume but keep their weight the same.
There are 3603 floats spread across the world’s oceans actively collecting data today (see the map below where each black dot represents the latest surface location of a float). Each year about 800 new floats are put in the water to replace those that have reached the end of their battery life. You can see by looking at the map that they are spread out over almost the entire global ocean, something that could never be achieved with only research vessels or commercial shipping. So they are creating a collection of data that far exceeds any measurement programme we have had before in terms of its geographical reach. The data from the Argo programme have many uses, ranging from working out how much heat is stored in the upper ocean (not just once, but every 10 days!), to process studies examining the extent of deep winter mixing in remote places or the pathways of mid-depth flows. The salinity data from Argo is starting to reveal changes in the global water cycle too.
The Argo programme is relatively recent – the first floats were launched in 2000, but it took until 2007 before the desired total of 3000 active floats was established. It is an international programme, with floats contributed by many nations including the UK. And it is a brilliant example of what we can achieve with a cooperative observation programme because all the data are freely available to everyone. It’s a good feeling to be contributing in our small way to such a great and important effort.
Images: Sending an Argo float on its way – Stefan carries it to the stern, Stefan and Brian prepare the line from which it is lowered into the water, Brian lowers it as we steam gently forward, releasing the line, and the float is free (all by Penny Holliday). Map of the distribution of currently active Argo floats (www.argo.ucsd.edu).