Current-tracking drone subs to help improve climate predictions

The UK’s Natural Environment Research Council (NERC) has announced funding for two projects in collaboration with the US to study the circulation of water in the Atlantic that keep Europe’s climate mild and how it could be affected by changing global temperatures.

One of the projects, known as OSNAP, involves mooring monitoring arrays that reach from the bottom to the surface of the ocean at key points across the northern Atlantic, and sending autonomous underwater gliders to gather data from in between.

Dr Sheldon Bacon of the National Oceanography Centre at Southampton University, who is leading the UK team for OSNAP, said this will help them better understand how geographical features of the seabed affect the currents that transfer heat across the Atlantic.

‘If we want models to represent these processes correctly so that we can have betters projection of future climate, if we want to understand regional details like how Britain is likely to be affected in coming decades, we have to understand these features that affect the decadal variability of the ocean,’ he told The Engineer.

The project’s gliders will spend up to four months at a time navigating the ocean currents, following a pre-planned route and surfacing around once a once a week to check its position, correct its course and transmit data back to base.

Each 150cm-long glider uses an external bladder filled and emptied with a reservoir of oil to change the device’s density, in order to sink to around 1km and then rise to the surface. As the glider moves up and down, its wings enable it to move forward at the same time.

The moored arrays will take measurements from the bottom of the ocean that can be used to calculate what happens along the flat part of the seabed. The gliders will navigate the warmer, upper currents in order to study how they are affected by topographical ridges and troughs.

The instruments are due to be deployed next summer and will be changed every one to two years until 2018. ‘These gliders have been around for a number of years but they’re only now becoming mature and reliable enough for long-term missions,’ said Bacon.

Western Europe has a relatively mild climate for a region so far north because of the so-called conveyor belt currents in the Atlantic that bring warm water at the top of the ocean from the tropics northwards.

As the water transfers heat to the atmosphere, it cools, becoming denser and saltier and sinking to the bottom of the ocean - a process known as overturning - where it returns southwards.

Researchers in the other NERC-funded project, known as RAPID, have been studying the conveyor belt between Florida and the Canary Islands using a series of moored arrays for a decade and the programme is now due to run for a further six years.

But in the part of the ocean between Britain and Canada, known as the North Atlantic Subpolar Gyre, the process is more complicated due to the wind-driven horizontal circulation of waters on the surface and the more complex seabed topography.

OSNAP (Overturning in the Subpolar North Atlantic Program) will enable researchers to study the relationship between the horizontal and vertical currents to build a better understanding of how the water returns to the southern Atlantic rather than just circulating in the north.

‘It turns out you get eddies spinning off at great depths and forming a pathway,’ said Bacon. ‘If we want models to represent these processes correctly so we can have a better projection of future climate, we have to understand these features that affect the decadal variability of the ocean.’

Total funding for the projects is worth £44m, supplied by NERC and the US’s National Science Foundation (NSF) and the National Oceanic and Atmospheric Administration (NOAA).