Cold water conundrum cracked

The scientists' work explains where the cold water comes up from the ocean bed

By Alex Kirby BBC News Online environment correspondent

British scientists have identified an area where cold water from the Antarctic deeps is mixed into the upper ocean.

It is the Scotia Sea, between the Antarctic and the tip of Latin America.

The sea acts like a blender to mix the very cold water with the warmer upper layers.

The finding will help climatologists to improve the models they use to predict change.

The scientists, from the University of East Anglia (UEA), report their findings in the journal Nature.

They found that the dense, cold waters which are formed in the Weddell Sea near the Antarctic continent, and which provide the cold layers at the bottom of many of the world's oceans, are mixed up much more thoroughly in the Scotia Sea than anyone had realised.

Riddle solved

The team describes the Scotia as "a basin with a rough topography, situated just east of the Drake Passage where the strong flow of the Antarctic Circumpolar Current is constricted in width".

The team measured a range of factors, including temperature, salinity and pressure of the water, at 170 different points around the Scotia Sea. This enabled them to calculate the amount of mixing taking place.

Marine life depends on mixing

The research was carried out as part of the Albatross project (Antarctic Large-Scale Box Analysis and the Role of the Scotia Sea) which was designed to measure the flow of water into and out of the Scotia Sea in order to determine the role it plays in ocean circulation.

One of the scientists is Dr Karen Heywood, senior lecturer in physical oceanography at UEA's School of Environmental Sciences.

She said: "Previously we didn't know where much of the cold water mixing took place.

Blender effect

"We've shown for the first time that the coldest water from the bottom of the Weddell Sea gets violently churned up as it passes through gaps in the ridge system found on the seabed in the Scotia Sea, mixing upwards towards the surface.

"This accounts for up to 20% of the water mass from the Antarctic that cools the global ocean."

There are other mixing hot spots

Some cold Antarctic water travels as far as the north Atlantic, where it is identified by its temperature, salinity and high silicate content.

Dr Heywood told BBC News Online: "One of the big puzzles is that we know cold water sinks at the poles, but we haven't known till now where it comes up.

"We've now found a hot spot for cold water mixing. The places where you seem to get a lot of mixing have a lot of narrow gaps.

Improved predictions

"We established what was happening in the Scotia Sea by indirect calculation, not by putting down an instrument to measure mixing directly.

"The search for more hot spots will continue, and the next steps may include more direct measurements."

The UEA team's findings will help climate scientists to improve the accuracy of global climate models, in which ocean circulation patterns are a key factor, by ensuring that they represent cold water mixing in the right places.