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The Catlin Arctic Survey: Thermohaline circulation

The views expressed are those of the author and are not necessarily those of Scientific American.


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If you look at a map of the world and draw a line through London, a latitude of about 50 degrees North and follow this line across the world, you’ll see that it passes through southern Siberia and skims the southern shores of Hudson Bay in Canada. The week before I came out to the Catlin Arctic Survey Ice Base, the temperature in Hudson Bay was lurking between -20°C and -15°C, whilst London was starting to nudge a balmy 8°C.

 

In general we think about temperatures becoming colder as you move further north. England, for example, enjoys a relatively mild climate compared to Northern America.

 

The reason for this are the global ocean currents, and specifically for the British Isles and North Western Europe, the current known as the Gulf stream which brings warm waters from the Caribbean. The ocean currents around the world have a huge impact on our climate. 


The movement of the ocean current in the North Atlantic is a bit like a swimmer doing a tumble turn. The warm salty water from the tropics moves northward releasing its heat close to the English coast. This current continues to move further north until it enters the Arctic Ocean. When Arctic sea ice is formed in the autumn and winter, cold brine (salt water) is rejected from the ice and the surface water becomes denser sinking down towards the seabed. This water then flows out of the Arctic back towards the equator, creating a dragging effect that sustains global ocean currents.

 

This mechanism is known as thermohaline circulation (thermo temperature + haline salt). Both salt and temperature play a big role in this process as these two factors effect the water’s density. Cold water is heavier than warm water. We know this if we have been swimming in a lake on a summer’s day and enjoyed the warmth of the upper layers and shivered as our feet reach down into the depths.  Salt water is heavier than fresh water, as there is not only water in it, but also dissolved salts.

 

In the North Atlantic the warm salt water cools as it moves north, and then sinks. Cold brine is the heaviest type of water and so this process is relatively rapid, speeding the flow of the current. However, the speed at which the water sinks in the North Atlantic could be affected by changes in the Arctic Ocean. Because of increased sea ice and glacier melting (ice is all fresh water), as well as more riverine input, there is an increased flow of fresh water from the Arctic that is mixing with the cold salt water from the Atlantic. By adding more buoyant fresh water to the mix, the Atlantic water does not sink as rapidly.

 

Scientists have put forward scenarios that show thermohaline circulation slowing abruptly over a variety of timeframes. This slowing of the ocean conveyor (the global system of ocean currents) could have dramatic climatic effects around the world. A Met Office Technical Note ‘Global climatic impacts of a collapse of the Atlantic thermohaline circulation’ predicts the impact if the ocean conveyor were to stop completely:

 

"In the first five decades after the collapse, surface air temperature response is dominated by cooling of much of the Northern Hemisphere (locally up to 8°C, 1-2°C on average) and weak warming of the Southern Hemisphere (locally up to 1°C, 0.2°C on average)."

 

More research is needed to understand better sea ice formation, movement and melt rate in the Arctic Ocean. If the sea ice melt-rate increases rapidly, then we could see a resulting slowing of the ocean currents that warm North Western Europe. A significant slowing 8,200 years ago heralded a mini Ice Age. 

 

The research that I am doing in the Arctic at the moment focuses on how solar energy is absorbed into the ocean. The more solar energy is absorbed into the ocean, the faster the rate of sea ice melt will be. This melting sea ice could then disrupt the climate in North Western Europe and beyond by slowing thermohaline circulation in the Atlantic.

Photo credits: The Catlin Arctic Survey, Copyright Martin Harley.

Editor’s Note: The Catlin Arctic Survey is a unique collaboration among polar explorers and scientists to gather data on the impacts of climate and environmental change in the Arctic.

This 10-week international scientific expedition will travel to the farthest reaches of the Arctic to research the impact of melting ice caps on the world’s oceans and weather systems. In recent years, the surface area of Arctic ice has declined to levels that were not expected until 2070. The Catlin Arctic team will seek to understand how climate and environmental changes affect ocean currents, which have a major impact on weather patterns throughout North America. Scientists are predicting that climate-related changes in the way that ocean currents circulate could result in a dramatic increase in the frequency and intensity of storms and cause extensive flooding, coastal erosion and damage to crops, homes and cities across the U.S. and around the world. The scientific team will be based at a unique research station located on sea ice in the Canadian Arctic shelf.

Simultaneously, a team of polar explorers will undertake two separate Arctic missions: the first across the Prince Gustav Adolf Sea, and the second from the North Geographic Pole toward Greenland.

Victoria Hill is research professor of ocean, earth and atmospheric sciences at Old Dominion University, Norfolk, Va.





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  1. 1. promytius 1:48 pm 04/18/2011

    wow – some of that I knew, but that was a very well-articulated explanation. Thanks!

    Link to this

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