Science

Thermohaline Circulation (2011)

The Arctic plays a crucial role in driving thermohaline circulation – powerful ocean currents that circulate warm and cold water around the world’s oceans. These currents have a major impact on Earth’s climate and weather patterns.

While a number of processes drive thermohaline circulation, there are a few that are unique to the Arctic. Further data on these will help scientists determine how important the Arctic is to thermohaline circulation, and what changes are taking place.

Ocean circulation

Diagram courtesy of Dr. Simon Boxall, National Oceanography Centre, Southampton

Thermohaline circulation is driven by differences in density. The word thermohaline comes from the two factors that affect density: thermo- refers to temperature and -haline refers to salinity.

Warm salty surface water flows north from the Atlantic Ocean towards the Arctic. As the water flows north, heat picked up in the tropics is transferred into the atmosphere and the temperature of northerly seas is raised. This means countries in northwest Europe such as Norway and Denmark experience more temperate climates than their latitude suggests they should.

The Atlantic water cools as it approaches the Arctic. A combination of evaporation into the atmosphere and cooler Arctic temperatures increase the water’s density. That density is further increased by brine (salt saturated water) dripping from the sea ice as it forms.

This water eventually becomes so dense it overturns – sinking down into the depths of the Arctic Ocean to begin flowing south again. This process is known as North Atlantic Deep Water formation. It is vital to thermohaline circulation.

However, a proportion of the Atlantic water flows further north still. This water is known to circulate beneath the Arctic sea ice for many years before flowing back out into the Atlantic.

Both the overturning North Atlantic Deep Water formation and the Atlantic water that continues circulating around the Arctic are thought to affect thermohaline circulation.

Climate change and ocean circulation

The Arctic environment is changing. Melting sea ice, thawing permafrost, increased rainfall and runoff from glaciers and rivers is increasing the amount of freshwater entering the Arctic Ocean. The rising level of freshwater is increasing the Arctic Ocean’s temperature.

This freshening and warming of the Arctic Ocean may interrupt the driving and sinking mechanisms of the Atlantic surface water. It could even stop the pull of warm tropical waters past northwest Europe. If this were to happen, it could significantly change the climate and weather patterns of the world and Earth’s ecosystems.

While current research suggests this scenario is unlikely to happen in the 21st century, a better understanding of the changes taking place in the Arctic will give scientists a better understanding of thermohaline circulation.

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