Ocean currents have a profound influence on climate

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Ocean currents have a profound influence on climate

Covering some 71 per cent of the Earth and absorbing about twice as much of the sun's radiation as the atmosphere or the land surface, the oceans are a major component of the climate system. With their huge heat capacity, the oceans damp temperature fluctuations, but they play a more active and dynamic role as well. Ocean currents move vast amounts of heat across the planet - roughly the same amount as the atmosphere does. But in contrast to the atmosphere, the oceans are confined by land masses, so that their heat transport is more localized and channeled into specific regions.

The present El Niño event in the Pacific Ocean is an impressive demonstration of how a change in regional ocean currents - in this case, the Humboldt current - can affect climatic conditions around the world. As I write, severe drought conditions are occurring in a number of Western Pacific countries. Catastrophic forest and bush fires have plagued several countries of South-East Asia for months, causing dangerous air pollution levels. Major floods have devastated parts of East Africa. A similar El Niño event in 1982/83 claimed nearly 2,000 lives and global losses of an estimated US$ 13 billion.

Another region that feels the influence of ocean currents particularly strongly is the North Atlantic. It is at the receiving end of a circulation system linking the Antarctic with the Arctic, known as 'thermohaline circulation' or more picturesquely as 'Great Ocean Conveyor Belt' (Fig. 1). The Gulf Stream and its extension towards Scotland play an important part in this system. The term thermohaline circulation describes the driving forces: the temperature (thermo) and salinity (haline) of sea water, which determine the water density differences which ultimately drive the flow. The term 'conveyor belt' describes its function quite well: an upper branch loaded with heat moves north, delivers the heat to the atmosphere, and then returns south at about 2-3 km below the sea surface as North Atlantic Deep Water (NADW). The heat transported to the northern North Atlantic in this way is enormous: it measures around 1 PW, equivalent to the output of a million power stations. If we compare places in Europe with locations at similar latitudes on the North American continent, the effect becomes obvious. Bodö in Norway has average temperatures of -2°C in January and 14°C in July; Nome, on the Pacific Coast of Alaska at the same latitude, has a much colder -15°C in January and only 10°C in July. And satellite images show how the warm current keeps much of the Greenland-Norwegian Sea free of ice even in winter, despite the rest of the Arctic Ocean, even much further south, being frozen.


Figure 1. Europe's heating system. This highly simplified cartoon of Atlantic currents shows warmer surface currents (red) and cold north Atlantic Deep Water (NADW, blue). The thermohaline circulation heats the North Atlantic and Northern Europe. It extends right up to the Greenland and Norwegian Seas, pushing back the winter sea ice margin. Reproduced from Rahmstorf 1997.

We now have computer models that give fairly realistic simulations of the ocean circulation, and these models can be used to examine the effects of the currents on climate. For the Atlantic 'conveyor belt' this task is made particularly straightforward by a peculiarity of the climate system: there are two stable climate states, one with the Atlantic conveyor, one without it. Just by using different initial conditions, all else remaining the same, the models can come up with either of these two different climates. This makes it easy to compare what the world would look like without the ocean circulation that warms Europe. Manabe and Stouffer 1988 were the first to analyze what happens when the familiar conveyor circulation is absent in an ocean-atmosphere circulation model. They found that the sea surface temperatures in the northern North Atlantic dropped up to 7°C in this case. Air temperatures dropped even more, up to 10°C over the Arctic seas near Scandinavia, even though the root cause for the atmospheric cooling was the lower sea surface temperatures. The reason for this amplification of the cooling was the advance of sea ice, which reflects sunlight back into space and thus led to further cooling. The air temperature changes in the model are roughly consistent with the observed difference between Bodö and Nome, confirming that this difference is indeed mainly caused by the warmth brought north by the Atlantic ocean currents in the present climate.

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