Ocean circulations are changing

Ocean circulations are changing

The oceans are warming and their salinity is changing, which has implications for ocean circulation patterns.

Concern has been expressed that global warming could lead to massive changes in ocean currents such as the Gulf Stream. Currents in the ocean are responsible for about half the work of the climate system in redistributing heat between the equator and the poles. This redistribution makes some parts of the world warmer or cooler than they would otherwise be, and therefore changes to currents could affect some regional climates—either magnifying or partially neutralising some of the global warming changes in those places.

The current system in the North Atlantic is particularly well studied, and is driven by convection which takes place in two areas, one in the Labrador Sea and the other in the Nordic Seas, as shown in the figure below. The system makes up part of the global thermohaline circulation that transports water across and between the world’s oceans in what is sometimes called the global ocean conveyer belt. It is driven by gradients in water density from the combined effects of heat (thermo—from the Greek thermos, meaning warm) and salinity (haline—from the Greek halinos, meaning of salt).

North Atlantic thermohaline circulation

North Atlantic thermohaline circulation

Source: Adapted from UK Department of Environment, Food and Rural Affairs, Met Office Hadley Centre, Climate change and the greenhouse effect: a briefing from the Hadley Centre, December 2005, p. 58. © British Crown Copyright 2005, the Met Office.

In both convection areas in the North Atlantic, the surface water is cooled by arctic winds. Cooler water is more dense, and therefore this water slowly sinks a few thousand metres to the bottom of the ocean. This cool dense water then flows southwards, with a flow equivalent to a hundred Amazon rivers, crossing the equator and heading south. The sinking cold water in the north has the effect of drawing northwards warm near-surface water from the Gulf of Mexico (producing the Gulf Stream), which travels across the North Atlantic (as the North Atlantic Drift). The heat which it transports towards north-west Europe is part of the reason why countries such as the British Isles and Norway are a lot warmer than, for example, those parts of western Canada at the same latitudes.

There is strong evidence that the Gulf Stream has switched off more than once over the last ten thousand years, due to natural causes. This could happen again. Several factors associated with climate change could increase this probability by reducing the density of surface waters and thereby making them sink more slowly. The factors are as follows:

  • Warming of surface waters in the convection areas will reduce their density.
  • Increased rainfall expected over the convection areas will act to reduce the density of surface waters (because rainwater, lacking salt, is less dense than seawater).
  • Increased precipitation already observed and expected to continue in high latitudes will increase the outflow of fresh water from rivers, further reducing density of surface waters.
  • As the amount of sea ice decreases, a further mechanism for driving convection (the seasonal freezing of sea ice which rejects salt and thus makes surface waters denser) will decrease.

What would happen if the North Atlantic thermohaline circulation did switch off? Modelling suggests that in the first decade the whole of the Northern Hemisphere would cool, especially in the Arctic Ocean and adjacent parts of the North Atlantic. The UK would cool by some 3–5°C. The effects on extremes, for example winter minimum temperatures over central England, would be very marked, and the effect of this on infrastructure would be likely to be far worse than that of a gradual global warming (although nothing like as great or as sudden as depicted in the Hollywood disaster movie The Day After Tomorrow).

Modelling of the temperature effects of the collapse of the thermohaline circulation (THC).

Modelling of the temperature effects of the collapse of the thermohaline circulation (THC).

Source: UK Department of Environment, Food and Rural Affairs, Met Office Hadley Centre, Climate change and the greenhouse effect: a briefing from the Hadley Centre, December 2005, p. 59. © British Crown Copyright 2005, the Met Office.

However, further modelling suggests that the complete collapse of the Gulf Stream is unlikely. Most scenarios indicate a decrease in the circulation strength of about 15-25 per cent by 2100 with no further decline. This reduction will lead to a corresponding reduction in the amount of heat transported towards the UK, but would be more than offset by the direct effects of greenhouse warming. Even worst-case scenario modelling suggests that the circulation will be maintained, with flows of about a 55 per cent of today’s flow strength.

Further reading:

Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report—the physical science basis, Chapter 5. Observations—oceanic climate change and sea level.


 

1 September, 2009

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