Global atmospheric and oceanic circulation

As we mentioned in Week 1 of the course, mid-latitude depressions are one way in which the Earth moves heat from the warm Tropics to the colder Poles.

Together with tropical storms, they move about 1/3 of the heat that is redistributed. Roughly another third is moved by the large scale circulation of the atmosphere and another third by the oceans. We’re going to look at these two circulations a little bit now.

Figure 1: A cross section through the atmosphere, showing air rising and forming deep clouds, at the InterTropical Convergence Zone and spreading polewards, and the Hadley, Ferrell and Polar cells.

Figure 2: Idealised representation of the general circulation of the atmosphere showing the positions of Polar Front; ITCZ (Inter Tropical Convergence Zone); Subtropical Jets (STJ) Polar Front Jets (PFJ) © Crown Copyright, Met Office

Figure 3: A simplified diagram showing the ocean’s thermohaline circulation

Figure 4: The thermohaline circulation of the oceans.©University of Reading. Adapted with kind permission from Climatica UK.

Figure 5: Currents in the North Atlantic

In the UK, the Gulf Stream and its eastward extension, the North Atlantic Drift, bring warm water from further south in the Atlantic, keeping us approximately 5^°C warmer than other countries at a similar latitude (Figure 5). The Gulf Stream is largely wind driven – by the westerly winds blowing over the surface of the Atlantic, whereas the North Atlantic Drift is part of the thermohaline circulation.

The effect of less sea-ice forming in the Arctic (as the climate there warms far more rapidly than most of the rest of the Earth) is expected to have an impact on the North Atlantic Drift. It is very likely that it will weaken and/or move. Although this would imply that the UK and other western European countries would lose a source of warmth, actually, the cooling is expected to be less than the warming caused by other aspects of climate change.