The Great Heattransporting Machine

The decrease in temperature towards the poles forms the basic pattern of the earth's climates. But this pattern is greatly altered by the global circulation of two fluids: air and water. Factoring in the circulation of air and water enables us to understand the present-day patterns of climate in more detail.

One useful way to think of the world's climate circulation is to view it essentially as a heat-transporting machine that takes heat from the tropics and moves it to higher latitudes. It operates by movement of warm ocean currents, and also movement of winds and air masses (those great "blobs" of air) that move across the surface.

Heat is transported not just as the temperature that one can easily measure (known as "sensible" heat, because it can easily be "sensed"), but also in the form of "latent heat". This latent heat is hidden energy that comes out only if you try to lower the temperature of moist air until a fog of water droplets appears. As you attempt to cool it, the air temperature drops, but nowhere near as fast as you would expect, because the water vapor condensing out as droplets gives off heat that keeps the air warm.

If it were not for this movement of heat in air masses and ocean currents, the high latitudes would be far colder than they actually are. Heat transport from the tropics "subsidizes" the higher latitudes, by as much as 40-60% more than the heat that they get from the sun (and the higher the latitude, the more important this heat subsidy is). I his draining of heat away from the tropics also makes them cooler than they would otherwise be.

Places in the high latitudes that arc close to the oceans, and receive especially strong ocean currents from the tropics, can be a lot warmer than places that do not. A warm current known as the Gulf Stream (mentioned above) crosses the Atlantic up from the Caribbean, and across to northwestern Europe. Largely because of the Gulf

Stream, Britain has a much warmer climate on average than Nova Scotia, the eastern tip of Canada which is at the same latitude on the west side of Atlantic. In England, grass stays green in January and palmettos can be grown outdoors by the coast because the winters are so mild. In Nova Scotia the snow lies deep all winter long, and temperatures can dip to -40 C\ As mentioned above, part of the reason that the Gulf Stream flows so strongly northwards and carries so much heat is that it is essentially "sucked" northwards by the sinking water of the thcrmohaline circulation in the north Atlantic.

There is a similar "gulf stream" reaching the western side of North America (e.g., on rainy Vancouver Island) which has a very mild climate compared with the harsh winters of Sakhalin/northern Japan at the same latitude on the western side of the Pacific. However, because there is no strong sinking /.one in the ocean to pull it in. its eficct on climates is not as strong as in the north Atlantic.

High-latitude places that arc isolated from tropical air masses and warm sea currents tend to be especially cold for most of the year. The most extreme example is

Antarctica. It is cut oil' from the rest of world by the belt of swirling currents and winds known as the "Roaring Forties". This prevents much heat transfer from lower latitudes, so Antarctica is colder than the North Pole region which receives air masses and warmer ocean currents from low latitudes.

In some places an especially cold area of ocean just oft* the coast makes a difference to the climate inland. Although Nova Scotia is at a disadvantage for heat because it does not receive the Gulf Stream, the frigidity of its climate is added to by a cold sea current that comes down along the west side of Greenland, bringing water straight down from near the North Pole. Across the other side of North America, the remarkable climate of San Francisco in California, which almost never gets hot—and almost never has frost either—is caused by a zone of upwclling of cool deep ocean water just off the coast. A similar cool upwclling zone occurs off the coast of Peru, where it brings about the extreme aridity of the Atacama Desert (sec below).

1.5.1 The "continental" climate

Areas far inland in the higher latitudes tend to experience wide seasonal swings in temperature, bccausc they are cut off from the moderating influence of the oceans. Seas have a very high capacity to store up heat - so their temperature docs not vary so much during the year. In contrast, the land cools down or heats up far more quickly. An area far inland gets less oceanic influence and is more at the mercy of the amount of heat received from the differing sun angle and day length at different times of year. Hence in such places the seasonal differences in temperature can be extreme. The coldest winters on earth outside Antarctica occur not at the North Pole but in the interior of northeastern Siberia, because of its isolation from the oceans. This is known as a "continental" climate, receiving little heat from the distant oceans, and not much warming water vapor in the atmosphere to release heat. The coldest temperature ever recorded in northeastern Siberia in winter was a bone-chilling —60 C. Yet, paradoxically, this same part of Siberia has warm summers too; temperatures can exceed 30 C. The summer warmth is the result of the same factor— isolation from cooling sea winds, which do not reach the interior of Siberia from the seas around its edges.

To a lesser extent, continental climates with wide seasonal temperature swings are found in central Canada and the USA, eastern Europe and central Asia.

1.5.2 Patterns of precipitation

Not only temperature patterns depend on ocean currents and winds. Patterns in the wetness or aridity of large parts of the world's land surface can be understood as a product of circulation.

Why is it, for instance, that the tropics are so moist? Just as with temperature, this is ultimately a result of sun angle. The band of rising air along the equator (the intertropical convergence zone or ITCZ) occurs due to intense solar heating, from the sun being directly overhead. The heating sets up convection in the air. and this rising air sucks in moist ocean winds and water evaporating from the forests. As the air rises

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