Consideration of the actual average patterns of air pressure and wind directions on the Earth reveals both similarities to and differences from the simple model outlined above. To a large degree, these differences arise because we neglected differences in the surface properties of the Earth in the model. In reality, the distribution of land and sea introduces large variations to the global energy balance.
The solar radiation reaching the Earth is reflected back to space in variable proportions according to the type of surface. This is why some surfaces appear darker or lighter than others. The proportion of radiation reflected back is called the albedo. Surfaces with a high albedo (e.g. snow) reflect back a lot of radiation, so absorb relatively little. Surfaces with a low albedo (e.g. shales) absorb a lot of the incoming radiation. Thus, the absorbed radiation at the surface depends on the spatial pattern of surface types. Large amounts of energy are absorbed by the the oceans and equatorial forests (dark, low albedo); whereas lower amounts of energy are absorbed by deserts, ice sheets, and snow surfaces (light: high albedo). The albedo of the Earth has seasonal variations, due to changes in vegetation and snow cover. In general, the mid- and high latitudes of the winter hemisphere will have high albedo and low net shortwave receipts due to extensive snow and sea-ice cover.
The pattern of received solar radiation is also influenced by the presence of clouds, which have high albedo and reflect energy back to space before it reaches the Earth.
Varying amounts of incoming radiation will be used to evaporate water over land and sea surfaces. Evaporation is much larger over ocean surfaces than dry land surfaces, absorbing more solar energy. Over land, this solar energy is used instead to warm the land, and hence the atmosphere. Thus, in the Tropics, where there is net gain of energy from solar radiation, daytime surface temperatures will be lower over oceans than over adjacent land surfaces.
Oceans have a higher heat storage capacity than land surfaces. Land surfaces take up and give up heat more readily to the atmosphere, whereas heat is taken up more slowly and retained for longer periods of time in the oceans. Outgoing radiation is also influenced by clouds, which trap longwave, enabling part of it to be re-radiated to the surface.
Because of the differences introduced by the distribution of land masses and oceans, the pattern of global net energy receipts differs from the simple latitudinal variation considered in the simple model described in Lecture 1. Latitudinal patterns are still evident, but considerable spatial and seasonal variations are introduced by land and sea differences. The distribution of land and sea also allows us to understand spatial and temporal variations in temperature, evaporation and atmospheric water vapour (which depends on both temperature and the availability of moisture).
We are now in a position to consider the actual general circulation of the atmosphere, and its causes. The following maps show the average global circulation for January (northern hemisphere winter) and july (northern hemisphere summer).
The Tropical Hadley cells are clearly apparent on the global circulation map. The zone of uplift between the two cells forms the Intertropical convergence zone (ITCZ). The energy for uplift is supplied by longwave radiation from the surface and the latent heat released during condensation in rising cumulus clouds. The associated low pressure is most intense over the tropical continents and the western Pacific archipelago: these are the main sites of ascending warm, moist air. The ITCZ and associated low pressure zones are pulled polewards in the summer hemisphere: southward into South America, Africa, and Indonesia/Australia in January, and northward into northern Africa and the Himalaya/Indian subcontinent in July. The most northward position of the ITCZ is over India in July, due to the intense heating of the subcontinent by the early summer sun. The northward and southward motion of the ITCZ causes a seasonal reversal of wind patterns. In India, winds are south-westerly in the summer, bringing moist air on to the land, and northerly in the winter, bringing cooler, dry air from the Himalayas. Similar patterns exist over East Asia, Africa, Australia, and South America. This seasonal reversal of weather patterns is known as a monsoonal climate. The torrential rains commonly associated with the word 'monsoon' are a characteristic of the summer monsoon season.
The equatorial low pressure is less intense over the tropical Indian, Atlantic and eastern Pacific Oceans, where more solar energy is consumed by evaporation. In many parts of these tropical seas, the uplift less intense, with only weak surface winds in the zone of uplift. These latitudes were known as the Doldrums in the days of sail, when sailing ships could drift for weeks with limp sails in blistering heat. However, intense storms (known as tropical cyclones or hurricanes) can form over the Tropical oceans.
The Inter Tropical Convergence Zone is fed by the low-level Trade winds, which are north-easterly in the northern hemisphere and south-easterly in the southern hemisphere. These are particularly clear in the January map. Coriolis deflection is weak at these low latitudes, so air flows across the isobars at a relatively low angle.
In the Sub-tropics, the descending limbs of the Hadley cells create areas of clear calm weather: these are the Sub-Tropical Highs. These form permanent high pressure zones over the subtropical oceans, and tend to be best developed in the winter hemisphere. Over the oceans, the calm sub-tropical high pressure zones were known as the Horse latitudes. The origin of the term is obscure, although it has been suggested that it comes from the frequent death of horses on ships becalmed en route to the colonies, so that the sailors would eat horse meat in these parts of the world. (In his fascinating book ‘In Patagonia’, Bruce Chatwin recounted the lurid tale of a hold-full of penguin meat that turned rotten as a northbound ship traversed these latitudes, with hellish consequences for all on board.)
Less intense high pressure cells are also developed over the subtropical continents, where the descending, dry air controls the location of many of the worlds deserts, including the Sahara, the Kalahari (Africa), the Atacama (South America), the Australian deserts, and the arid south-west of the USA.
A polar cell is best developed over Antarctica, where a large, relatively uniform ice-covered continent occupies the polar region. This, as predicted by the model, consists of a cold, high-pressure region, which spreads outward at low altitudes. The southern polar cell expands and contracts on a seasonal basis as sea ice expands in the winter and breaks up in summer. A polar cell is less well developed over the arctic, which is an ocean area fringed by land masses such as the Canadian arctic islands, Greenland, and high-latitude Eurasian archipelagos such as Svalbard and Franz Josef Land. The northern polar cell is best developed in the northern winter, but breaks down in the spring and summer as the sea ice breaks up.
Mid latitude circulation is driven by interactions between subtropical and polar air, as predicted by the model. This interaction is simplest in the southern hemisphere, where oceans completely circle the globe, and little land occurs between 40° S (south Australia) and 65° S (Antarctica). A low pressure zone extends right round the southern ocean, between the Sub-Tropical Highs and the Antarctic high pressure system. This zone, known asthe polar front, is characterised by a series of dynamic, cyclonic weather systems of vigorous uplifting warm air interacting with cold polar air, swept along in the southern westerlies. Thesecyclones (also known as depressions) are rotating storms, formed where cold and warm air meet, and wrap round each other in the process of mixing.In the northern hemisphere, the situation is more complex, and strongly influenced by the distribution of land and ocean. Over the oceans, two low-pressure zones occur in the North Pacific (Aleutian low) and North Atlantic (Icelandic low). These zones are not permanent features like the Sub Tropical Highs, but are average states arising from the frequent passage of low pressure cyclones along the polar front. In maritime mid-latitude regions, such as the Pacific North-west of the USA and Canada, and the west coasts of Northern Europe, the passage of cyclones is a major feature of the weather, bringing frequent cloudy, wet conditions.
The northern hemisphere continents have distinctive annual climatic patterns. In winter, a cell of cold, high pressure air develops over Siberia (the Siberian High), and to a lesser extent over Canada. These air masses are extremely cold and dry, and temperatures may fall to below -50° C. Essentially, these behave like cold, polar cells, and originate because of the effect of snow cover on albedo and the consequent very low temperatures. Conversely, warm, low pressure air masses develop over these land masses in summer. Westerly weather systems originating over the oceans may penetrate the continents, especially in North America, carrying in moisture.
Upper-level airflow is westerly, not easterly as predicted by the model. Upper air flow in both hemispheres is characterised by high velocity westerlies along the polar fronts, centred on a core or jet stream, which changes position thoughout the year. These upper-level westerlies have a wavelike motion, snaking around the planet in meanders. The longer meanders (about 4 around the whole earth) are known as Rossby or Planetary waves, and shorter waves, superimposed on the Planetary waves, are known as Short Waves (these, of course should not be confused with shortwave radiation). The existence of westerlies at altitude as well as the surface is due to vertical and horizontal patterns of air pressure. Over the Sub-Tropics, upper level air (at around 10,000 metres above sea-level) is relatively warm, and has a relatively high air pressure. In contrast, at higher latitudes, air at this level is colder and with lower pressure. The difference in temperature and pressure drives a wind: this would be towards the poles, but it is deflected by the Coriolis Force, rightwards in the northern hemisphere, and leftwards in the southern hemisphere. The deflection means that in both hemispheres, the jet stream winds are westerlies.
The jet streams show up clearly on this image, which is of the winds at about 10,000 metres above sea level around the northern hemisphere for the 14th November 2000. The Sub-Tropical jet stream almost completely encircles the planet, and exhibits a series of Planetary Waves. Further north is the more fragmentary Polar Front Jet Stream, which is distorted into a series of short waves.
Both Planetary and Short Waves change position over days or weeks, and exert an important control on the surface weather.
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