The atmosphere and hydrosphere are in constant motion. The weather is in constant flux on all timescales: by the hour, day by day, and seasonally. Rain falls, rivers flow, water passes through lakes and under ground in a constant flux. The oceans themselves undergo constant cycling. Viewed on some timescales, the motion of the atmosphere and oceans is chaotic, with no repeating pattern; turbulence is one example. Within the chaos, however, is a deep, underlying order. There are persistent spatial and seasonal patterns of weather conditions which we expect to find in particular places and times of the year, and these patterns underlie the hydrological cycle, the distribution of animals and plants, the action of earth-surface processes such as chemical or periglacial weathering, and human activities such as agriculture and recreation.
Almost all of the energy that drives the atmosphere and oceans comes from the sun. This energy is in the form of shortwave radiation, consisting of visible light + ultraviolet (UV) and near-infrared (IR). About 70% of this shortwave radiation is reflected straight back out to space by clouds, ice sheets, land and ocean surfaces. The remainder is absorbed by land surfaces, the upper layer of the oceans, and a small amount by atmosphere. Heated land, sea, and atmosphere re-radiate radiation, but at longer wavelengths (infrared or longwave radiation), which is absorbed by the atmosphere, especially water vapour and CO2. Solar shortwave radiation is also used to evaporate water from the oceans, lakes, and moist or vegetated land surfaces. When this water condenses to form cloud droplets, the energy is re-released as heat into the atmosphere. Thus shortwave radiation heats the atmosphere mainly indirectly, either through (1) conversion to longwave radiation and subsequent heating, or (2) through the evaporation and condensation of water (latent heat).
The overall pattern of atmospheric motion is known as the General Circulation. In detail this is quite complex, but the main features of this circulation are summarised on the idealised diagram below:
The real circulation of the atmosphere is more complex than this, due to the influence of landmasses and oceans on heating, cooling, and air flow, but the essence is the same. The main features are:
Before going on to look at the real circulation in more detail, we will examine why the earth's general circulation has this overall form. Essentially, the atmosphere is a massive system whose motion transfers energy and mass around the planet. The general circulation of this system can be understood in terms of surprisingly few principles:
The curvature of the Earth ensures that more shortwave radiation is received near the equator than near the poles. This is because the the sunÕs rays hit the ground surface at a lower angle at high latitudes, and so the energy is Ôspread outÕ over a larger area. Thus, the energy receipt per unit area is less. The earthÕs axis is tilted relative to the plane of its orbit around the sun, but (on human timescales) is fixed relative to the distant stars. This means that the amount of solar radiation received at any given point on Earth varies seasonally. In the northern hemisphere summer, the north pole is tilted towards the sun, so the zone of maximum heating lies north of the equator. Meanwhile, the southern polar regions are in the depths of the winter darkness. The situation is reversed during the northern hemisphere winter (southern hemisphere summer). Averaged throughout the year, there are net gains of heat at low latitudes (near the equator), and net loss of heat near the poles, with important variations introduced by seasonal cycles and the distribution of land and sea. The circulation of the atmosphere and hydrosphere serve to redistribute this heat, balancing the heat budget of the Earth.
More accurately, a packet of air which is warmer than the surrounding air will rise, while a cooler packet will sink. This is because warmer air expands and becomes less dense than its surroundings, and the lower density makes it buoyant. Thus, parts of the atmosphere which are gaining heat will rise, whereas those which are losing heat will sink or subside. On a planetary scale, air rises in the tropics and subsides at the poles. On smaller scales, air will rise or sink in response to variations in heating or cooling, such as between land and sea, or between land surfaces with different characteristics. Buoyancy in the ocean is also controlled by temperature. Over much of the planet, the surface layers of the ocean are the warmest, so there is little tendency for vertical mixing. However, ocean density is also influenced by the salt content (salinity): saltier water is denser. Thus, cold, salty water is dense, and will tend to sink, whereas warm, fresh water is less dense and is buoyant.
Air exerts pressure on its surroundings. At sea-level, air exerts a pressure of about 1000 millibars, due to the weight exerted by the overlying air (this is equal to 10,000 kg of air overlying one square metre of surface, pulled downwards by gravity). This huge pressure does not crush us because it is exactly balanced by outward pressure from the inside of our bodies. Ears popping due to a change in altitude are the result of the pressure difference between the inside of our heads and the surrounding air. At any given altitude, winds will blow from areas of high air pressure to areas of low pressure, attempting to equalise the pressure distribution. Uplift of air creates areas of high pressure aloft (at altitude), because air is imported up to that altitude from below. This air will therefore tend to spread outwards at this high-level, thus reducing the pressure at the surface (because there will be less air above a cetrain point than before). Thus, surface winds will blow towards the area of surface low pressure, and upper level winds will blow away from the corresponding area of high pressure above. Important note: in this context, the terms ÔhighÕ and ÔlowÕ pressure refer to the air pressure relative to the surrounding air at that altitude. In absolute terms, upper level air is at lower pressure than lower level air (because there is less air pressing down in it from above). This pattern of uplift, pressure variations, and lower- and upper-level winds accounts for sea-breezes (warm land, cool sea, onshore surface wind), as well as large scale patterns of wind circulation such as the Hadley Cells.
If the Earth did not rotate, the circulation would consist of simple cells in both hemisphere with rising warm air at the Tropics, sinking cold air at the poles, linked by equatorward low-level flow and poleward upper level flow. The Earth, however, is a rotating sphere, and this affects the motion of anything (including winds) across the Earth. On a fairground merry-go-round, we feel an outward pull due to the tendency of our bodies to keep travelling in a straight line (inertia): this is the centrifugal force. The same force operates on the rotating Earth, but we do not feel it because it is exactly balanced by gravity (or, more correctly, the component of gravity acting towards the Earth's axis). For a moving body (or wind) this balance is upset, causing the body to veer to one side. In the northern hemisphere this is to the right; in the southern hemisphere it is to the left. The apparent force causing this deflection is called the Coriolis Force. It is at a maximum at the poles and non-existent on the equator. The Coriolis Force has a profound effect on the motion of the winds and oceans. For example, instead of blowing straight towards an area of low pressure, winds are deflected to the right (in the northern hemisphere), causing the wind to rotate anticlockwise around the low. Similarly, winds blowing outwards from an area of high pressure are deflected to the right (northern hemisphere) setting up a clockwise circulation. These motions are reversed in the southern hemisphere. The effect of the Coriolis force is to break up the global circulation into more than one cell: on earth it is into the three cells; on larger, faster rotating planets such as Jupiter (which rotates every 9.8 earth hours), there are more cells.
Water currents are also affected by the Coriolis force. Winds blowing over a water surface exerts a drag on the water and sets it in motion. The motion, however, will veer to the right of the wind (in the northern hemisphere), an effect that will increase with depth. Thus, wind-driven ocean currents will differ from the surface winds, a phenomenon first observed by the arctic pioneer Fritjof Nansen but explained by and named after Eckman.
Together, these simple principles control the circulation of the atmosphere. The zone of maximum heating near the equator produces uplift of air and outward flow at high altitudes. Moisture is drawn in on winds from the sub-Tropics (Trade winds), helping to fuel the uplift. This uplift zone: the Inter-Tropical Convergence, is characterised by high rainfall, usually in the form of intense thunderstorms. The descending air at the poleward limits of the Hadley Cells produces the Sub-Tropical Highs, characterised by dry, stable air, and clear, hot sunny weather. At the poles (the area of minmum solar heating), cold air spreads out at the surface (like cold air falling out of a home freezer), and is replaced by air returning polewards at high altitudes. These are the Polar Cells. Between the Hadley and Polar Cells, are the Mid Latitude Ferrel Cells, which are secondary motions set up by the other two cells. Motions in the three cells are deflected to the right in the northern hemisphere, and to the left in the southern hemisphere. This is clearly seen in the Trade Winds and the Mid-Latitude Westerlies. In reality, Mid Latitude circulation is more complex than this, as we shall see in the next lecture. In Lecture 2, we will examine how these principles influence the actual circulation pattern, and look at how and why the real circulation differs from this simple model.