GG2003: ENVIRONMENTAL SYSTEMS, 2001. Climate, Lecture 1.
Environmental Systems: the Atmosphere and Oceans
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Solar (short-wave, SW) radiation which passes through the atmosphere is absorbed by the Earth’s surface, which warms up as a result.
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The atmosphere takes up heat from the Earth’s surface. Warm air emits terrestrial (long-wave,
LW) radiation, which escapes to space.
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At the scale of the entire globe, LW radiation outputs balance SW radiation receipts, so we can speak of a global energy balance.
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This global balance disguises a series of energy transfers which shunt mass (air and water) and heat around the globe, transfers familiar to us as winds and ocean currents. Energy taken up from SW radiation at place A will often be carried away to place B before it is lost to space as LW radiation.
E.g. a parcel of solar radiation arriving at the Equator warms the ground, which in turn warms the air above it. This parcel of air moves north (i.e. as a wind) towards Aberdeen. Here it cools - i.e. it loses energy - and emits LW radiation, some of which (eventually) escapes to space.
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This persistent, large-scale system of winds and ocean currents is called the GENERAL
CIRCULATION. It is set up by the temperature gradient, caused by differential solar heating, which exists between the Equator and the Poles. Low latitudes (areas of heat surplus) export heat to high latitudes (areas of heat deficit). This transfer of heat cannot fully correct the temperature imbalance (heat gain at the Equator and heat loss at the Poles is too great) but it does stabilise the difference.
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Contrasts in heat supply change the distribution of mass within the atmosphere and oceans. This uneven distribution of mass creates pressure gradients (‘slopes’). Fluids such as air and water cannot withstand applied force - such as this pressure gradient force . This is why we get winds and ocean currents. Winds blow (and ocean currents flow) from high pressure (HP) to low pressure (LP), so parcels of air (water) - which carry heat and moisture - are shifted between different parts of the atmosphere (ocean basins). The steeper the pressure gradient (i.e. the closer together the isobars on a weather map) the faster the speed of this wind or current.
With respect to atmospheric motion (I exclude oceans here for the sake of clarity, but the principles of ocean circulation are very similar):
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LP represents a relative deficit of atmospheric mass; HP represents a relative excess of atmospheric mass.
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Surface LP is caused by uplift of air. This often reflects supply of heat from below, as at the
Equator (the warm ground surface triggers thermal convection). However, uplift of air can also be induced by juxtaposition of warm and cold air masses, in conjunction with the behaviour of strong high-level winds ( jet streams ) which pull air upwards. We will look at these process, characteristic of the mid-latitude Polar Fronts, in the next lecture.
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Surface HP tends to reflect sinking air: i.e. atmospheric mass which pushes its way towards, and collects close to, the Earth’s surface.
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Air which rises (warm, buoyant air) expands and cools, and gives rise to rainfall if it contains moisture. This is why it is wet at the Equator (convectional rainfall) and in the UK (frontal rainfall). Air which sinks contracts and warms up. This does not favour rainfall: e.g. the semi-arid and arid zones which lie beneath the extensive sub-tropical high pressure cells.
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LP and HP systems form linked circulation cells: e.g. the Hadley Cell of the tropics, or the sequence of depressions and anticyclones characteristic of the mid-latitudes. These links occur because there is a fixed quantity of atmospheric mass to move around. If air piles up in zone A, zone B must lose air in compensation. We cannot have HP or LP everywhere at the same time.
Surface LP usually means surface convergence of air, and divergence of air aloft (i.e. upper troposphere wind directions are reversed). [NB: the troposphere is the lowest layer of the
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GG2003: ENVIRONMENTAL SYSTEMS, 2001. Climate, Lecture 1. atmosphere, within which the general circulation is confined.] Similarly, surface HP usually means surface divergence, and convergence aloft.
Two further factors tend to exert a major influence on the general circulation:
CORIOLIS ‘FORCE’. The Earth spins at a constant rate (one turn every day). However, because its surface is curved, different points travel at different linear speeds: the daily straight-line distance travelled by a point located at the North Pole is zero, but a point on the Equator travels ~40,000 km per day. This means that things moving over the Earth’s surface tend to follow curved paths. It is possible to lose a lot of sleep over the Coriolis force; the things to remember are: 1) Coriolis force is zero at the
Equator, and reaches its maximum at the Poles; and 2) Coriolis force turns winds and currents to the right in the northern hemisphere, and to the left in the southern hemisphere.
DISTRIBUTION OF LAND AND SEA. This has two important effects:
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The topography of the ocean basins constrains the flow of ocean currents.
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Land heats up and cools down faster than does water. This means that land tends to be warmer than the sea in summer, but colder than the sea in winter. Thus it is common to differentiate between extreme (high temperature range) continental climates, and equable (low temperature range) maritime climates. Perhaps the best example of such thermal contrasts is the monsoon of the Indian sub-continent. Intense summer heating creates a major zone of LP, whereas winter cooling creates a zone of HP. Thermal response of the adjacent ocean is much less marked (
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‘middling’ atmospheric pressures all year round), so the pressure gradient force - and so the wind direction - is subject to seasonal reversal (i.e. ‘onshore’ winds in summer, ‘offshore’ winds in winter).
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