Climate system: Movement of wind and water
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Ocean currents and large ‘convection cells’ in the atmosphere transport heat from the Equator towards the poles. The Earth’s rotation causes prevailing winds that would otherwise blow northwards or southwards to curve round, creating easterly ‘trade winds’ in the tropics, westerly winds in mid-latitudes and easterly winds at the poles.
The intense sunlight at the Equator is absorbed by the Earth’s surface, causing it to give off heat and warm the air above it. The air expands as it warms – becoming less dense – and rises, cooling as it does so. This cooler air moves higher and flows both north and south, causing a stream of air flowing poleward at high altitude. The Earth’s rotation causes this airflow to curve round until – at about 30 degrees latitude – it’s flowing from west to east rather than poleward, at which point it begins to sink. The rising air at the Equator leaves an area of low pressure at the surface, drawing in cooler air from the north and south. These processes drive the two tropical convection cells and much of the climate’s large-scale circulation.
Scientists consider the atmosphere in terms of different layers, each defined by the way air temperature changes with height. In the lowest layer – the troposphere – temperature tends to decrease with height. This is known as a positive lapse rate. In contrast, the layer above the troposphere – the stratosphere – has a negative lapse rate, becoming warmer with increasing altitude. The top of the troposphere is defined as the height at which the lapse rate ceases to be positive. This height varies with latitude, because the intense surface heating at the Equator drives deeper convection there, causing the two tropical convection cells to extend higher into the atmosphere than the mid-latitude and polar cells. As a result, the troposphere is up to 15 km high at the Equator, declining to less than 10 km high at the poles.
If our planet didn’t rotate, the tropical convection cells would extend uninterrupted to the poles. However, the atmosphere and oceans are fluids resting on the Earth’s solid rotating surface. This influences their apparent direction of travel, causing moving fluids to curve to the right in the northern hemisphere and to the left in the southern hemisphere – known as the Coriolis effect. So air can’t flow directly between the Equator and poles, because it gets deflected along the way. This breaks up the circulation into three cells encircling the globe in each hemisphere – the tropical cells from 0 to 30 degrees latitude, the mid-latitude cells from 30 to 60 degrees and the polar cells from 60 to 90 degrees.
There are three large atmospheric convection cells in each hemisphere. Two of these – the tropical and polar cells – are characterised by poleward airflow at high altitude and surface airflow towards the Equator. So the dominant surface winds in these cells blow from north to south in the northern hemisphere and from south to north in the southern hemisphere. But these winds are curved round by the Earth’s rotation. As a result, tropical and polar surface winds blow mainly from east to west – known as easterlies. Small-scale weather features mean that not all winds follow the same path. But in the tropics the prevailing easterly winds became known as ‘trade winds’ because of their reliable direction and importance for commercial sailing routes.
In the tropical and polar convection cells, the direction of airflow is driven by the temperature differences between the Equator and the poles. Sandwiched between these is the mid-latitude convection cell, which is driven by pressure differences created by the neighbouring cells. Airflow in the mid-latitude cell is in the opposite direction to the other cells, travelling towards the Equator at high altitude. So the prevailing surface winds at mid-latitudes blow towards the poles, curving round in a westerly direction under the influence of the Earth’s rotation. However, the prevailing winds are far less dominant here than in the tropical and polar cells, and mid-latitude regions are characterised by highly variable weather patterns.
Covering 70% of the Earth’s surface, the oceans play a major role in the redistribution of heat around the globe. Warmer waters flow toward the poles at the surface, while colder, denser water sinks at high latitudes, flowing back towards the Equator at depth. Heat released by the warm surface currents can have a pronounced effect on regional climates – for example, the Gulf Stream warms parts of the UK. Because the Earth’s rotation affects the movement of all fluids on its surface, such currents aren’t a uniform mass of moving water. Instead, the currents are composed of many short-lived whirlpools, known as eddies, all swirling along the same overall path. Ocean currents move very slowly and on average it takes a water molecule about 1000 years to travel round the whole circuit.
Faye Davies is trying to understand how the Earth’s surface and the atmosphere interact with each other. She uses LIDAR and radiometers to measure the temperature, humidity and movement of air, upwards through the atmosphere. This gives clues as to how the Earth’s surface affects the atmosphere and vice versa. ‘We’re used to thinking about climate in terms of how hot or cold we feel,’ says Faye. ‘But these effects on the ground are controlled by things happening much higher up in the atmosphere.
‘My job can be tough. The weather never quite seems to do what’s expected. My instruments are often mounted on tall masts, which means lightning strikes can be a problem.’