The Earth's Climate

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Atmosphere and oceans

The driving force behind the Earth's atmospheric movements is the Sun. Solar radiation heats the surface of our planet, which in turn heats the air that surrounds it. Masses of air are warmed contact with the Earth's surface, thus tending to rise as warm air is less dense than cold air. A cyclone, also called a depression or low-pressure zone, then forms at ground level. Conversely, cold air masses tend to drop lower in the atmosphere and form anticyclones, or high-pressure zones, at ground leve.l
The warm air cools as it rises, thus descending towards the ground, where it is warmed again. This air circulation cycle occurs on a planetary scale in patterns determined by the Earth's energy balance. Overall the planet's energy balance is zero, but there is an accumulation of energy at the lower latitudes and a deficit at the poles. Masses of air circulate at ground level from the polar high-pressure zones towards the equatorial low-pressure zones, and back again at high altitudes. In addition, each hemisphere has not just one, but three zonal air circulation cells.
The warm, humid air that rises from the ground in the equatorial low-pressure regions moves toward the North and South Poles on either side of the Equator, getting gradually cooler. At about 30 degrees latitude this tropical air meets the cold polar air, drops back towards the surface and returns to the Equator in the form of trade winds. This tropical cell transfers heat from the Equator to the tropics. Between 30 and 60 degrees latitude an inverse cell forms, characterized by winds that blow from south to north at ground level. Further north, the cold, high-density air flows towards the temperate latitudes, forming the third cell.
In addition, the Earth's rotation affects the movements of these air masses. The winds blowing from the high-pressure towards the low-pressure zones are deviated to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The warm, humid air that rises from the ground in the equatorial low-pressure regions is deflected to the east as it moves north, transforming at about 30 degrees North into a powerful jet stream that flows over the region where the tropical and polar air meet on the ground. This region is characterized by an unstable thermal front that causes atmospheric disturbances whose activity transfers heat very efficiently from south to north.
Thermal energy is also transferred from the Equator to the poles by the oceans, in which a system of currents offsets the uneven distribution of thermal energy received on the surface. Surface currents in the oceans are primarily caused by wind action and are sensitive, like the winds, to the rotational "Coriolis Force." They are also affected by variations in sea level and pressure fields.
Generally, the oceans convey heat from the Equator to the poles via the major western boundary currents, the Gulf Stream and the Kuroshio in the Northern Hemisphere and the Brazil and Agulhas currents in the Southern Hemisphere. These waters are cooled, causing them to sink in the temperate latitudes and then return towards the Equator as deep currents. However, this general principle is modified by the specific geographic characteristics of each region. In fact, only the Pacific Ocean follows the pattern exactly. The Indian Ocean, blocked to the north by the barrier of the Indian subcontinent, transfers heat southwards at all latitudes, and the Atlantic Ocean, which opens onto the Arctic Ocean, transfers heat northwards at all latitudes.
This characteristic of the Atlantic, which plays a key role in the endless "conveyor belt" of oceanic circulation, is linked to its capacity to form deep waters in the Subarctic region. One stream of the warm, saline water of the Atlantic rises towards the Arctic along the coastlines of Europe, gradually cooling and becoming denser. When the water reaches freezing point, part of it transforms into pack ice, releasing its salt into the surrounding water and thus further increasing its density. Gravity pulls this cold, high-salinity, high-density water down to depths of between 2,000 and 4,000 meters, where it forms a deep current that conveys the cold northern water southward. The result is a deep transport in the North Atlantic comparable to that provided by surface currents.