Global Atmospheric and Ocean Circulation

Global Atmospheric Circulation

Global atmospheric circulation contributes to the understanding of the location of the Earth's climatic zones. Atmospheric circulation is the movement of air masses in the atmosphere to distribute energy across the planet. Atmospheric cells are driven by the heat of the sun, meaning that latitude is an important factor in atmospheric circulation.   The surface of the Earth is curved; the equator receives much more sunlight than the poles. Sunlight is concentrated at the equator because sunlight hits a parallel surface. At the poles, sunlight is sparse and widely dispersed due to its angle from the sun. This makes latitude an important factor in climate. 

The air at the equator is heated and becomes less dense, rising to a higher altitude. This creates a zone of low pressure; following this, the air spreads out towards the poles The lack of heating at the poles causes air to be cold and dense, and so it sinks towards the ground, spreading towards the equator and creating a zone of high pressure. The spreading of both air masses forms a convection cell in which air is circulated.  There are three different convection cells that operate on each side of the equator; Polar cells, ferrel cells, and Hadley cells. 

Convection Cells

Low pressure at the equator causes a Hadley cell to form. Hadley cells bring warm trade winds At the equator, trade winds from the Northern and Southern hemisphere meet, causing thunderstorms. Where these two winds meet is called the Inter-Tropical Convergence Zone (ITCZ).   Low pressure at the Tropics of Cancer and Capricorn cause Ferrel cells to form. Ferrel cells bring warm southwesterly winds in the Northern hemisphere and northwesterly winds in the Southern hemisphere. The Coriolis effect causes the cells to motion to the left in the Southern hemisphere and to the right in the Northern hemisphere.   High pressure at the poles forms polar cells.    The rotation of the Earth causes strong, high-altitude jet streams to form.  The tilt of the Earth causes the position of convection cells to shift seasonally. 

How Atmospheric Circulation affects Precipitation:

Rainfall is high and constant around the year at the equator. Hot air can hold more water vapour than colder air. This brings convectional rain to the areas surrounding the equator. The ITCZ is a low pressure area. As it moves, it brings tropical cyclones and heavy rainfall with it.  Rainfall is higher in coastal areas in Western Europe due to the movement of the jet stream over the Atlantic. Rain baring systems often surround the jet stream, bringing stormy conditions to the UK's west coast. Arid conditions are experienced at the tropic of Cancer and Capricorn due to dry air descending over these areas in the Hadley cell. Precipitation is generally low in polar areas as the cold air cannot hold much water vapour. 

Ocean Circulation:

Oceans transfer up to 20% of the total heat energy from the tropics to the poles.  Patterns of surface currents are referred to as gyres. They are produces as masses of water move from one climatic zone to another. Ocean circulation is aided by surface winds created by atmospheric circulation. In the Northern hemisphere currents move in a clockwise direction and in the Southern hemisphere currents move in an anti-clockwise direction. The strongest currents are on the Western side of the oceans.