1 0º Equator90º Pole LP HP  At the Equator the atmosphere is heated  Air becomes less dense and rises.  Rising air creates low pressure at the equator.  Air cools as it rises because of the lapse rate.  Air spreads.  As air mass cools it increases in density and descends.  Descending air creates high pressure at the Poles.  Surface winds blow from HP to LP. SINGLE CELL MODEL

2  warm air is less dense therefore lighter  air rises in the Tropics  this creates a zone of LOW PRESSURE  air spreads N and S of the Equator  air cools and sinks over the Poles  this is a zone of HIGH PRESSURE  air returns as surface WINDS to the Tropics

3 ITCZ = Inter-tropical convergence Zone (Low Pressure) STH = Sub-tropical High (High Pressure) HADLEY CELL ITCZ

4 0º Equator90º Pole30º60º LP HP Hadley Cell Polar Cell Ferrel Cell THREE CELL MODEL

5 Warm air rises at the Equator - Inter-Tropical Convergence Zone (ITCZ). Equatorial air flows to ~30º N then sinks to the surface and returns as a surface flow to the tropics. This is the Hadley cell. Cold air sinks at the North Pole. It flows S at the surface and is warmed by contact with land/ocean, by ~60º N it rises into the atmosphere. This the Polar cell. Between 60º N and 30º N there is another circulation cell. This is the Ferrel cell. The Hadley cell and the Polar cell are thermally direct cells. The Ferrel cell is a thermally indirect cell. ENERGY TRANSFER

6 Hadley Cell Polar Cell Ferrel Cell Heat energy is transferred from the Hadley Cell to the Ferrel Cell and from the Ferrel Cell to the Polar Cell. In this way heat is transferred from the Equator where there is an energy surplus to the Poles where there is an energy deficit.

7 0º Equator90º Pole30º60º LP HP convergence divergence convergence divergence winds blow from high pressure zones to low pressure zones WINDS

8 CONVERGENCE and …………DIVERGENCE

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10 Coriolis occurs because the Earth rotates. Earth rotates about its axis every 24 hours. Distance around the equator is ~25,000 miles the earth is travelling east at ~ 1,000 miles per hour. Distance around the Earth at 40ºN ~19,000 miles the earth is travelling east at ~800mph. The Coriolis effect results from this difference in velocity. In the Northern hemisphere the Coriolis effect deflects movement to the right. In the Southern hemisphere Coriolis effect deflects movement to the left. The combination of atmospheric cells and Coriolis effect lead to the wind belts. Wind belts drive surface ocean circulation

11 WIND Coriolis effect High Pressure Low Pressure pressure gradient force Winds are named by the direction they blow from. PLANETARY WINDS

12 90ºS 90ºN 0º Equatorial Low - Doldrums LP 30ºN Sub-tropical High - Horse Latitudes HP30ºS Sub-tropical High - Horse Latitudes HP 60ºS Temperate Low LP 60ºNTemperate Low LP Polar easterlies South westerlies NE Trades Polar easterlies North westerlies SE Trades WIND BELTS

13 90ºS Polar easterlies South westerlies NE Trades Polar easterlies North westerlies SE Trades 0º convergence Inter-tropical convergence zone LP 30ºS divergence Sub-tropical High HP 30ºN divergence Sub-tropical High HP 60ºN convergence LP 60ºS convergence LP WIND BELTS

14 WIND BELTS

15 Northern Hemisphere Polar Easterlies Blowing from the Polar High Pressure zone to about 60ºN Westerlies Blowing from Sub-Tropical High Pressure zone to about 60ºN Northeast Trade Winds Blowing from Sub-Tropical High Pressure zone to Equatorial Low Pressure zone. Southern Hemisphere Southeast Trade Winds Blowing from Sub-Tropical High Pressure zone to Equatorial Low Pressure zone. Westerlies Blowing from Sub-Tropical High Pressure zone to about 60ºS Polar Easterlies Blowing from the Polar High Pressure zone to about 60ºS WIND BELTS

16 Series of High and Low pressure centres approx. every 30º latitude High pressure zones associated with descending air (divergence) Low pressure zones associated with rising air (convergence) Three circulation cells in each hemisphere: Hadley Cell thermally direct Ferrel Cellthermally indirect Polar Cellthermally direct Wind is the horizontal movement of air arising from differences in pressure. Very little wind at the Equator (Doldrums) because air is being convected upward. Little wind at 30ºN and S (Horse Latitudes) because direction of air movement is down. Winds always blow from an area of High Pressure to Low Pressure. Winds are affected by the Coriolis Effect. Coriolis is a consequence of motion on a rotating sphere. Acts to the Right of direction of motion in Northern Hemisphere Acts to the Left of direction of motion in the Southern Hemisphere Major wind belts of the Earth surface 0 to 30ºN Northeast Trades 0 to 30ºS Southeast Trades 30 to 60ºN/S Westerlies 60 to 90ºN/S Polar easterlies

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18 4 forces: > solar heating > surface winds > Coriolis effect > and surface winds result in a clockwise circulation of water in the Northern hemisphere. This circulation is known as a GYRE.

19 OCEAN CURRENTS IN THE NORTH ATLANTIC 0º Equator 90º Pole NORTH EQUATORIAL CURRENT GULF STREAM NORTH ATLANTIC DRIFT LABRADOR CURRENT CANARIES CURRENT NORTH ATLANTIC DRIFT

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