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Transcript
Chapter 6 [part I] Wind and Ocean Circulation [The atmosphere] © 2006 Jones and Bartlett Publishers • Density of air [~1.2 kg/m3] is controlled by: – Temperature – Pressure [Is this also true for water?] – Moisture content [Why? Compare with full fat milk.] • Warm air is less dense than cold air and moist air is less dense than dry air. • Air pressure is the weight of the air from Earth’s surface to the top of the atmosphere. – It equals 1.04kg/cm2 (standard air pressure, one atmosphere) at sea level. [~10 m water, ~10 000kg/m2, ~1000hPa/m2] 6-1 Atmospheric Processes • Low pressure zone is where air density is lower than in surrounding areas because the air: – is warmer or – has a higher moisture content • High pressure zone is where air pressure is higher than in surrounding area because the air: – is cooler or – has lower moisture content 6-1 Atmospheric Processes • Fluids (air and water) flow from areas of high pressure to areas of low pressure. • Change in pressure across a horizontal distance is a pressure gradient. • Greater the difference in pressure and the shorter the distance between them the steeper the pressure gradient and the stronger the wind. 6-1 Atmospheric Processes Figure 6-1 Air Pressure 6-1 Atmospheric Processes • Winds are designated according to the direction from which they come. • In contrast, ocean currents are designated according to the direction towards which they travel. • Global winds blow in response to variation in pressure related to uneven solar heating (insolation) of Earth’s surface. 6-1 Atmospheric Processes Rotation of the Earth strongly influences winds. Coriolis deflection is the apparent deviation of objects moving across Earth’s surface. – Objects are deflected to the right of direction of travel in the northern hemisphere, – Objects are deflected to the left of direction of travel in the southern hemisphere. – [The amount of deflection is a function of latitude and current speed.] 6-1 Atmospheric Processes Figure 6-2 Coriolis Deflection 6-1 Atmospheric Processes Three major convection cells are present in each hemisphere • The Coriolis effect causes wind in these cells to bend to the east or west to form the (easterlies), westerlies, and the trade winds. 6-1 Atmospheric Processes Global [Average] Wind Circulation Figure 6-3a Global Wind Pattern 6-1 Atmospheric Processes Global [Average] Wind Circulation Figure 6-3b Air Pressure and Prevailing Winds 6-1 Atmospheric Processes Chapter 6 [part II] Wind and Ocean Circulation [Surface currents] © 2006 Jones and Bartlett Publishers Wind-driven currents are produced by the interaction between the wind and the water • As wind moves across the water, air molecules collide with water molecules. • When they collide, energy [momentum] is transferred from the air to the water. • Water moves at about 3-4% of the wind speed. 6-2 Surface Ocean Currents • Zonal wind flow is wind moving nearly parallel to latitude. – This is a result of Coriolis deflection. • A gyre is a circular current caused by: – Westerly-driven ocean currents in the trade winds – easterly-driven ocean currents in the Westerlies – deflection of the ocean currents by the continents 6-2 Surface Ocean Currents Surface Ocean [Average] Currents Figure 6-4 Surface Ocean Currents 6-2 Surface Ocean Currents • The sea surface is warped into broad mounds and depressions with a relief of about one meter. • They cause pressure gradients to develop in the ocean. • Mounds on the ocean’s surface are caused by converging currents. • Depressions on the ocean’s surface are caused by diverging currents. • Water flows down pressure gradients on the ocean’s irregular surface, from mounds to depressions [as in the atmosphere]. • The flowing water is deflected by the Coriolis effect [as in the atmosphere]. – The amount of deflection is a function of latitude and current speed. 6-2 Surface Ocean Currents • With time, wind-driven surface water motion extends downward into the water column. – speed decreases – direction changes because of Coriolis deflection • Ekman Spiral is the pattern caused by changes in water direction and speed with depth. [Ekman (1905)] 6-2 Surface Ocean Currents • Ekman transport is the net transport of water by wind-induced motion. • Net transport of the water in an Ekman spiral has a Coriolis deflection of 90o to the direction of the wind. • Along coastal areas, Ekman transport can induce: – downwelling by driving water towards the coast, or – upwelling by driving water away from the coast [The angles for surface current and net transport depend on the circumstances.] Figure 6-6b Map View 6-2 Surface Ocean Currents Figure 6-6a Ekman Spiral in the Northern Hemisphere 6-2 Surface Ocean Currents Convergence & Divergence of Water Currents in the Northern Hemisphere [Where are the richest fishing waters? Compare with global wind pattern!] Figure 6-7a Coastal Divergence & Convergence 6-2 Surface Ocean Currents Figure 6-7b Ocean Divergence & Convergence 6-2 Surface Ocean Currents Geostrophic flow allows currents to flow long distances with no apparent Coriolis deflection. • Coriolis effect deflects water into the center of the gyres, forming a low mound of water. • As height of the mound increases, the pressure gradient steepens pushing the water outward to level the mound. • When the pressure gradient equals Coriolis deflection, the current flows parallel to the wind around the mound. – This is called geostrophic flow. 6-2 Surface Ocean Currents Geostrophic Currents (a) Stacking of water in center of ocean (b) Effect of pressure gradient (c) Geostrophic currents Figure 6-9 6-2 Surface Ocean Currents • The current flow pattern in gyres is asymmetrical: – narrow, deep and swift currents along the basin’s western edge. – broad, shallow slower currents along the basin’s eastern edge Figure 6-10 Flow Asymmetry Around Circulation Gyres 6-2 Surface Ocean Currents • The geostrophic mound is deflected to the western part of the ocean basin because of the eastward rotation of the Earth on its axis. • The Sargasso Sea is a large lens of warm water: – encircled by the North Atlantic gyre – separated from cold waters below and laterally by a strong thermocline 6-2 Surface Ocean Currents • Western boundary currents, such as the Gulf Stream, form a meandering boundary. • These boundaries separate coastal waters from warmer waters in the gyre’s center. • Meanders can be cut off to form warmcore and cold-core rings. 6-2 Surface Ocean Currents Gulf Stream Meanders and Rings Figure 6-12a Heat Photo of Gulf Stream 6-2 Surface Ocean Currents Figure 6-12b Formation of Rings 6-2 Surface Ocean Currents