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S Surface Ocean Circulation and Geostrophy Fig. CO7 Ocean currents • Moving seawater • Surface ocean currents – Transfer heat from warmer to cooler areas – Similar to pattern of major wind belts because wind-driven – Affect coastal climates P Ocean currents and climate ZONAL ISOTHERMS August temperatures P Basically zonal S Measuring surface currents Fig. 7.2 Surface currents • Frictional drag between wind and ocean • Wind plus other factors such as – Distribution of continents – Gravity – Friction – Coriolis effect cause • Gyres or large circular loops of moving water P Ekman spiral • Surface currents move at angle to wind • Ekman spiral describes speed and direction of seawater flow at different depths • Each successive layer moves increasingly to right (N hemisphere) Fig. 7.5 P Ekman transport • Average movement of seawater under influence of wind • 90o to right of wind in Northern hemisphere • 90o to left of wind in Southern hemisphere Fig. 7.6 P Geostrophic flow • Ekman transport piles up water within subtropical gyres • Surface water flows downhill (gravity) and • Also to the right (Coriolis effect) • Balance of downhill and to the right causes geostrophic flow around the “hill” Fig. 7.7 P Western intensification • Top of hill of water displaced toward west due to Earth’s rotation • Western boundary currents intensified – Faster – Narrower – Deeper Eastern Boundary Currents • Eastern side of ocean basins • Tend to have the opposite properties of Western Currents • Cold • Slow • Shallow • Wide P P orange arrows = wind yellow arrows = water P Ocean gyres Subtropical gyres Centered about 30o N or S Equatorial current Western Boundary currents Northern or Southern Boundary currents Eastern Boundary currents Fig. 7.3 P • • • • • • • Atlantic Ocean circulation North Atlantic Subtropical Gyre North Equatorial Current Gulf Stream North Atlantic Current Canary Current South Equatorial Current Atlantic Equatorial Counter Current Climate effects of North Atlantic currents • Gulf Stream warms East coast of U.S. and Northern Europe • North Atlantic and Norwegian Currents warm northwestern Europe • Labrador Current cools eastern Canada • Canary Current cools North Africa coast P Atlantic Ocean circulation Fig. 7.14 South Atlantic Subtropical Gyre Brazil Current Antarctic Circumpolar Current Benguela Current South Equatorial Current P Other surface currents • Equatorial Countercurrents • Subpolar gyres Fig. 7.4 P One Sverdrup = 1,000,000 cubic meters/sec =1,000,000 tonnes/sec Harald Sverdrup 1888-1957 • Wellborn road to Texas Ave to University to George Bush Drive to the height of the O&M Building in 2.75 minutes • Peak flood flow of the Mississippi River is 0.03 Sv. PP > 2 knots < 2 knots S P Coastal upwelling • Ekman transport moves surface seawater offshore (upwelling) Fig. 7.11a 07_12a-c S Equatorial upwelling Figure 7.10 P Other surface currents • Equatorial Countercurrents • Subpolar gyres Fig. 7.4 P Low concentrations except in equatorial Pacific and high latitudes at the surface PP Purple areas are permanently stratified Satellite view of surface chlorophyll purple areas are low values red and yellow areas are high - Note the large surface area with low chlorophyll Antarctic circulation Antarctic Circumpolar Current (West Wind Drift) Encircles Earth Transports more water than any other current East Wind Drift Antarctic Divergence Antarctic Convergence Fig. 7.14 S P Rings, or eddies, are important because they transfer heat, salt, plants and animals across strong boundaries. P Gulf Stream Best studied Meanders or loops Warm-core rings Cold-core rings Unique biological populations Fig. 7.16 S Cold-core Rings S P PP North Atlantic Gyre South Atlantic Gyre S P One sverdrup = 1,000,000 cubic meters/sec • Wellborn road to Texas Ave to University to George Bush Drive to the height of the O&M Building in 2.75 minutes • Peak flood flow of the Mississippi River is 0.03 Sv. Peak flood flowof the Mississippi River is .03 sv. S Numbers are sverdrups (millions of cubic meters per sec) and the flow on the eastern boundary must equal that on the west, therefore, slower broader flow on the east means narrower, faster flow in the west. S S