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Ocean Currents I

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Ocean currents I Ocean currents are driven by surface effects such as the surface motion driven by the drag interaction of wind and water and the vertical motion driven by density and salinity gradients in the deeper ocean layers. These motions couple with the Coriolis force, which results from the influence of Earth’s rotation on a northward (or southward) moving bodies with velocity v. The directional forces Wind-water interaction The drag forces Circular pattern of ocean currents Earth rotates counter-clockwise The ocean currents run clockwise on the northern hemisphere and turn counter-clockwise on the southern hemisphere! The Coriolis Force The Coriolis force is a result of classical mechanics on a moving object in a rotating frame of reference! The Coriolis force pushes a moving object to the right for counterclockwise rotation of the reference frame or to the left for clockwise rotation. The force is a vector cross product between velocity vector of the moving object and the orbital momentum vector of the rotation. The Coriolis force represents a vector cross product Fc f zˆ v zˆ,v are unit vectors they give the direction of componts, and v 2sin a s1 Coriolis parameter with as latitude f ma zˆ v cross product in vector terms Fc ~ zˆ v points in perpendicular direction Fc ~ zˆ v defines vector direction of transport Coriolis Force on Earth The earth rotates with angular velocity around its axis. A body located on earth with mass m has a tangential linear velocity of vx=r in the external reference frame . The angular momentum is: L m r2 If the body moves pole-wards with a velocity v=vy , the angular momentum L changes, generating the torque : dL dmr 2 dt dt dL dr r 2mr r Fc m dt dt R dr dy sin v sin dt dt is latitude Fc 2mv sin The Coriolis force accelerates a body moving south on the southern hemisphere to the west or or a body moving north on the northern hemisphere to the east. Fc mac Fc 2mv sin ac 2v sin 3600 rad 2 7.292105 s1 day day A motor yacht crosses the 20o N latitude moving northwards in the Northern Atlantic with a constant speed of 10 knots (1knot=1.852 km/h). What is the strength of the Coriolis force Fc acting on the boat, if the yacht has a mass of 20 tons? 1.85104 m F 2 2104 kg7.292 105 s1 sin 200 c 3600s kgm F 5.127 5.127N c s2 0 for 20 S Fc 5.127N The boat needs one full minute to accelerate from zero to its 10 knots speed. Calculate the force Fm, the engine has to generate for bringing the boat up to the speed of 10 knots. 10knots F ma 2104 kg m 1min m 5.14 F 2104 kg s 1713N m 60s The Coriolis force is relatively small but becomes larger at higher latitudes. Calculate the Coriolis force acting on the boat when it crosses the 60o N latitude on its way to Iceland? 1.85104 m F 22104 kg7.292 105 s1 sin600 c 3600s kgm F 12.98 13N c s2 Coriolis force on air or water mass forms inertial cycles! Air or water moving with speed v without any external forces acting upon them, except for the Coriolis force Fc, will take a circular clockwise flow trajectory on the northern hemisphere, and a circular counter-clockwise flow trajectory on the southern hemisphere, balanced by the centripetal force Fcf. v2 F m 2m v sin F cf R c v v v R 2 sin 20.263h1 sin 0.525h1 sin The Gulf Stream is the fastest ocean current in the world with peak velocities near 3m/s. What is the curvature at 35o north latitude? m 3 v R s 1 1 0.525h sin 1.46104 sin s m 3 R s 35,825m 36km 1 1.46104 sin 350 s Only small eddies form at that low flow speed! Reality Realityis more iscomplex more complex because becauseof additional of additional forces due forces to friction due toand the drag temperatureforces effects, provided which by the add atmospheric to the eddy windformation circulation phenomenon! and by the friction forces exerted by deeper water layers! Winds as pressure phenomenon Air moves; less dense (higher temperature) air pockets rise to higher altitude, cools and sinks back towards earth surface. Airflow rushes toward low pressure regions and away from high pressure zones. As larger the pressure gradient, as larger is the force acting upon an air mass m, increasing the wind velocity. m F P The force points from a higher pressure p towards a lower pressure range dP g Upwards and downwards motion is determined by dz the hydrostatic equation leading to the lapse rate. Isobars indicate lines of constant pressure P=0! Wind direction points perpendicular to isobars (lines of constant pressure) Global isobar map Presence of large continental land areas in the northern hemisphere affects the ‘ideal’ isobar patterns observed at southern latitudes. During the summer land is warmer than ocean causing the development of low pressure areas, which change northward/southward winds patterns. During winter land is colder than ocean water, causing the development of low pressure areas over the ocean regions. The emergence of localized weather zones is further enhanced by the geographical affects from lakes or mountains. Geostrophic winds on non-rotating earth Friction effects are neglected Wind direction would be defined entirely by the latitudinal isobars Wind convection cells form because of the equatorial heating effects and the polar cooling effects. The ITCZ (intertropical convergence zone) shows only a negligible horizontal wind component, and is also called the doldrums, where the seasonal lack of wind often delayed a sailing ship’s journey. The intertropical convergence zone Area of cloud formation because of extended condensation and uprising air masses. Also origin of thunderstorms and cyclones if a rotational friction component is added! The effects of rotation, Coriolis force Wind continues to move perpendicular to isobars, but air masses are subject to the Coriolis force, which prevents winds to reach the poles, forming three circulation patterns. At 30o latitude the air cools and sinks, moving back to equator in an inertial cycle in north easterly directions (the trade winds). Winds that continue to move north are deflected eastwards by increasing Coriolis force (prevailing westerly winds). Radius of circular wind patterns v v R 2sin 0.525h1 sin m 100 v R s 1194km 1 1 0.525h sin 1.46104 sin 350 s Reality shows larger radii (2500 km) due to The trade winds blow persistently westward the impact of pressure gradients! and toward the Equator as part of the geostrophic wind pattern from the subtropical high-pressure belts toward the intertropical convergence zone (ITCZ). At high altitude the average speed over the oceans is about 50-100 m/s. On low altitude the speed slows down due to friction. The trade winds were named by the crews of sailing ships that depended on the winds during westward ocean crossings. Westerlies Westerlies trade winds trade winds Hurricanes (northeasterly) (northeasterly) doldrums doldrums doldrums trade winds (southeasterly) trade winds Cyclones (southeasterly) Westerlies Trade routes for Guano supply Coriolis force on low pressure area This low-pressure system spins counter-clockwise due to the balance between the Coriolis force and the pressure gradient force. The air tends to flow towards it, but is deflected perpendicular to its velocity by the Coriolis force. A system of equilibrium can then establish itself creating circular movement, or a cyclonic flow. Fc Fp Wind-Water surface interaction Water motion at the surface of the ocean (mixed layer) is driven by wind effects. Friction causes drag effects on the water, transferring momentum from the atmospheric winds to the ocean surface water. The drag force Wind generates vertical and horizontal motion in the water, triggering convective motion, causing turbulent mixing down to about 100m depth, which defines the isothermal mixed layer. The drag force FD on the water depends on wind velocity v: 2 FD CD A a v CD drag coefficien t dimensionless constant for wind water interactio n CD 0.002, A cross sectional area depending on surface roughness,and particularly the emergence of waves! The Beaufort Scale is an empirical measure describing wind speed based on the observed sea conditions (1 knot = 0.514 m/s = 1.85 km/h)! m Conversion from scale to wind velocity: v 0.836 B3/ 2 s A strong breeze of B=6 corresponds to wind speed of v=39 to 49 km/h at which long waves begin to form and white foam crests become frequent. The drag force can be calculated to: kg km m F C A v2 1.2 v 45 12.5 C 0.001 D D a a m3 h s D 2 kg 2 m 2 FD 0.0011.2 Am 12.5 0.188 A N or FD A 0.188 N m m3 s For a strong gale (B=12), v=35 m/s, the drag stress on the water will be: 2 kg m 2 FD / A 0.00251.2 35 3.68 N m m3 s kg m 1.2 v 35 C 0.0025 a m3 s D.
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