8-OcFA11-SurfDeepCircWEB

8-OcFA11-SurfDeepCircWEB - Ocean Circulation: Introduction...

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Ocean Circulation: Introduction Surface Ocean Circulation Mixed layer ~ top 200m (10%) Controlled by wind plus Coriolis Deep Ocean Circulation Below ~ 200m (90%) Controlled by density (T and S) variation Ultimate Driver–Differential Heating by Sun (Energy Source) + Force of Gravity Direct methods Float meters (drifters or moored) Propeller meters Float meters Inadvertent Indirect method – via Satellite since 1996 (based on altimeter derived sea level data) Wind pushes on ocean surface Friction sets water into motions Flows at ~1-2% of the wind speed Wind ~ 5-20 m/s, Currents ~ 0.1 – 0.5 m/s Fridtjof Nansen first to research
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Ekman Transport Polar Easterlies Predominant Winds (named for direction coming from) Currents (named for direction going to) Net Surface Water Movement = Ekman Transport Creates a Gyre: Closed, circular flow around the periphery of an ocean basin Polar Easterlies Because of Coriolis Surface water flows at ~45° to the wind direction Northern Hemisphere: 45 o to right Southern Hemisphere: 45 o to left Net effect Each deeper layer pushes layer below + coriolis deflection = Ekman spiral Net effect = transport of the upper ocean at 90° to the wind 90° to right of the wind (North Hem.) 90° to left of the wind (South Hem.)
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N. Atl. gyre gyre N. Pacific gyre S. Pacific gyre gyre 5 gyres
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Geostrophic Flow Ekman Transport will pile water up in the center of the gyre (about 1 – 1.5 m higher in the center) Gyre ‘hills’ can’t keep getting bigger Gravitational force will cause flow back down the hill = a pressure gradient (also deflected by Coriolis) NE Trades Ekman Westerlies gravity gravity Ekman flow flow
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Geostrophic Flow Result = Geostrophic Flow A balance between forces of the wind (Ekman + coriolis) and gravity (pressure gradient + coriolis) Flow around the periphery of an ocean basin (geostrophic flow) is maintained by the balance between the wind which builds up a hill, and gravity (which knocks it down) Boundary currents flow North or South, Transverse currents flow east or west Transverse Currents Eastern Boundary Currents Western Boundary Currents Eastern Boundary Currents Western Boundary Currents Transverse Currents Transverse Currents
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Boundary Currents Western Boundary Currents Warm, transports heat to higher latitudes Strong, fast, narrow, brings moisture 100’s km/day <100 km wide e.g. Gulf Stream Surface current that flows like a river Carries amount of water ~ 100x the discharge of all rivers Depth of the Gulf stream ~ 450 m Width ~ 70 km Separates warm, nutrient-depleted water from cold, nutrient-rich water Has Eddies Cold core and warm core eddies 100’s of miles wide Surface to seafloor (abyssal storms) Duration – several years
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8-OcFA11-SurfDeepCircWEB - Ocean Circulation: Introduction...

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