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Steady State General Ocean Circulation “steady state” means: constant in time, no accelerations or Sum of all forces = 0 Outline: 1. Ekman dynamics (Coriolis~Friction) 2. Geostrophic dynamics (Coriolis~Pressure gradients) 3. Ekman+Geostrophy with Coriolis as f=f0+by The subtropical gyre circulation is a geostrophic flow (with many eddies) From WHP Pacific Atlas (Talley, 2007) http://www-pord.ucsd.edu/whp_atlas Geostrophy Coriolis balances pressure gradient Geostrophic Degeneracy Ekman balance: Coriolis ~ Friction QuickTime™ and a decompres sor are needed to s ee this picture. QuickTi me™ a nd a de com press or are need ed to se e th is p icture. QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. a-1=(2Az/f) is a vertical decay scale ~ 20m-60m from Stewart, 2005 Ekman velocity spiral • • • • Surface velocity to the right of the wind (northern hemisphere, due to Coriolis) Surface layer pushes next layer down slightly the right, and slightly weaker current Next layer pushes next layer, slightly to right and slightly weaker current Producing a “spiral” of the current vectors, to right in northern hemisphere, decreasing speed with increasing depth • Details of the spiral depend on the vertical viscosity (how frictional the flow is, and also whether “friction” depends on depth) Ekman transport The wind stress on the ocean surface is the vector = ( (x) , (y) ) Integrate the Coriolis/friction balances in the vertical x: -fv = /z(AVu/z) -> -fVEK= AVu/z = (x) / y: fu = /z(AVv/z) -> fUEK= AVv/z = (y) / •UEK and VEK are the “Ekman transport” ∫udz, ∫vdz •Ekman “transport” is exactly to the right of the wind stress (northern hemisphere ). •Ekman transport does not depend on the size or structure of AV (but the detailed structure of the spiral DOES depend on it) Ekman layer “transport” • “Transport”: 90° to wind, to right in northern hemisphere • UEk= /f (units are m2/s, not m3/s so technically this is not a transport; need to sum horizontally along a section to get a transport). • Typical size: for wind stress 0.1 N/m2, UEk= 1 m2/s. Integrate over width of ocean, say 5000 km, get total transport of 5 x 106 m3/sec = 5 Sv. Ekman layer depth • Depth: depends on eddy viscosity AV (why?) Dek = (2AV/f)1/2 • Eddy viscosity is about 0.05 m2/sec in turbulent surface layer, so Ekman layer depth is 20 to 60 m for latitudes 80° to 10°. Ekman layer velocity • Velocity: spirals with depths and magnitude depends on eddy viscosity. If AV is constant, surface velocity is 45° to wind • For eddy viscosity 0.05 m2/sec, and wind stress of 1 dyne/cm2 (.1 N/m2), surface velocity is 3 cm/sec at 45°N. Observations of Ekman layer Direct current measurements in California Current region revealed excellent Ekmantype spiral (Chereskin, JGR, 1995) Global surface wind velocity Westward Eastward Ekman divergence (Ekman upwelling) at equator and at land boundaries Equatorial Land boundary (southern hemisphere, like Peru) QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Ekman transport convergence and divergence Ekman transport convergence and divergence Ekman Pumping Geostrophic flow Vertical velocity at base of Ekman layer: order (10-4cm/sec) (Compare with typical horizontal velocities of 1-10 cm/sec) Pacific winds (mean) • Ekman pumping Gray: downwelling White: upwelling from Talley 2006 Ekman Pumping continuity QuickTime™ and a decompressor are needed to see this picture. vertical integral Ekman transports Convergence in Ekman transport Recall MEx = y/f MEy = -x/f Vertical velocity Ekman pumping Ekman transport 90 degrees to wind