Lecture 10: Ocean Circulation Ekman Transport Ekman Pumping Wind-Driven Circulation ESS228 Prof. Jin-Jin-YiYi Yu Basic Ocean Structures Warm up by sunlight! Upper Ocean (~100 m) Shallow, warm upper layer where light is abundant and where most marine life can be found. Deep Ocean Cold, dark, deep ocean where plenty supplies of nutrients and carbon exist. ESS228 No sunlight! Prof. Jin-Jin-YiYi Yu Basic Ocean Current Systems Upper Ocean surface circulation Deep Ocean deep ocean circulation ESS228 (from “Is The Temperature Rising?”) Prof. Jin-Jin-YiYi Yu Vertical Structure of Ocean Mixed Layer: T and S well mixed by winds Temperature Thermocline: large gradient of T and S Salinity Deep Ocean: T and S independent of height cold salty high nutrient level (from Climate System Modeling) ESS228 Prof. Jin-Jin-YiYi Yu Mixed Layer Processes The depth of the mixed layer is determined by (1) the rate of buoyancy generation and (2) the rate of kinetic energy supply. The atmosphere can affect the mixed layer through three ppggrocesses: heating, wind forcing, and freshening (P-E). The global-average depth of the mixed layer is about 70 m . The heat capacity of the mixed layer is about 30 times the heat (from Global Physical Climatology) capacity of th e at mosph ere. ESS228 Prof. Jin-Jin-YiYi Yu Seasonal Variation of Mixed Layer Summer: warm and thin. Winter: cold and deep (several hundred meters). (from Global Physical Climatology) ESS228 Prof. Jin-Jin-YiYi Yu Two Circulation Systems density-driven wind-driven ciltiirculation (Figure from The Earth System) circulation ESS228 Prof. Jin-Jin-YiYi Yu Global Surface Currents (from Climate System Modeling) ESS228 Prof. Jin-Jin-YiYi Yu Six Great Current Circuits in the World Ocean 5 of them are geostrophic gyres: North Pacific Gyre South Pacific Gyre North Atlantic Gyre South Atlantic Gyre Indian Ocean Gyre The 6th and the largest current: Antarctic Circumpolr Current (also called West Wind Drift) (Figure from Oceanography by Tom Garrison) ESS228 Prof. Jin-Jin-YiYi Yu Characteristics of the Gyres (Figure from Oceanography by Tom Garrison) Currents are in geostropic balance Each gyre includes 4 current components: two boundary currents: western and eastern two transverse currents: easteward and westward Western boundary current (jet stream of ocean) the fast, deep, and narrow current moves warm water polarward (transport ~50 Sv or greater) Eastern boundary current the slow, shallow, and broad current moves cold water equatorward (transport ~ 10-15 Sv) Trade wind-driven current the moderately shallow and broad westward current (transport ~ 30 Sv) Westerly-driven current the wider and slower (than the trade wind-driven Volume transport unit: current) eastward current 1 sv = 1 Sverdrup = 1 million m3/sec ESS228 (the Amazon river has a transport of ~0.17 Sv) Prof. Jin-Jin-YiYi Yu Major Current Names Western Boundary Current Trade Wind-Driven Current Gulf Stream (in the North Atlantic) North Equatorial Current Kuroshio Current (in the North Pacific) South Equatorial Current Brazil Current (in the South Atlantic) Eastern Australian Current (in the South Pacific) Agulhas Current (in the Indian Ocean) Eastern Boundary Current Westerly-Driven Current Canary Current (in the North Atlantic) North Atlantic Current (in the North Atlantic) California Current (in the North Pacific) North Pacific Current (in the North Pacific) Benguela Current (in the South Atlantic) Peru Current (in the South Pacific) Western Australian Current (in the Indian Ocean) ESS228 Prof. Jin-Jin-YiYi Yu Gulf Stream A river of current Jet stream in the ocean Speed = 2 m/sec Depth = 450 m Width = 70 Km Color: clear and blue (Figure from Oceanography by Tom Garrison) ESS228 Prof. Jin-Jin-YiYi Yu Surface Current ––GeostroGeostroppyhic Gyre Mixed Layer Currents controlled by frictional force + Coriolis force Æ wind-driven circulation Æ Ekman transport (horizontal direction) Æ convergence/divergence Æ downwelling/upwelling at the bottom of mixed layer Thermocline downwelling/upwelling in the mixed layer Æ pressure gradient force + Coriolis force Æ geostrophic current Æ Sverdrup transport (horizontal) ESS228 Prof. Jin-Jin-YiYi Yu Step 1: Su rface Winds (Figure from Oceanography by Tom Garrison) ESS228 Prof. Jin-Jin-YiYi Yu Winds and Surface Currents Polar Cell FlCllFerrel Cell Hadley Cell (Figure from The Earth System) ESS228 Prof. Jin-Jin-YiYi Yu Steppy 2: Ekman Layer (frictional force + Coriolis Force) (Figure from Oceanography by Tom Garrison) ESS228 Prof. Jin-Jin-YiYi Yu Ekman Sp iral – ARA Resu lt o fCf Cori oli s F orce (Figure from The Earth System) ESS228 Prof. Jin-Jin-YiYi Yu Formula for Ekman Transport ESS228 Prof. Jin-Jin-YiYi Yu How Deep is the Ekman Layer? D ∝ (ν/f)1/2 ν = vertical diffusivity of momentum f = Coriolis parameter = 2Ωsinφ (from Climate System Modeling) ESS228 Prof. Jin-Jin-YiYi Yu Ekman Transport (Figure from The Earth System) ESS228 Prof. Jin-Jin-YiYi Yu Step 3: Geostrophic Current (Pressure Gradient Force + Corioils Foce) NASA-TOPEX Observat io ns o f Sea-Level Hight (from Oceanography by Tom Garrison) ESS228 Prof. Jin-Jin-YiYi Yu Ekman Transport Æ Convergence/Divergence (Figure from The Earth System) Surface wind + Coriolis Force Ekman Transport Convergence/divergence Thermocline (in the center of the gyre) Pressure Gradient Force GhiCGeostrophic Currents ESS228 Prof. Jin-Jin-YiYi Yu Geostrophic Current Forces Geostrophic Gyre Currents (Figure from The Earth System) ESS228 Prof. Jin-Jin-YiYi Yu Sverdrup Transport • Continuity equation for an incompressible flow: Ekman layer pumping Î vertical depth decreases • Assume the horizontal flows are geostrophic: Î move equatorward to conserve absolute vorticity. • Replace the geostrophic flow pressure gradients: Ekman layer suction • The continuity equation becomes: Î vertical depth increases Î move poleward to conserve absolute vorticity. ESS228 Prof. Jin-Jin-YiYi Yu Sverdrup Transport • Continuity equation for an incompressible flow: • Integrate the equation from the bottom of the upper ocean (Dw) to the bottom of the Ekman layer (DE): assume zero • Assume the horizontal flows are geostrophic: • Ekman pumping (ѡE) is related to the convergence of the Ekman transport: • Replace the geostrophic flow pressure gradients: • Therefore, we obtain: • The continuity equation becomes: geostrophic Sverdrup - (Ekman transport transport Transport) • Therefore, ESS228 Sverdrup transport = Geostrophic transportProf. + Jin-JinEkman-YiYi Yu transport Ekman and Sverdrup Transports Eq E W E 90°N Ekman Layer Ekman Pumping EkSikman Suction (w<0) (w>0) Equatorward Poleward Sverdrup Transport Sverdrup Transport ESS228 Subtropical Gyre Prof. Jin-Jin-YiYi Yu Conservation of Potential Vorticity Pole Potential Vorticity PV = f + ζ f = planetary vorticity = 2Ωsinφ f1 H (ζ <0) 1 ζ = relative vorticity = ∂v/∂x- ∂u/∂y f1 + ζ1 = f2 + ζ2 since f1 > f2 Æ ζ1 <ζ2 If ζ <0< 0, the vortex decreases rotation when f2 H moves toward lower latitudes and increases (ζ2> ζ1) rotation when moves toward higher latitudes. Equator ESS228 Prof. Jin-Jin-YiYi Yu Boundar y Cu rrents (Figure from The Earth System) ESS228 Prof. Jin-Jin-YiYi Yu Step 4: Boundary Currents (Figure from Oceanography by Tom Garrison) ESS228 Prof. Jin-Jin-YiYi Yu Boundary Currents Eastern boundary currents: broad and weak Western boundary currents: narrow and strong ESS228 Prof. Jin-Jin-YiYi Yu Eastern Boundary Current Cold water from higher latitude ocean. Costal upwelling associated with subtropical higgph pressure sy stem. Atmospheric subsidence produce persistent stratiform clouds, which further cool down SSTs by blocking (from Global Physical Climatology) solar radiation. ESS228 Prof. Jin-Jin-YiYi Yu Costal Upwelling/Downwelling A result of Ekman transport and mass continuity. (Figure from Oceanography by Tom Garrison) ESS228 Prof. Jin-Jin-YiYi Yu Global Surface Currents ESS228 Prof. Jin-Jin-YiYi Yu.