Ekman ≠ Geostrophic Ekman ≠ Geostrophic Ekman Geostrophic

Home , Ekman spiral, Ekman transport, Geostrophic current

Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Ekman " Geostrophic Ekman " Geostrophic • Ekman transport is a • Geostrophic current is a long-term pattern short-term phenomenon – Integrates conditions over months to years – Coriolis & friction – Equilibrium between Coriolis & pressure – Upper water layer • Slow to change with wind fluctuations responds to change in • Local variations occur with storms, etc. wind speed and/or direction • Takes a few hours to a few days • Transient, not equilibrium

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– Turn on the wind Step 2: Current turns as Ekman transport builds a sea water hill – Within hours-days, the Ekman Spiral develops & pressure • Ekman is the agent of its own demise – Ekman transport pushes water 90˚ to the wind – Builds a hill of sea water to one side Right north, left south Step 3: Geostrophic current parallels wind as pressure from sea • water hill balances Coriolis – The hill slope creates pressure (gradient) force • Counteracts Coriolis & Ekman • Current turns to parallel the wind

3 – Geostrophic flow 4 Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Waves vs. Currents Time Scale of Waves • 3 main differences: • Waves arise within a few hours – Waves are periodic, currents are continuous – ”Full developed seas” within 2-3 days – Waves move energy, but no net movement of • Depending on speed & fetch water (until they break) • Continue as long as wind blows • Currents move water and energy – Additional energy goes into net water – Waves arise more quickly, currents more slowly movement (currents)

Geostrophic current Geostrophic current gy gy Ener Ener

Waves Ekman Transport Waves Ekman Transport

Hours Days Weeks Months-Years Hours Days Weeks Months-Years 5 Time that wind blows 6 Time that wind blows

Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Time Scale of Currents Time Scale of Currents • Ekman Transport • Geostrophic current arises in 1–2 weeks – Arises in 1–2 days – Sea slope rises in response to steady wind • Depending on speed & fetch • Resulting pressure balances Coriolis – Persists for days-weeks • Current turns to become more parallel to wind • But changes to geostrophic as sea level adjusts – Stores tremendous energy over seasons-years • Momentum carries it after wind dies or reverses

Geostrophic current Geostrophic current gy gy Ener Ener

Waves Ekman Transport Waves Ekman Transport

Hours Days Weeks Months-Years Hours Days Weeks Months-Years 7 Time that wind blows 8 Time that wind blows Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Time Scale of Sea Level Time Scale of Sea Level • Note a match between atmospheric • But what about storm surge? pressure & sea level? – Low atmospheric pressure = high sea level – High atmospheric pressure – High pressure = low sea level – High sea level (& pressure acting on current) – Wind also pushes water to raise sea level

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Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Time Scale of Sea Level Time Scale of Sea Level • The difference: time scale • So long-term sea level matches long-term – Imagine a hurricane (low pressure) arrives average atmospheric pressure • Hours-days: barometric effect dominates – Low atmospheric pressure • Sea level rises – Low sea level – Imagine the hurricane stayed in one place • Days-months: Ekman & geostrophic take over

Geostrophic current Barometric effect

Sea Level Undisturbed sea level

Wind effect

Hours Days Weeks Months-Years 11 Time that pressure is low 12 Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Two Key Distinctions Two Key Distinctions • #1 High atmospheric pressure = high sea • #2 Note high atmospheric pressure is surface pressure centered over the ocean basin – But low atmospheric pressure also = high sea – But high sea level is toward the west side of surface pressure the basin – High sea surface pressure results from steep sea surface slope – Match is between atmos pressure & sea level

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Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington The Asymmetrical Oceans Western Boundary Currents • Currents differ on the east & west sides • On the west sides of ocean basins of ocean basins – Gulf Stream (N. Atlantic), Kuroshio (N. Pacific) – Western Boundary Currents – Agulhas (Indian), Brazil (S. Atlantic), • Fast, narrow, deep, & well-defined E. Australia (S. Pacific) • Weak upwelling • Warm – Eastern Boundary Currents • Slow, broad, shallow, less distinct • Strong upwelling • Cold

15 16 Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Eastern Boundary Currents “Western Intensification” • On the east sides of ocean basins • Western Boundary Currents – Canary (N. Atlantic), California (N. Pacific) – Warm, fast, narrow, deep, clearly defined – Peru (S. Pacific), Benguela (S. Atlantic), • Eastern Boundary Currents W. Australia (Indian) – Cool, slow, wide, shallow, less defined

Figure 8.5

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Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington “Western Intensification” Western Intensification • Western Boundary Currents • Ultimate causes are: – Warm, fast, narrow, deep, clearly defined – Blocking effect of continents • Eastern Boundary Currents – Earth’s rotation – Cool, slow, wide, shallow, less defined • Generally weaker in S. hemisphere – Shape & size of basins & continents – Orientation of coastline – E. Australia, Brazil, Agulhas Currents

19 20 Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Blocking effect of continents Blocking effect of continents • Gulf Stream is • Kuroshio is stronger stronger than E. Australia than Brazil Current Current – Shape of N. America – Asia solid • Funnel-shaped • Strongly blocks • Forces sharper turn N. Equatorial Current of Equatorial Current – Australia & New – Shape of S. America Zealand islands • Diverts S. Equatorial • Only weakly Current into N. block S. Equatorial Atlantic Current

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Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Effect of Earth’s Rotation Simplest Description • Too technical to • Volume continuity examine details of – Same volume of Coriolis water must be – More descriptive traveling all the explanation way around the gyre. – Picture the “hill” of sea surface – On west it must squeeze through • Ideally in the center of the ocean basin a narrow width, • But actually skewed so travels faster. toward the western side – East wider &

23 24 slower Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Volume Continuity Sea Surface Profile: Ideal • Same volume of water must be traveling all • Cross-sectional schematic view the way around the gyre. – From the Equator looking north in N. – On west it must squeeze through a narrow Hemisphere width, so travels faster. • Idealized sea surface: gray scallops – East wider & slower • Water diverges from coasts • Water converges in ocean center • Upwelling & downwelling

Convergence West Upwelling Downwelling Upwelling East shore Divergence Divergence shore 25 26

Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Sea Surface Profile: Ideal Sea Surface Profile: Actual • Cross-sectional schematic view • Earth rotates (accelerates) eastward – From the Equator looking north in N. – Water “lags” on trailing edge Hemisphere – Water accumulates on the western side of • Idealized sea surface: gray scallops ocean basins • Water diverges from coasts • (Backward sloshing of water in a dishpan when • Water converges in ocean center you take a quick step forward) • Upwelling & downwelling • Cross-sectional view

Earth’s rotation Downwelling Upwelling Upwelling Convergence West East shore Divergence Divergence shore 27 28 Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Sea Surface Profile: Actual Sea Surface Profile: Actual • Asymmetry in sea level & sea surface slope • Asymmetry in convergence & – Sea level is higher on west & lower on east divergence, upwelling & downwelling – Summit of sea level “hill” shifts westward – Steeper slope, greater pressure inhibit – Slope is steeper on west & less steep on east divergence & upwelling on west side – Current is narrower on west & wider on east – Gentler slope, weaker pressure enhance divergence & upwelling on east side

Convergence & Weaker Downwelling Divergence Stronger Divergence

West East West Weaker Stronger East shore shore shore Upwelling Upwelling shore 29 30

Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Western Intensification Technical Explanation • Fast, narrow geostrophic current on west • Western flank of "hill" is steeper – Steeper slope, greater pressure, faster speed – Gravity/pressure gradient force is greater – Coriolis increases with speed = pressure – Water tends to accelerate downhill faster • Slower, wider geostrophic current on east – Greater speed increases Coriolis effect – Gentler slope, weaker pressure, slower speed – Turning tendency is stronger – Coriolis decreases with speed = pressure – Stronger Coriolis balances stronger gravity/ pressure force. Pressure Coriolis Pressure – Geostrophic current Coriolis

West East shore shore 31 32 Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Oceanography 101, Richard Strickland! ! Lecture 14! ! © 2006 University of Washington Western Boundary Currents Western Boundary Currents • Sharp temperature, • Strong western speed, & color boundaries are changes at unstable & tend western edges to “wobble.” give impression – This creates of "river” “eddies.” – Gulf Stream • Warm-core rings – (Photo shows • Cold-core rings sea surface temperature)

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