Ocean Circulation II Ekman transport Geostrophic currents Ocean

Ocean Circulation 1. Surface Circulation Wind Driven – – – Ekman Transport and Geostrophic

Surface Ocean Circulation • • Ekman Spiral Geostrophic Currents Open Ocean Currents Global Surface

Ekman Spiral • 45 o surface transport offset • 90 o net water transport

Consequences of Ekman Transport • Ekman affects picnocline layer • Coastal Upwelling – Cold

Pressure Gradient and Coriolis equilibrium: Geostrophic currents

Geostrophic currents follow lines of equal pressure / height

Open Ocean Surface Currents Ekman surface transport

Upwelling and Downwelling Convergence and Divergence • Downwelling – Convergence – Subtropical Gyres –

Oceanic Gyres Western Boundary Currents • Gulf Stream, Kuroshio, Agulas currents Eastern Boundary Currents

Western Boundary Current Intensification Stronger Coriolis force at higher latitudes

Western Boundary Current Intensification Subtropical Gyres displaced to the West

Slides: 33

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Ocean Circulation II Ekman transport & Geostrophic currents

Ocean Circulation 1. Surface Circulation Wind Driven – – – Ekman Transport and Geostrophic Currents Surface layer and Picnocline zone 0 – 50, 100 m / 50, 100 m - ~1000 m Affects ~25% of total water mass Fast (1 -2 m/s) 2. Deep Circulation Density Driven – – Thermohaline Circulation Below Picnocline zone (>~1000 m) Affects ~75% of total water mass Slow (~ m/day)

Surface Ocean Circulation • • Ekman Spiral Geostrophic Currents Open Ocean Currents Global Surface Circulation

Ekman Motion

Ekman Spiral

Ekman Spiral • 45 o surface transport offset • 90 o net water transport • Stronger winds cause deeper water movement – Wind 10 m/s 50 – 100 m watercolumn – Wind 20 m/s 100 – 200 m watercolumn

Thermo, halo & Picnoclines

Coastal Ekman Transport

Consequences of Ekman Transport • Ekman affects picnocline layer • Coastal Upwelling – Cold surface water, high nutrients – High phytoplankton productivity – Great fisheries (Sahara bank, Peru) • Coastal Downwelling – Warm surface water, low nutrients – Low phytoplankton – Poor fisheries (Northern Brazil)

Pressure Gradient and Coriolis equilibrium: Geostrophic currents

Geostrophic currents follow lines of equal pressure / height

Open Ocean Surface Currents Ekman surface transport

Open Ocean Geostrophic currents

Geostrophic currents and continents

Upwelling and Downwelling Convergence and Divergence • Downwelling – Convergence – Subtropical Gyres – Sargasso Sea example of Geostrophic current • Upwelling – Divergence

Global Surface Currents

Oceanic Gyres Western Boundary Currents • Gulf Stream, Kuroshio, Agulas currents Eastern Boundary Currents • California, Canary, Peru currents • • • Narrow (100 km) Deep (<2000 m) Swift (1. 5 m/s) Warm High Volume (50 -75 Sv) Broad (1000 km) Shallow (<500 m) Sluggish (0. 3 m/s) Cool Low Volume (10 -15 Sv) 1 Sverdrup = 100000 m 3/s

Western Boundary Current Intensification Stronger Coriolis force at higher latitudes

Western Boundary Current Intensification Subtropical Gyres displaced to the West

Gulf Stream 150 Sv 30 Sv

Gulf Stream

Cold and Warm Water Eddies

SST – Gulf Stream

California Seasonal Reversal

The End