The Circulation of the Oceans Geos 110 Lectures: Earth System Science Chapter 5: Kump et al 3 rd ed. Dr. Tark Hamilton, Camosun College

Ocean Water is a Special Fluid Dense & Viscous but variable due to salinity & temperature Density also varies with suspended sediment load High thermal mass & High heat capacity While the Ocean is the “burner” under the Troposphere, it is mainly heated from above 90% of Sunlight/heat is absorbed in the top 100 m Large insolation changes achieve minor T°C change The sea is thermally and density stratified, convecting as a 2 layer system This is very slow at depth ~ several thousand years Biology matters, especially for ocean chemistry

Winds & Surface Currents: Sea in a Box Winds have mass, momentum and vector directions Wind Drift Currents: Friction at the sea surface & excites wave motion & lateral flow causing convection & advection in the sea Continent   Continent Continents deflect flow to North and South in Gyres aided by Westerlies at mid latitudes Easterly Equatorial Trades cause westwards flow in Tropics

Winds and Surface Currents: Sea in a Box Wind Drift Currents confined to upper m Except big turbulent gyres: Gulf stream, Kuroshio current 1- 2 km deep and 100’s of km wide Coriolis force plays a role: Clockwise N, Anti-Clock S Continent   Continent Kuroshio Current Gulf Stream Cold Labrador   Warm Iceland/Scotland

Major Ocean Surface Currents Westwards Equatorial flow and gyres as predicted Reality is similar but more complex: warm & cold Esp. Polar Regions, Northern Indian Ocean, Straits

Current Convergence & Divergence Water does not generally pile up along the East coast of land in tropics Mid ocean pile up: wind driven currents, rotation & friction all contribute Nansen 1890’s noted drift at 20-40° to the right of the wind! Ekman Spiral: viscous shear, thermal dissipation & coriolis, greater angular deflection with depth but less speed, currents at 100 m depth can reverse! Eckman Transport: net advection at 90° down wind

The Ekman Transport & Spiral Wind Friction Currents & Earth’s rotation The shallow flow drags & shears the water just below it Heat losses make each deeper layer flow slower Each layer still feels Coriolis Force

The Ekman Transport & Spiral W/strong wind, surface current is <45° to wind Flow slows & reverses by ~100 m depth Net transport is 90° to wind, into gyre! In Southern Hemisphere, this reverses as Coriolis force and gyres are Widdershins!

 Divergence  &  Convergence  In Equatorial North Atlantic, NE Trades & Ekman Transport to right of wind deflects water North! This Produces the North Equatorial Current Conversely, SE Trades in Equatorial South Atlantic & Ekman Transport to left deflects water South! This Produces the South Equatorial Current W Mexico & W Africa have significant divergence Also diverge off W Ecuador & W South Africa

Major Ocean Surface Currents: Divergence Westwards Equatorial flow and gyres as predicted Reality is similar but more complex: warm & cold D D D D D D

 Upwelling  &  Downwelling  Where Divergence occurs, the sea thins and sea level drops ~2 m below the GEOID (geopotential surface) This causes upwelling of cold, micro-nutrient rich deep water: marine biology usually thrives here Where Convergence occurs, the sea thickens and piles up ~2 m above the GEOID This causes downwelling of warm, acidic, dust and carbon rich plankton bearing surface waters creating deep sea drifts (linear sediment deposits)

Major Ocean Surface Currents: Convergence Westwards Equatorial flow and gyres as predicted Reality is similar but more complex: warm & cold C C C C C C

 Divergence  &  Convergence   Upwelling  &  Downwelling 

Sea Surface Relief & Geostrophic Flow Gradients are a few m over 100 to 10,000 km! Slopes of 1 in create outwards Pressure▼ This results in circular geostrophic current ↑‘gyre,↓

Sea Surface Relief & Geostrophic Flow Northern Hemisphere wind stress currents set up the sub-tropical gyres & geostrophic currents

Sea Surface Relief & Geostrophic Flow Pressure gradient force opposes Coriolis force for net outwards deflection

Sea Surface Relief & Geostrophic Flow Pressure gradient force opposes Coriolis force for net outwards deflection tangential & clockwise around the gyre in the Northern Hemisphere

Major Ocean Boundary Current Gyres Westwards Equatorial flow & Subtropical gyres Clockwise Northern & Anti-clockwise Southern G G G G G G G

G G G G Canary Current Boundary Currents are Asymmetric - The Gulf Stream is a Fast Western Boundary Current >20°C, km wide & 3-10 km/hr Up to 1 km deep <10°C, 1000 km wide & <4 km/hr < 500m deep

Vorticity: The Tendency for Fluid to Rotate under the influence of Body Forces Positive vorticity  Counterclockwise (from above) Negative vorticity  Clockwise Planetary vorticity increases towards the poles due to rotation = Coriolis Force Relative vorticity Cyclonic Low P wind shear + AntiCyclonic Hi P -negative Ocean current shear in gradients parallel to coasts

Current Shear Producing + or - Vortex + Positive Vorticity when current increased to Right - Negative Vorticity when current increases to Left Whirlpools & Rip Tides tend to take you offshore!

So why are Eastern Boundary Currents Weaker than Western Ones? The divergence to the east & Slowing Westerlies Equator bound Canary Current Slowing Speeding

Ocean Circulation & Sea Surface T°C The Labrador Current - Cold outflow from Arctic North Atlantic Drift – Warms Iceland & Norway C W C

Ocean Circulation & Sea Surface T°C The Cold Humbolt Current – makes the Namib Erg The Cold Benguela Current – makes the Atacama C W C Humbolt current Benguela current

Ocean Circulation & Sea Surface T°C