The Circulation of the Oceans

• Geos 110 Lectures: Earth System Science• Chapter 5: Kump et al 3rd 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 50-100m• 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

ContinentKuroshio 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

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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

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Divergence & Convergence

Upwelling & Downwelling

Sea Surface Relief & Geostrophic Flow

• Gradients are a few m over 100 to 10,000 km!

• Slopes of 1 in 105 - 108 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

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Boundary Currents are Asymmetric - The Gulf

Stream is a Fast Western Boundary Current

>20°C, 50-70 km wide & 3-10 km/hrUp to 1 km deep

<10°C, 1000 km wide & <4 km/hr< 500m deep

Vorticity: The Tendency for Fluid to Rotateunder 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 vorticityCyclonic 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

CWC

Ocean Circulation & Sea Surface T°C

• The Cold Humbolt Current – makes the Namib Erg

• The Cold Benguela Current – makes the Atacama

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Ocean Circulation & Sea Surface T°C