the circulation of the oceans

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



DD

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



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 105 - 108 create outwards Pressure▼

• This results in circular geostrophic current ↑‘gyre,↓


• Northern Hemisphere wind stress currents set up the sub-tropical gyres & geostrophic currents


• Pressure gradient force opposes Coriolis force for net outwards deflection


• 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

GG

G

G

GCa

nary

Cur

rent

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


• The Cold Humbolt Current – makes the Namib Erg

• The Cold Benguela Current – makes the Atacama

C

W

C

Hum

bolt

curr

ent

Beng

uela

cur

rent


• E-W Circulation in Equatorial Troposphere• Where does Rainfall happen the most?• With respect to the atmospheric circulation?• With respect to sea versus land?

Ocean Surface Layer – Tropical PacificLa Niña every 2-10 years (enhanced normal)

• Strong Easterlies make for upwelling in E Pacific

• Colder December-February off Ecuador & Peru

ENSO – El Niño Southern Oscillation Pattern of Easterlies Breaks Down

• Reverse of Trades: no upwelling in E Pacific

• Warmer December-February off Ecuador & Peru

• Rains, high tides, coastal flooding

Which Comes 1st the Chick or the Egg

• Positive feedback loop: Easterlies make warm west

• Warm west Pacific makes strong Easterlies!

El Niño versus La Niña

• Correlated Circulation Events (1997-1998 ENSO)

• Sea Surface Temperature Maps (& 1989 La Niña)

ENSO - Atmospheric Circulation

• Shift of Warm water to Central Pacific

• Loss of Upwelling & micronutrients in E. Pacific

• Leads to massive die back of marine life & seabirds

• Droughts/Famines in Africa, Australia & Indonesia


• Normal E-W Circulation in Equatorial Troposphere• Rains in South American jungle not Andes• Rains in central Africa• Rains in northern Australia

SOI Index – Sea Level Pressures

• Negative SOI at Tahiti vs Darwin in warm ENSO

• Note extreme ENSO in 1982-83 & 1997-98

Consequences of 82-3, 97-8 ENSO Events

• Ecuador & Peru: floods, landslides, 600 dead

• crop & property losses > $400M

• Guayaquil X 20 normal rainfall

• Major erosion, soil loss & sediment transport

• Indonesia crop failure & famine

• E. Australia worst drought of 100 years

• Livestock slaughter

• 11,000 tons of dust on Melbourne & worst bushfires

• Tahiti got 100 year Typhoon & 6 others in 5 months

• Flooding of Mississippi & California landslides

ENSO Anomalies in Rainfall & T°C

• Consistent Pattern in Tropics but variable intensity

• Highly Variable Mid Latitude Effects Wet 82Dry76

Salinity =g solute per Kg solventi.e. per mil

Seas 35, 1.035

• Anions: Cl- > SO4 -2 > HCO3

- > Br- > Boric>F-

• Cations: Na+ > Mg+2 > Ca+2 > K+ > Sr+2

The Salt in the Seas• Total salt content is 5 x 1019 kg

• John Joly assumed this accululated since Earth formed and got 80-89 Ma

• Salts are removed by submarine weathering, biosedimentation, subduction, uplift and evaporation

• The modern flux is 4 x 1012 kg/day

• Our estimate would be: 5 x 1019 kg / 4 x 1012 kg/day

or 13 x 106 yr

This is not “The Age of the Earth” but the residence time for salts in the ocean! ~ 1/400 of Earth’s age!

Thermohaline Structure & CirculationThermal, Salinity & Density Structure

• Shallow water: warmer, fresher & lighter• Deep water is pretty uniformly dense & stable• Deep density currents are slow• Vertical convection is limited, 2 layer convection

Water is Special & so is Seawater• As per graphs:

– density & salinity decrease as T°C increases

• Pure H2O max density at 4°C

• Above 4° increase in T°C means decrease in density

• But density also decreases below 4°C down to 0°C

• For saline H2O max density at 2°C for 10 per mil

• & at the Freezing point for 24.6 per mil

• The freezing point drops as salinity increases, but density decreases somewhat, still defining freezing


• Surface zone/Mixed Layer = low density in upper 60-100 m

• Maximum interaction with atmosphere: energy, kinetics, friction, evaporation, concentration, dilution

• Thermal: absorption of solar radiation, emission of long wave IR


• Rapid increase in density with depth ~1 km = pycnocline

• Transition zone is Pycnocline Zone

• Where density increase is driven by salinity increase, this is Halocline

• Most regions the density increase is driven by the temperature decrease thus the Thermocline

• Either structure stops mixing and is stable despite seasons


• Deep Ocean has only slight increase in salinity with depth while temperature continues to decrease slightly for a net constant salinity

• Bottom water is the densest & slight lateral salinity gradients drive deep circulation across isopycnals like isobars hi to low in atmosphere

• Where dense cold or salty water is produced, the deep ocean is fed

• Polar ice margins -1.9°C dense water + salts excluded by freezing

Thermohaline Structure & Circulation

• The thermocline is evident in the tropics

• At high latitude, deep water extends to the surface


• Salinity structure is more complex than thermal

• Salinity maxima at surface in tropics (Why?) & deep


• Salinity structure is more complex than thermal • Salinity maxima at surface in tropics (Why?) & deep• Which ocean has greater salinity layering?

Circumpolar Flow & Weddell Sea

• Wintertime Ice Factory – Antarctic Outflow Winds

• Ice forms at Shore and disperses to north

• Thick ice shelf at Weddell Sea from W. Ant. Cap.

• Antarctic Bottom Water forms here & sinks

4000 M Ocean Depth/Configuration

• NADW forms off Greenland, higher salinities W AT• AABW forms off Weddell Sea• These dominate flow at 4 km depth

the circulation of the oceans

Documents

sea surface

water south

surface waters

water north

boxwind drift currents

tropicsmid ocean

ne trades ekman transport

sea level drops