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  Continent Easterly Equatorial Trades cause westwards flow in Tropics Continents deflect flow to North and South in Gyres aided by Westerlies at mid latitudes Continent  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

Winds and Surface Currents: Sea in a Box Warm Iceland/Scotland Cold Labrador  Kuroshio Current Gulf Stream  Continent Continent  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

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

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

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

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

Sea Surface Relief & Geostrophic Flow The pressure force of a gyre 50 cm high in the middle acts to counteract the inwards directed Ekman transport. The net result is right handed circular flow (Gulf Stream) in N. Atlantic and Kuroshio in NW Pacific! There are similar features set up in the southern oceans between the South Equatorial current and the West Wind Drift. In practice since the Ekman force is a little less than 90°, there is a build up and downwelling in the center of the gyre. 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

>20°C, 50-70 km wide & 3-10 km/hr Up to 1 km deep G G <10°C, 1000 km wide & <4 km/hr < 500m deep Canary Current Boundary Currents are Asymmetric - The Gulf Stream is a Fast Western Boundary Current G G

Vorticity: The Tendency for Fluid to Rotate under the influence of Body Forces 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 Positive vorticity  Counterclockwise (from above) Negative vorticity  Clockwise

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? Speeding Vorticity makes the gyres themselves asymmetric. Wind over water is driven by heat picked up. Slowing The divergence to the east & Slowing Westerlies Equator bound Canary Current

Ocean Circulation & Sea Surface T°C W C C T The North Atlantic Drift also makes Europe’s Ice Cap Grow first as Earth enters an Ice Age! The Ocean Heat has a long memory and influences climate fluctuations on annual to decadal time scales! The Labrador Current - Cold outflow from Arctic North Atlantic Drift – Warms Iceland & Norway

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

Ocean Circulation & Sea Surface T°C 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 Pacific La 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

Ocean Circulation & Sea Surface T°C 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 solvent i.e. per mil Seas 35, 1.035 Anions: Cl- > SO4 -2 > HCO3- > Br- > Boric>F- Cations: Na+ > Mg+2 > Ca+2 > K+ > Sr+2 Iodide is less than Fluoride Soluble ions come from chemical weathering of common rock forming minerals.

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 & Circulation Thermal, 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

Thermohaline Structure & Circulation Thermal, Salinity & Density Structure 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

Thermohaline Structure & Circulation Thermal, Salinity & Density Structure 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

Thermohaline Structure & Circulation Thermal, Salinity & Density Structure 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

Thermohaline Structure & Circulation The thermocline is evident in the tropics At high latitude, deep water extends to the surface

Thermohaline Structure & Circulation Salinity structure is more complex than thermal Salinity maxima at surface in tropics (Why?) & deep

Thermohaline Structure & Circulation 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

Δ14C Values: Low = young Hi = old The difference value is referenced to modern day ocean surface waters. Off of BC, the reservoir age for modern sea water is 400 years Not just atmospheric CO2 The reference for the difference value is the modern carbon content of upper oceans. Here off of BC, the ocean carbon reservoir today gives a radiocarbon age of 400 years -40 permil off Greenland NADW young -140 permil off Weddell Sea Antarctica AABW -220 permil off Bering Sea – N. Pacific = OLD

Radioactive Decay – 14 C Biota take up Carbon including small amounts of radiocarbon as they live and grow. They stay in equilibrium with their surface reservoir. After they die they become decay clocks 14N  14C in upper atmosphere by cosmic rays Oxidation to CO2 deposits this in lower atmosphere 14C  14N by β emission at T1/2 = 5730 years

Radioactive Decay – 14 C You are what you eat! We need to correct 14C dates for reservoir effects! Tree ring analysis & counts. Coral annual growth bands. Living biota: clams, forams etc. BC Reservoirs: Wood ~400 year Clams ~6000 year correction Planktonic Forams ~400 year Ocean returns Carbon to deeps 14N  14C in upper atmosphere by cosmic rays Oxidation to CO2 deposits this in lower atmosphere 14C  14N by β emission at T1/2 = 5730 years

Thermohaline Structure & Circulation NADW AABW 2 big Deep Water Factories: NADW & AABW Return flow is slow over whole ocean pycnocline & fast in areas of upwelling: W. Broecker LDGO

Thermohaline Structure & Circulation AABW isopycnals: observed & modelled Note the role of Ice shelves in making dense water

Nimbus 7 Coastal Zone Scanner detects pigments from Clorophyll compounds. Light Colours = greatest plankton & plants North Atlantic North Pacific Bahamas Galapagos South Atlantic South Indian Dead biota recycle nutrients to deep ocean

False-colour satellite image of the world's oceans, showing the distribution of phytoplankton in the surface water. The colours represent varying phytoplankton densities from red (most dense) through yellow, green and blue to violet (least dense). Grey areas are data gaps. This image is an average of distributions for October- December 1979 and shows the seasonal build-up of phytoplankton along the equator, especially off the western coasts of Africa & South America. The image was produced from data acquired by the Coastal Zone Colour Scanner, one of the instruments on NASA's Nimbus-7 research satellite. False-colour satellite image of the world's oceans, showing the distribution of phytoplankton in the surface water. The colours represent varying phytoplankton densities from red (most dense) through yellow, green and blue to violet (least dense). Grey areas are data gaps. This image is an average of distributions for October- December 1979 and shows the seasonal build-up of phytoplankton along the equator, especially off the western coasts of Africa & South America. The image was produced from data acquired by the Coastal Zone Colour Scanner, one of the instruments on NASA's Nimbus-7 research satellite. Dr. Gene Feldman, NASA GSFC Photo Library

1000 m Depth T°C & Salinity ◦/◦◦ Mediterranean is shallow, warm & salty through evaporation & dissolution of Miocene salt beds See how it makes the halocline & sets the floor for the Canary Current

Thermohaline Structure & Circulation As NADW & AABW from and sink near poles Warm water flows poleward at the surface This is negative feedback in ice ages, buffering climate

The Moist End of Chapter 5!