Chapter 7: Ocean Circulation

current

current

water masses in motion, wind driven or density driven

surface currents

horizontal ocean currents derive their energy from wind, and horizontal motion

deep currents

density driven, temp and salinity. vertical motion, initiated by creation of dense surface water in high latitudes of atlantic ocean where temps are low, and salinity is increased during winter months

where do deep currents form

Circulation is initiated by creation of dense surface water in high latitudes of Atlantic where temp low, and salinity increases in winter due to ice

how do you measure currents directly

release floating device transported by currents and tracked through time, or by lowering into water from stationary position with flow meters

indirect current measuring methods

pressure gradients: bulges and depressions in earth’s surface

radar altimeters

doppler flow meter

radar altimeters

dynamic topography maps are produced from which current flow is inferred

doppler flow meter

low frequency sound signals sent through ocean to determine differences in pressure which indicate current

argo floats

global array of free drifting profile floats. tracked through time, sinks to a depth, drifts for 10 days, resurfaces and transmits, then sinks again

chemical tracers

for detecting deep currents. tritium (produced by uclear bomb tests in 1950’s and 60’s)

chlorofluorocarbons

where do surface currents occur

above pycnocline and affect only 10% of ocean water on earth

surface current formation

friction btwn wind and ocean surface.

2% of wind energy transferred to ocean surface

50 knot wind creates 1 knot current

knot to miles conversion

1 knot = 1.15 miles or 1.85 km

what are surface currents affected by

wind belts and patterns. also affected by distribution of continents. if no continents, they would follow trade winds, westerlies, and easterlies

other current influences

gravity, friction, coriolis effect, geometry of ocean basins, seasonal changes

gyres

large circular loops of moving water

subtropical gyres

around 30 degrees latitude

5 subtropical gyres

Columbus, Navigator, Turtle, Heyerdahl, Majid

North Atlantic Gyre

Columbus Gyre

South Atlantic Gyre

Navigator Gyre

North Pacific

Turtle Gyre

South Pacific

Heyerdahl Gyre

Indian Ocean

Majid Gyre

what are subtropical gyres consisted of

equatorial current, western border current, N or S boundary current, E. boundary current

equatorial current

trade winds from east to west set in motion water between tropics. N or S, travel westward along equator. warm from equatorial, west edge ocean basins

western boundary currents

gulf stream and brazil currents

northern or southern boundary currents

North Atlantic \n Current from west to east, Antarctic Circumpolar current

Eastern Boundary currents

Canary current, Benguela current \n Subtropical Gyres. cool waters

equatorial countercurrents

narrow, easterly flow of water that occurs counter to and between adjoining equatorial currents. E flow btwn N and S equatorial currents

prominent in Pacific, due to minimal coriolis at equator

subpolar gyres

Driven in the westerly direction by polar easterlies \n – Rotate opposite subtropical gyres \n – Smaller and fewer than subtropical gyres \n Other Surface Currentsf

4 factors affecting ocean surface circulation

ekman spiral, ekman transport, geostrophic currents, western intensification of subtropical gyres

ekman spiral

observation that arctic ice moves 20-40 degree angle to right of wind. S hemisphere movement to left of wind

who discovered ekman spiral and when

V. Walfrid Ekman in 1905

What does the ekman spiral describe

balance between friction and coriolis effect

direction and flow of surface waters at different depths

current speed decreases w/ increasing depth bc energy of wind is passed through water column

ekman transport

average movement of surface waters:

90 deg to right in N hemis.

90 deg left in S hemis.

real conditions (ekman spiral <45 deg, and ekman transport in open ocean is ~70 deg from wind direction

subtropical convergence

piling up of water in middle of gyre. hill of water within all subtropical gyres ~6.6 ft high.

geostrophic current

surface water flows downhill and is deflected (right in N, left in S). coriolis effect opposes gravity, balance of coriolis effect and gravitational forces, moves in circular path downhill

western intensification

top of hill of water displaced toward west due to Earth’s rotation. coriolis effect contributes to it.

how do western boundary currents work in both hemispheres

intensify on eastern side of ocean basins (opposite properties of W boundary currents), become faster narrower, deeper, warmer

upwelling

vertical movement of cold, nutrient rich water to surface, creates high biological productivity, abundance of algae at base of food web

downwelling

vertical movement of surface water downward in water column, carries oxygen-rich water

equatorial upwelling

divergence of currents at equator generates upwelling and high productivity

what is converging surface water

gulf stream, labrador current, east greenland current all come together in same region causing convergence. water piles up, low biological productivity

coastal upwelling

ekman transport moves surface seawater away from shore. cool nutrient rich waters come up (west US and san fran)

coastal downwelling

Ekman transport moves surface seawater toward shore \n • Water piles up, moves downward in water column \n • Lack of marine life \n Coastal Upwelling and Downwelling

what are other causes of upwelling

offshore winds

Seafloor obstruction \n Coastal geometry change \n Lack of pycnocline \n High latitude oceans \n Other Causes of Upwelling

Antarctic circulation

dominated by movement of water masses in S Atlantic, Indian, and Pacific oceans. also called West Wind Drift, Penguin Gyre

antarctic circumpolar current

west wind drift, West to East. Strong westerlies (Roaring Forties, Furious Fifties, Screaming Sixties) \n Only current to completely encircle Earth \n Moves more water than any other current \n Antarctic Circulation

Antarctic convergence

Cold, dense Antarctic waters converge with warmer, less dense \n sub-Antarctic waters \n – Northernmost boundary of Antarctic Ocean

East Wind Drift

Polar easterlies, creates surface divergence w/ opp flowing Antarctic circumpolar current

Antarctic divergence

abundant marine life

North Atlantic Subtropical Gyre

rotates clockwise due to trade winds and coriolis and these currents:

North Equatorial Current \n Gulf Stream \n North Atlantic Current \n Canary Current \n – Atlantic Equatorial

S. Atlantic subtropical gyre

rotates counterclockwise, S Equatorial current splits into Brazil current, and canary current. Antarctic circumpolar current, W. Wind Drift. Benguela Current

Gulf stream

N along East Coast, moderates climates, formed by N Equatorial current and S. which splits into Antilles current and Caribbean. merges with sargasso sea

West Indies geography

Caribbean basin, region of N Atlantic ocean which includes island countries and surrounding H2O of Greater Antilles, Lesser Antilles, and Lucayan Archipelago (bahamas)

greater antilles

comprising the islands of Cuba, Jamaica, Hispaniola (Haiti and the Dominican Republic), and Puerto Rico;

lesser antilles

including the Virgin Islands, Anguilla, Saint Kitts and Nevis, \n Antigua and Barbuda, Montserrat, Guadeloupe, Dominica, \n Martinique, Saint Lucia, Saint Vincent and the Grenadines, \n Barbados, and Grenada; and those of the South American \n shelf, including Trinidad and Tobago, Aruba, Curaçao, and \n Bonaire

lucayan archipelago

bahama archipelago, an island group comprising the commonwealth of the bahamas and british overseas

warm core rings

warmer sargasso sea water trapped in loop surrounded by cool H2O. rotates clockwise in gulf stream

Cold-core rings

cold water trapped in loop, surrounded by warmer water; rotate counterclockwise

north atlantic currents

labrador, irminger, canary (cold, move south), norwegian, N Atlantic (both warm, move north)

loop current

warm ocean surface current in Gulf, generates warm loop current eddies, hurricanes intensify when passing over warm cores

Monsoons

seasonal reversal of winds over N indian ocean, heat capacity differential, NE monsoon is in winter

Indian Ocean Monsoon

during winter, NE trade winds called NE monsoons. rapid air cooling over asian mainland strengthens them. high pressure cell forces atm. masses off continent into ocean with low pressure.

Indian ocean circulation

driven by subtropical gyre and agulhas current, australian current, leeuwin current

pacific ocean circulation

2 large subtropical gyres

North pacific subtropical gyre

N. Equatorial Current: flows westward

Kuroshio: Warms japan

N Pacific

California: cool

Alaskan

South pacific subtropical gyre

S. Equatorial current: flows westward into E. Australian C

E Australian Current: western intensified

Antarctic circumpolar current

peru current: humboldt current

equatorial counter current: better developed bc pacific ocean basin is larger and more unobstructed than atlantic ocean

Humboldt Current System (HCS)

peru current (major east boundary upwelling ecosystem). Flows along W coast of S America. Eastern boundary current flows in direction of equator.

highly productive, many species; sardines, anchovies, jack mackerel

mainly pelagic

walker circulation cell (normal conditions)

E. Pacific: high pressure, sinking air, coastal regions of W South america, cloudy, precipitation, thermocline.

El Nino Southern Oscillation ENSO

walker cell circulation disrupted (high pressure in E pacific weakens. Low biological productivity, reverse southeast trades

la nina ENSO cool phase

Increased pressure difference across equatorial Pacific \n • Cooler equatorial Pacific ocean \n • Like normal conditions, only intensified! \n • Stronger trade winds \n • Stronger upwelling in eastern Pacific \n • Shallower thermocline \n • Cooler than normal seawater \n • Higher biological productivity \n La Niña – ENSO Cool Phase

occurrence of ENSO events

El Nino every 2-10 years, irregular, last 12-18 months.

Deep ocean currents

low latitude regions, temp is too high to become dense and sink. deep currents only in high latitudes. sometimes only occur due to ice formation.

thermohaline circulation

deep ocean circulation driven by temp density differences in water. below pycnocline, 90% of all ocean water, slow velocity (10-20km)

Deep water masses

antarctic bottom water, N Atlantic deep water, Antarctic intermediate water, oceanic common water

power from currents

cold water dissolves more oxygen than warm water, deep water is o2 and nutrient rich, currents carry more energy than winds, Florida gulf stream current system. underwater turbines