ocean currents water masses

It all begins with the sun……

Resultant flow which gives rise to vertical motion in and below the Ekman layer -Upwelling-Downwelling

Ekman pumping

© 2011 Pearson Education, Inc.

Geostrophic FlowEkman transport piles

up water within subtropical gyres.

Surface water flows downhill and to the right.

Geostrophic flow – balance of Coriolis Effect and gravitational forces

Ideal geostrophic flowFriction generates actual

geostrophic flow

Geostrophic flow and western intensification

Geostrophic flow causes a hill to form in subtropical gyres

The center of the gyre is shifted to the west because of Earth’s rotation

Western boundary currents are intensified Figure 7-7

30o

30o

60o

60o

90o

90o

0o

Forces

1. Solar Heating (temp, density)

2. Winds

3. Coriolis

Surface Currents

Factors Influencing Nature and Movement of Ocean Currents:1. Factors related to the earth’s rotation: Gravitational force and force of deflection. 2. Factors originating within the sea: Atmospheric pressure, winds, precipitation, evaporation and insolation. 3. Factors originating within the sea: Pressure gradient, temperature difference, salinity, density and melting of ice. 4. Factors modifying the ocean currents: Direction and shape of the coast, seasonal variations and bottom topography.

Surface and Deep-Sea Current Interactions

“Global Ocean Conveyor Belt”

Global ocean circulation that is driven by differences in the density of the sea water which is controlled by temperature and salinity.

White sections represent warm surface currents. Purple sections represent deep cold currents

http://www.whoi.edu/institutes/occi/currenttopics/climatechange_wef.html

http://www.whoi.edu/institutes/occi/currenttopics/climatechange_wef.html

Formation of Antarctic Bottom Water (AABW)

Weddel Sea (Flichner ice shelf) and Ross Sea (Ross Ice Shelf)

Weddel Sea – partially isolated embayment -greatest contributor

There is less entrainment than with NADW so AABW is densest water in ocean.

Cold wind blows ice offshore (polyna) allowing ice to continually form.

During freezing, salts are left behind (brine formation) resulting in water that is more saline.

Surface waters are chilled to temperature of ~ -1.9°C, salinity is 34.6 psu.

This cold dense water collects on the Antarctic shelf and sinks to the bottom of the adjacent deep-ocean basin.

In the process of mixing, mixes with other waters and is warmed.

Resulting water is ~ -0.4-1°C and 34.6 to 34.8 psu.

Formation of North Atlantic Bottom Water (NABW)


Antarctic CirculationAntarctic Circumpolar

CurrentAlso called West Wind

Drift and Penguin GyreOnly current to

completely encircle Earth

Moves more water than any other current


Antarctic CirculationAntarctic Convergence

Cold, dense Antarctic waters converge with warmer, less dense sub-Antarctic waters

Northernmost boundary of Antarctic OceanEast Wind Drift

Polar EasterliesCreates surface divergence with opposite flowing

Antarctic Circumpolar CurrentAntarctic Divergence

Abundant marine life


Atlantic Ocean CirculationEquatorial Atlantic circulationAt the Equator, Atlantic extends from 10° E to 45 °

W – 6000 kmMain currentsNorth equatorial counter current (NECC) flowing

to east from 8° -3° N South Equatorial current (SEC) flowing west from

3°N to 8°SEquatorial undercurrent (EUC) flowing east at

equator about 50-300m Brazil coastal current


Atlantic Ocean CirculationNorth Atlantic

Subtropical GyreRotates clockwise –

Coriolis effectSeparated from

South Atlantic gyre by Atlantic equatorial counter current


Atlantic Ocean CirculationNorth Atlantic

Subtropical GyreNorth Equatorial

CurrentGulf StreamNorth Atlantic CurrentCanary CurrentSouth Equatorial

CurrentAtlantic Equatorial

Counter Current


Atlantic Ocean CirculationSouth Atlantic

Subtropical Gyre Brazil Current Antarctic Circumpolar

Current Benguela Current South Equatorial

Current

South Atlantic – upper water gyre – extends from surface to a depth of 200 m near the equator to 800m southern limits of gyre at Subtropical convergence

Wind stress of South East trade winds between equator and 10-15° S – main driving force

Acts on sea and forms South Equatorial current (SEC) – greatest strength just below equator – flows west towards American side of South Atlantic

Spills by topographic interference by eastern prominence of Brazil. Part of SEC moves off northeastern coast of South America towards Caribbean and North Atlantic, rest is turns southwards as brazil current

Brazil current coming from the tropics is warm and saline, turns east and continues across Atlantic as Antarctic Circumpolar current (WWD) and moves eastward. The Brazil current is much smaller than the Northern Hemisphere counterpart i.e. the Gulf stream due to the splitting of SEC

WWD than turns north up on African side as the Benguela Current which flows equatorward along Africa’s western coast

Benguela current is slow drifting cold current because of the contribution of Subantartic water and of upwelling along the African coast

Falkland current

Falkland current – is outside the South Atlantic gyre, but is a significant north bound flow of cold water. Current flows from Drake passage and moves along the western margin of South Atlantic up the coast of South America. Falkland current impart cold current that moves along the coast of Argentina as far as north as 30°S thus separating Brazil current from coast at this point.

South Atlantic circulation is bounded on south by Subtropical Convergence.


Gulf Stream Best studied of all ocean currents Meanders and loops Merges with Sargasso Sea

Circulates around center of North Atlantic Gyre Unique biology – Sargassum


Gulf Stream Meanders or loops may cause loss of water volume and generate:

Warm-core rings – warmer Sargasso Sea water trapped in loop surrounded by cool water

Cold-core rings – cold water trapped in loop surrounded by warmer water

Unique biological populations


Other North Atlantic CurrentsLabrador CurrentIrminger CurrentNorwegian CurrentNorth Atlantic Current


Climate Effects of North Atlantic CurrentsNorth-moving currents – warm Gulf Stream warms East coast of United States and

northern EuropeNorth Atlantic and Norwegian Currents warm

northwestern EuropeSouth-moving currents – cool Labrador Current cools eastern CanadaCanary Current cools north African coast


Indian Ocean CirculationMonsoons – seasonal reversal of winds over northern

Indian OceanHeat Capacity DifferentialNortheast monsoon – winterSouthwest monsoon – summer


Indian Ocean Circulation


Indian Ocean MonsoonAffects

seasonal land weather

Affects seasonal Indian Ocean current circulation

Affects phytoplankton productivity


Indian Ocean CirculationIndian Ocean Subtropical Gyre

Agulhas CurrentAustralian CurrentLeeuwin Current


Pacific Ocean CirculationNorth Pacific Subtropical Gyre

KuroshioNorth Pacific CurrentCalifornia CurrentNorth Equatorial CurrentAlaskan Current


Pacific Ocean CirculationSouth Pacific Subtropical Gyre

East Australian CurrentAntarctic Circumpolar CurrentPeru CurrentSouth Equatorial CurrentEquatorial Counter Current


Pacific Ocean Circulation

Upwelling and downwellingVertical movement of water

Upwelling = movement of deep water to surface Hoists cold, nutrient-rich water to surface Produces high productivities and abundant marine life

Downwelling = movement of surface water down Moves warm, nutrient-depleted surface water down Not associated with high productivities or abundant

marine life

upwelling

downwelling

UpwellingCauses cold, nutrient rich water from the deep ocean to rise to the surface.

El Nino and La NinaEl Nino is a change in water temperature in the

Pacific ocean that produces a warm current.La Nina is a change in temperature in the Eastern

Pacific that causes surface water temperature to be much colder than usual

BOTH El nino and La Nina can cause flooding (too much rain) and drought (too little rain) in different places on Earth. Upwelling does not occur where it normally would and this affects fish and sealife.

El Niño-Southern Oscillation (ENSO)El Niño = warm surface current in equatorial eastern

Pacific that occurs periodically around DecemberSouthern Oscillation = change in atmospheric

pressure over Pacific Ocean accompanying El NiñoENSO describes a combined oceanic-atmospheric

disturbance

• Oceanic and atmospheric phenomenon in the Pacific Ocean

• Occurs during December• 2 to 7 year cycle

Sea Surface Temperature

Atmospheric Winds

Upwelling

El NiñoNon El Niño

1997

Non El Niño

El Niño

Thermocline – layer of ocean right beneath the “mixed layer” where temperatures decrease rapidly.

upwelling

El Niño events over the last 55 years

El Niño warmings (red) and La Niña coolings (blue) since 1950. Source: NOAA Climate Diagnostics Center

http://www.cdc.noaa.gov/ENSO/enso.mei_index.html

http://www.cdc.noaa.gov/ENSO/enso.mei_index.html

El Nino Animation

World Wide Effects of El Niño

• Weather patterns

• Marine Life

• Economic resources

http://esminfo.prenhall.com/science/geoanimations/animations/26_NinoNina.html

Effects of severe El Niños

Coriolis EffectBecause of the coriolis effect, winds appear to deflected to the east or west depending on the direction winds are traveling.

A buoy records data about surface ocean temperature and transmits (sends) the information to a satellite in space that then transmits(sends) the information to scientists.

Land breeze and sea breeze

Water has a much higher heat capacity (absorbs and lets go of heat more) slowly than land, water temperature will increase and decrease less than land temperature.

e.g. during daytime, land temperatures might change by tens of degrees,water temperature change by less than half a degree.

i.e. coastal land temperatures don’t fluctuate (go up and down) extremely (a lot) because the ocean water nearby doesn’t fluctuate much.


Chapter OverviewOcean currents are moving loops of water.Surface currents are influenced by major

wind belts.Currents redistribute global heat.Thermohaline circulation affects deep currents.Currents affect marine life.


Types of Ocean CurrentsSurface currents

Wind-drivenPrimarily horizontal motion

Deep currents

Driven by differences in density caused by differences in temperature and salinity

Vertical and horizontal motions


Measuring Surface CurrentsDirect methodsFloating

device tracked through time

Fixed current meterIndirect methods

Pressure gradientsRadar altimetersDoppler flow meter


Global Surface Current Flow


Measuring Deep CurrentsFloating devices tracked through timeChemical tracers

TritiumChlorofluorocarbons

Characteristic temperature and salinity

Argo


Argo


Surface CurrentsOccur above pycnoclineFrictional drag between wind and oceanGenerally follow wind belt patternOther factors:

Distribution of continentsGravityFrictionCoriolis effect


Subtropical Gyres

Large, circular loops of moving water

Bounded by: Equatorial currentWestern Boundary

currentsNorthern or Southern

Boundary currentsEastern Boundary

currentsCentered around

30 degrees latitude


Five Subtropical GyresNorth Atlantic – Columbus GyreSouth Atlantic – Navigator GyreNorth Pacific – Turtle GyreSouth Pacific – Heyerdahl GyreIndian Ocean – Majid Gyre


Subtropical Gyres and Currents


Subtropical Gyre CurrentsFour main currents flowing into one another:Equatorial Currents

North or southTravel westward along equator

Western Boundary Currents – warm watersNorthern or Southern Boundary Currents – easterly

water flow across ocean basinEastern Boundary Currents – cool waters


Gyres and Boundary Currents


Other Surface CurrentsEquatorial Countercurrents – eastward flow between

North and South Equatorial CurrentsSubpolar Gyres

Rotate opposite subtropical gyresSmaller and fewer than subtropical gyres


Western IntensificationTop of hill of water displaced toward west due

to Earth’s rotationWestern boundary currents intensified in

both hemispheresFasterNarrowerDeeperWarm

Coriolis Effect contributes to western intensification


Eastern Boundary CurrentsEastern side of ocean basinsTend to have the opposite properties of Western

Boundary CurrentsColdSlowShallowWide


Eastern and Western Boundary Currents


Ocean Currents and ClimateWarm ocean currents warm the air at the coast.Warm, humid airHumid climate on adjoining landmass

Cool ocean currents cool the air at the coast.Cool, dry airDry climate on adjoining landmass


Ocean Currents and Climate


Upwelling and DownwellingUpwelling – Vertical movement of cold, nutrient-rich

water to surfaceHigh biological productivity

Downwelling – Vertical movement of surface water downward in water column


Diverging Surface WaterSurface waters move

away from areaEquatorial upwellingCoastal upwelling


Coastal UpwellingEkman transport

moves surface seawater offshore.

Cool, nutrient-rich deep water comes up to replace displaced surface waters.

Example: U.S. West Coast


Other Types of UpwellingOffshore windsSeafloor obstruction Coastal geometry

change


Converging Surface WaterSurface waters move

toward each other.Water piles up.Low biological

productivity


Coastal DownwellingEkman transport

moves surface seawater toward shore.

Water piles up, moves downward in water column

Lack of marine life


Atmospheric-Ocean Connections in the Pacific OceanWalker Circulation Cell – normal conditions

Air pressure across equatorial Pacific is higher in eastern Pacific

Strong southeast trade windsPacific warm pool on western side of oceanThermocline deeper on western sideUpwelling off the coast of Peru


Normal Conditions, Walker Circulation


El Nino – Southern Oscillation (ENSO)

Walker Cell Circulation disrupted High pressure in eastern Pacific weakensWeaker trade windsWarm pool migrates eastwardThermocline deeper in eastern PacificDownwellingLower biological productivity

Peruvian fishing suffers


ENSO Conditions in the Pacific Ocean


La Nina – ENSO Cool PhaseIncreased pressure difference across equatorial PacificStronger trade windsStronger upwelling in eastern PacificShallower thermoclineCooler than normal seawaterHigher biological productivity


La Nina Conditions


Occurrence of ENSO EventsEl Niño warm phase about every

2–10 yearsHighly irregularPhases usually last 12–18 months10,000-year sediment record of eventsENSO may be part of Pacific Decadal Oscillation

(PDO) Long-term natural climate cycleLasts 20–30 years


ENSO Occurrences


ENSO has Global Impacts


Notable ENSO Events1982 – 19831997 – 1998 Flooding,

drought, erosion, fires,tropical storms, harmful effects on marine life

Unpredictable


Predicting El Nino Events Tropical Ocean−Global Atmosphere (TOGA) program

1985Monitors equatorial South PacificSystem of buoys

Tropical Atmosphere and Ocean (TOA) projectContinues monitoring

ENSO still not fully understood


Deep-Ocean CurrentsThermohaline Circulation – deep ocean circulation

driven by temperature and density differences in water

Below the pycnocline90% of all ocean waterSlow velocity


Thermohaline CirculationOriginates in high latitude surface oceanCooled, now dense surface water sinks and changes

little.Deep-water masses identified on temperature–

salinity (T–S) diagram Identifies deep water masses based on temperature,

salinity, and resulting density


Thermohaline Circulation


T–S Diagram


Thermohaline CirculationSome deep-water masses

Antarctic Bottom WaterNorth Atlantic Deep WaterAntarctic Intermediate WaterOceanic Common Water

Cold surface seawater sinks at polar regions and moves equatorward


Conveyor Belt Circulation


Power From CurrentsCurrents carry

more energy than winds

Florida–Gulf Stream Current System

Underwater turbinesExpensiveDifficult to maintainHazard to boating

Measuring surface currentsDirect methods

Float meters Intentional Inadvertent

Propeller meters

Indirect methodsPressure gradientsSatellitesDoppler flow meters Figure 7B

Surface currents closely follow global wind belt pattern

Trade winds at 0-30º blow surface currents to the east

Prevailing westerlies at 30-60º blow currents to the west

Figure 7-3

Figure 7-4

Current gyresGyres are large circular-moving loops of

waterSubtropical gyres

Five main gyres (one in each ocean basin): North Pacific South Pacific North Atlantic South Atlantic Indian

Generally 4 currents in each gyre Centered at about 30º north or south latitude

Current gyresGyres (continued)

Subpolar gyres Smaller and fewer than subtropical gyres Generally 2 currents in each gyre Centered at about 60º north or south latitude Rotate in the opposite direction of adjoining subtropical

gyres

Western intensification of subtropical gyres

The western boundary currents of all subtropical gyres are:FastNarrowDeep

Western boundary currents are also warmEastern boundary currents of subtropical gyres have

opposite characteristics

Currents and climateWarm current � warms

air � high water vapor � humid coastal climate

Cool current � cools air � low water vapor � dry coastal climate

Figure 7-8a

Upwelling and downwellingVertical movement of water (� )

Upwelling = movement of deep water to surface Hoists cold, nutrient-rich water to surface Produces high productivities and abundant marine life

Downwelling = movement of surface water down Moves warm, nutrient-depleted surface water down Not associated with high productivities or abundant

marine life

Coastal upwelling and downwellingEkman transport moves surface water away from

shore, producing upwellingEkman transport moves surface water towards shore,

producing downwelling

Figure 7-11

Other types of upwellingEquatorial upwellingOffshore windSea floor obstructionSharp bend in coastal

geometry

Figure 7-9

Equatorial upwelling

Figure 7-13

Figure 7-14

Figure 7-15

The Gulf Stream and sea surface temperatures

The Gulf Stream is a warm, western intensified current

Meanders as it moves into the North Atlantic

Creates warm and cold core rings

Figure 7-16

Figure 7-17

El Niño-Southern Oscillation (ENSO)El Niño = warm surface current in equatorial eastern

Pacific that occurs periodically around ChristmastimeSouthern Oscillation = change in atmospheric

pressure over Pacific Ocean accompanying El NiñoENSO describes a combined oceanic-atmospheric

disturbance

Figure 7-18a

Figure 7-18b

Figure 7-18c

The 1997-98 El NiñoSea surface

temperature anomaly map shows warming during severe 1997-98 El Niño

Internet site for El Niño visualizations

Current state of the tropical Pacific

Figure 7-19a

El Niño recurrence intervalTypical recurrence interval for El Niños = 2-12

yearsPacific has alternated between El Niño and La

Niña events since 1950

Figure 7-20

Figure 7-21

Figure 7-23

Northeast monsoon Southwest monsoon

Deep currentsDeep currents:

Form in subpolar regions at the surfaceAre created when high density surface water sinksFactors affecting density of surface water:

Temperature (most important factor) Salinity

Deep currents are also known as thermohaline circulation

Deep ocean characteristicsConditions of the deep ocean:

ColdStillDarkEssentially no productivitySparse lifeExtremely high pressure

Identification of deep currentsDeep currents are

identified by measuring temperature (T) and salinity (S), from which density can be determined

Figure 7-24

Figure 7-25

Figure 7-27

ocean currents water masses

Education

pearson education

sea water

densest water

water nabw

cold dense water

antarctic shelf

dense antarctic waters

movement of ocean currents