ocean currents water masses
-
Upload
arabinda-pal -
Category
Education
-
view
153 -
download
0
Transcript of 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
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)
© 2011 Pearson Education, Inc.
Antarctic CirculationAntarctic Circumpolar
CurrentAlso called West Wind
Drift and Penguin GyreOnly current to
completely encircle Earth
Moves more water than any other current
© 2011 Pearson Education, Inc.
Antarctic CirculationAntarctic Circumpolar
CurrentAlso called West Wind
Drift and Penguin GyreOnly current to
completely encircle Earth
Moves more water than any other current
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Atlantic Ocean CirculationNorth Atlantic
Subtropical GyreRotates clockwise –
Coriolis effectSeparated from
South Atlantic gyre by Atlantic equatorial counter current
© 2011 Pearson Education, Inc.
Atlantic Ocean CirculationNorth Atlantic
Subtropical GyreNorth Equatorial
CurrentGulf StreamNorth Atlantic CurrentCanary CurrentSouth Equatorial
CurrentAtlantic Equatorial
Counter Current
© 2011 Pearson Education, Inc.
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.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Other North Atlantic CurrentsLabrador CurrentIrminger CurrentNorwegian CurrentNorth Atlantic Current
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Indian Ocean CirculationMonsoons – seasonal reversal of winds over northern
Indian OceanHeat Capacity DifferentialNortheast monsoon – winterSouthwest monsoon – summer
© 2011 Pearson Education, Inc.
Indian Ocean Circulation
© 2011 Pearson Education, Inc.
Indian Ocean MonsoonAffects
seasonal land weather
Affects seasonal Indian Ocean current circulation
Affects phytoplankton productivity
© 2011 Pearson Education, Inc.
Indian Ocean CirculationIndian Ocean Subtropical Gyre
Agulhas CurrentAustralian CurrentLeeuwin Current
© 2011 Pearson Education, Inc.
Pacific Ocean CirculationNorth Pacific Subtropical Gyre
KuroshioNorth Pacific CurrentCalifornia CurrentNorth Equatorial CurrentAlaskan Current
© 2011 Pearson Education, Inc.
Pacific Ocean CirculationSouth Pacific Subtropical Gyre
East Australian CurrentAntarctic Circumpolar CurrentPeru CurrentSouth Equatorial CurrentEquatorial Counter Current
© 2011 Pearson Education, Inc.
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
El Nino Animation
World Wide Effects of El Niño
• Weather patterns
• Marine Life
• Economic resources
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.
© 2011 Pearson Education, Inc.
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.
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Measuring Surface CurrentsDirect methodsFloating
device tracked through time
Fixed current meterIndirect methods
Pressure gradientsRadar altimetersDoppler flow meter
© 2011 Pearson Education, Inc.
Global Surface Current Flow
© 2011 Pearson Education, Inc.
Measuring Deep CurrentsFloating devices tracked through timeChemical tracers
TritiumChlorofluorocarbons
Characteristic temperature and salinity
Argo
© 2011 Pearson Education, Inc.
Argo
© 2011 Pearson Education, Inc.
Surface CurrentsOccur above pycnoclineFrictional drag between wind and oceanGenerally follow wind belt patternOther factors:
Distribution of continentsGravityFrictionCoriolis effect
© 2011 Pearson Education, Inc.
Subtropical Gyres
Large, circular loops of moving water
Bounded by: Equatorial currentWestern Boundary
currentsNorthern or Southern
Boundary currentsEastern Boundary
currentsCentered around
30 degrees latitude
© 2011 Pearson Education, Inc.
Five Subtropical GyresNorth Atlantic – Columbus GyreSouth Atlantic – Navigator GyreNorth Pacific – Turtle GyreSouth Pacific – Heyerdahl GyreIndian Ocean – Majid Gyre
© 2011 Pearson Education, Inc.
Subtropical Gyres and Currents
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Gyres and Boundary Currents
© 2011 Pearson Education, Inc.
Other Surface CurrentsEquatorial Countercurrents – eastward flow between
North and South Equatorial CurrentsSubpolar Gyres
Rotate opposite subtropical gyresSmaller and fewer than subtropical gyres
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Eastern Boundary CurrentsEastern side of ocean basinsTend to have the opposite properties of Western
Boundary CurrentsColdSlowShallowWide
© 2011 Pearson Education, Inc.
Eastern and Western Boundary Currents
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Ocean Currents and Climate
© 2011 Pearson Education, Inc.
Upwelling and DownwellingUpwelling – Vertical movement of cold, nutrient-rich
water to surfaceHigh biological productivity
Downwelling – Vertical movement of surface water downward in water column
© 2011 Pearson Education, Inc.
Diverging Surface WaterSurface waters move
away from areaEquatorial upwellingCoastal upwelling
© 2011 Pearson Education, Inc.
Coastal UpwellingEkman transport
moves surface seawater offshore.
Cool, nutrient-rich deep water comes up to replace displaced surface waters.
Example: U.S. West Coast
© 2011 Pearson Education, Inc.
Other Types of UpwellingOffshore windsSeafloor obstruction Coastal geometry
change
© 2011 Pearson Education, Inc.
Converging Surface WaterSurface waters move
toward each other.Water piles up.Low biological
productivity
© 2011 Pearson Education, Inc.
Coastal DownwellingEkman transport
moves surface seawater toward shore.
Water piles up, moves downward in water column
Lack of marine life
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Normal Conditions, Walker Circulation
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
ENSO Conditions in the Pacific Ocean
© 2011 Pearson Education, Inc.
La Nina – ENSO Cool PhaseIncreased pressure difference across equatorial PacificStronger trade windsStronger upwelling in eastern PacificShallower thermoclineCooler than normal seawaterHigher biological productivity
© 2011 Pearson Education, Inc.
La Nina Conditions
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
ENSO Occurrences
© 2011 Pearson Education, Inc.
ENSO has Global Impacts
© 2011 Pearson Education, Inc.
Notable ENSO Events1982 – 19831997 – 1998 Flooding,
drought, erosion, fires,tropical storms, harmful effects on marine life
Unpredictable
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Deep-Ocean CurrentsThermohaline Circulation – deep ocean circulation
driven by temperature and density differences in water
Below the pycnocline90% of all ocean waterSlow velocity
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Thermohaline Circulation
© 2011 Pearson Education, Inc.
T–S Diagram
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Conveyor Belt Circulation
© 2011 Pearson Education, Inc.
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