7 Ocean Circulation(1)

8/10/2019 7 Ocean Circulation(1)

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Physical Oceanography

(Atmosphere & Ocean Circulation)

!"#$%&https://www.facebook.com/CornellOceanography !'())(* (, -*#..%/& "CUOceanography

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Outline1. Overview of Observed Physical Patterns

Winds, Ocean Temperature, Salinity, Surface + Deep Currents

2. Atmospheric Circulation

Coriolis Force, Atmospheric Convection, Hadley Circulation, Surface Wind Patterns

3. Density Structure of the Ocean

Salinity Variations, Ocean Heating and Cooling, Seawater Density, Density Stratification,Permanent and Seasonal Thermocline.

4. Wind-Driven Surface Circulation

Acting Forces, Ekman Transport in the Surface Ocean, Geostrophic Currents in theSubsurface Ocean, Subtropical Gyre Circulation, Equatorial and Coastal Upwelling

5. Thermohaline Circulation of the Deep Ocean

Water Mass Identification by T-S Signature, Rate of Movement with 14C, Location ofDeepwater Formation, Global Conveyor Belt Circulation and Global Heat Transport2

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Definition of Latitudinal Regions of the Words Oceansyou do not need to memorize these, but some of them will come up in this lecture

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Observed Patterns

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Global Surface WindPatterns

1. Three primary wind belts

2. Contained within latitudebands

0-3030-6060-90

3. Wind belts alternate direction

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Ocean SurfaceTemperature

Degrees Celsius

1. Warmest water alongthe equator in the

western ocean basins

2. Coldest water nearthe poles

3. Tongue of colderwater along the

eastern equatorialregion

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1. Warm thin surface layer

2. Cold thick deep layer

3. Thermocline separates the warm and cold layers

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Atlantic Ocean

Temperature at Depth

Meridional Section from Iceland to Antarctica

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PartsperThousand

Ocean Surface

Salinity

1. Saltiest water in the subtropics

2. Atlantic considerably more salty than the other oceans

3. North Pacific the freshest of all regions

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Atlantic Ocean

Salinity at Depth

1. High salinity in surface layer in subtropics

2. Deep-ocean salinity more uniform and generally matches the surfacesalinity at higher latitude - especially northern high latitudes.

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1. Large subtropical gyres rotating clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.

2. Subpolar gyres rotating counterclockwise in the northern hemisphere and absentin the southern hemisphere

3. Antarctic Circumpolar Current moving eastward around the Antarctic Continent.

Ocean Surface

Currents

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Deep Circulation in

the Ocean

1. Cold surface water sinks specifically at high latitudes and movesto lower latitudes at depth.

2. Deep-water returned to the surface layer through broad diffusiveupward vertical mixing

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1. Sir Isaac Newtons famous F=malaw only works for a reference

coordinate system that is at rest(or at the very least not accelerating)

2. When the reference frame is itself accelerating (e.g., a car turning acorner, a rotating merry-go-roundor a rotating earth), then all

bets are off on Newtons law working unless the acceleration of the

reference frame itself is also taken into account.

3. Addition of a Coriolis force is needed to account for the acceleration

of the reference coordinate system (i.e., the latitude longitude grid ona rotating earth) before Newtons Law can be used.

The Need for a Coriolis Force

Sir Isaac Newton1642-1727

Gustave Gaspard de Coriolis1792-1843

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Qualitative Description of theCoriolis Force

1. Northward movement of a water parcel (greencircle) brings it closer to the axis of rotation.

2. By the principle of the conservation of a

angular momentum, the water parcel must spinfaster around axis of rotation (i.e., eastward).

4. If the earth is treated as a non-rotating reference frame (which is what we want to do) then

an apparent forcemust be added to explain the apparent push on the parcel in theeastward direction - this apparent force is the Coriolis Force!

5. The eastward-directed Coriolis Force in this example is also viewed as an apparent forcedirectedto the right of northward motion.

3. A close analogyto this process is an ice skater that spins faster whenthey draw their hands close to their body.

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All You Really Need to Know About the Coriolis Force For

This Class...

1. It arises from real physics related to the conservation of angular moment

2. In thenorthern hemispherethe Coriolis Force is always directed to therightof the water parcel motion

3. In the southern hemispherethe Coriolis Force is always directed to theleftof the water parcel motion

4. The Basic Ruleto determine the direction of the Coriolis Force is to1) point your nosein the direction of the ocean current (or wind) is

moving and then2)stick your hand directly out from your sideto get thedirection of the Coriolis force - right hand for the northern hemisphere orleft hand for the southern hemisphere

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Atmospheric Circulation

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1. Material is warmed when it absorbs radiant

energy(e.g., absorbs sunlight energy)

2. Much of the visible radiant energy from the

sun (short wavelength radiation)passesright through the atmosphere without beingabsorbed and, consequently,without muchdirect heating of the atmosphere from the sun.

3. The earths surface absorbs a lot of thesunlight energy and so it becomes warm.

4. The warm earth surface radiates infra red

radiation(long wavelength radiation) fromits surface and this energy isabsorbed by theatmosphere and, consequently warms the

atmosphere.

5. So the atmosphere is heated from belowbyabsorbing long wavelength radiation emittedfrom the warm earth surface.

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Atmospheric Convection1. The warm ocean surface heats the

atmosphere from below

2. Warm surface air is able to hold a lot ofwater vapor and so it becomes moist.

3. Warm moist surface air has low densityso it rises(like a hot air balloon) and asit moves aloft it expands and cools

4. Cooling of air aloft causes water vapor tocondense and precipitate

5. Condensation releases latent heat thatdrives the upward convection even higher

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Solar Heating of the Earth Surface

1. Solar heating of the earthsurface is most intense atthe equator where the

solar radiation is mostdirect.

2. Consequently,atmospheric convectionand precipitation is

strongest along theequator

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Intertropical

Convergence Zone(ITCZ)

1. Strong surface heating by the sun along the equator drives upward atmosphericconvection, condensation and precipitation.

2. The result is an obvious band of thick clouds that forms along the equator

3. This region migrates north and south with summer/winter movement of the sunoverhead

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Question...

Given that theweight of a column of airis directly proportional tothe average molecularweight of the mix of all molecules making up the aircolumn...

Assume dry air is mainly nitrogen gas (N2with a molecular weight of 28) with some variableamount of water vapor gas (H2Owith a molecular weight of 18) mixed in with it.

Which type of air column wouldweigh less?(i.e, produce lower sea level pressure underneath it)

a) air column of dry air

b) air column withhigh amounts of water vapor

The correct answer is (b)since the average molecular weight of the mixture of moleculesmaking up the air column is on average lower when overall fraction of water vapor gas is high.

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Idealized HadleyCirculation

Low SLPHigh SLP High SLPLow SLP

1. Surface air at the equator warms and rises aloft. Air aloft spreads north/south and becomes more denseas it cools and dries (due to precipitation) and then sinks at about 30 latitude.

2. Surface air at 30 spreads out north and south, warms and picks up moisture - by 60 latitude it haswarmed and moistened to the point where it rises, cools, precipitates and spreads out aloft north/south.

3. Near the poles the air aloft becomes very cold and very dense so it sinks over the poles and spreads outtoward the equator SLP= Sea Level Pressure

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Global Sea Level Pressure Resulting from IdealizedHadley Circulation

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Surface Winds Resulting from Pressure Differencesand Turning By the Coriolis Force

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Idealized Versus Actual Pattern of Global Sea Level Pressure AndResulting Surface Winds

1. Differences due to unequal warming of land and ocean by the sun

2. Land warms more strongly than adjacent ocean in summer and cools more deeply than adjacentocean in winter - due to differences in specific heat of rock versus seawater.25

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Physical Oceanography

(Atmosphere & Ocean Circulation)

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Outline

1.Overview of Observed Physical Patterns

Winds, Ocean Temperature, Salinity, Surface + Deep Currents

2.Atmospheric Circulation

Coriolis Force, Atmospheric Convection, Hadley Circulation, Surface Wind Patterns

3.Vertical Density Structure of the Ocean

Salinity Variations, Ocean Heating and Cooling, Seawater Density, Density Stratification,Permanent and Seasonal Thermocline.

4.Wind-Driven Surface Circulation

Acting Forces, Ekman Transport in the Surface Ocean, Geostrophic Currents in theSubsurface Ocean, Subtropical Gyre Circulation, Equatorial and Coastal Upwelling

5.Thermohaline Circulation of the Deep Ocean

Water Mass Identification by T-S Signature, Rate of Movement with 14C, Location ofDeepwater Formation, Global Conveyor Belt Circulation and Global Heat Transport

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Vertical Distribution of SeawaterDensity

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Why worry about the distribution of seawater density in the

ocean?

1. Vertical distribution of density directly influences the vertical movementof water in the ocean.

2. Horizontal ocean currents are forced by the 1) direct action of windsblowing over the ocean surface and by 2) internal pressure forces created, inpart, by the unequal distribution of density in the ocean.

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NOTE: salt concentrationandwater temperaturejointly determine seawater

density, consequently,the concepts of ocean salinity and ocean heat content arepresented in some detail.

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Surface Salinity

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Partsp

erThousand

Ocean

SurfaceSalinity

Much more will be said later in the semester, but surface salinity is set at the ocean surface as aresult of the exchange of fresh water between the ocean and atmosphere through evaporation and

precipitation. Rain lowers ocean salt concentration. Evaporation raises salt concentration

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The Exchange of Heat Between theOcean Surface and the Atmosphere

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Main Heat FluxComponents In and Outof the Surface Ocean

1. Ocean temperature is a measureof the heat energy contained in

the ocean.

2. When more energy enters than

leaves the ocean surface, theocean surface warms. Similarly,when more heat leaves thanenters the ocean surface, theocean surface cools.

3. Sunlightenergy always adds heat to the ocean (during daylight hours).

4. There are lots of ways to lose heat with Evaporative Cooling often being the dominant method

of heat loss

5. For temperate (mid-latitude) ocean regions in spring/summer, more heat enters the oceanfrom sunlight than leaves to ocean and so the surface ocean warms. In winter, more heat

leaves than comes in from sunlight and the surface ocean cools

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The Depth of Light/Energy Penetration Into the Surface Ocean IsRelatively Shallow

1. In the clearestopen-ocean waters:Light penetrates

down to about150-200 meters

2. In turbid coastalwaters:light mayonly penetrate a fewmeters to maybe 50meters

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Typical Vertical Profile of Temperature

1. Because sunlight energy is quickly absorbed in theocean, all directsolar heating takes place verynear the ocean surface (0-30 meters).

2. Vertical mixing of near-surface water can movesome of the warm water deeper, but mixing onlyreaches at most to about 500 meters.

3. Seawater below 500 meters is uniformly cold.

4. The broad region centered at around 500 meterswhere seawater temperature changes from warmto uniformly cold is referred to as thepermanentthermocline

5. In some ocean regions, there is a much shallowerthermocline that only forms in summer and is thenis erased in winter. This temporary thermocline isreferred to as the seasonal thermocline 35

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Typical Vertical Density Profile

1. In a stably stratifiedocean, the least densewater floats above the more dense water.

2. Theseasonal pycnocline(region of strong

change in density with depth) comes and goeswith net heat gains in spring and summer andnet heat losses in fall and winter

3. Thepermanent pycnoclineremains in place(at around 500 meters) and is the result of the

long-term balance between the downwardheating/mixing at the surface and upwardmixing of cold water from below.

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The Seasonal Thermocline

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Main Heat FluxComponents In and Out

of the Surface Ocean

1. Ocean temperature is a measureof the heat energy contained in

the ocean.

2. When more energy enters than

leaves the ocean surface, theocean surface warms. Similarly,when more heat leaves thanenters the ocean surface, theocean surface cools.

3. Sunlightenergy always adds heat to the ocean (during daylight hours).

4. There are lots of ways to lose heat with Evaporative Cooling often being the dominant method

of heat loss5. For temperate (mid-latitude) ocean regions in spring/summer, more heat enters the ocean

from sunlight than leaves to ocean and so the surface ocean warms. In winter, more heatleaves than comes in from sunlight and the surface ocean cools

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Seasonal Variation of Heating and Cooling of the Surface Ocean atMid-Latitude Regions

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Annual Range of Sea Surface Temperature

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Seasonal Change in Surface Layer Temperature and Surface LayerThickness

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Cartoon Depicting the Latitudinal Range of the Seasonal andPermanent Thermocline

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1. Ekman Transporta slab of surface ocean water (30-60 meters thick) moved bywind forcing

the slab of water moves at exactly 90 degrees to the direction

of the wind forcing

2. Geostrophic Currents

currents move along lines of constant pressure)

3. Formation of Subtropical Gyres

4. Deep Ocean (Abyssal) Circulation

5. The Conveyor Belt Circulation

Ocean Circulation

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Ekman Transportof Water in the

Surface Friction Layer of the Ocean

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Ekman TransportDue to Wind Force,Friction Force and Coriolis Force

1. Ekman Spiral: the spiraling of thin oceancurrents within the Ekman Layer

2. top-most current moves at 45-degrees to the windforcing. Becauseof Wind+Coriolis+Friction

3. The bottom-most current moves directly oppositethe wind direction.

4. Ekman Transport:is the rate of total water transported in the Ekman Layer (a layer that is about50 to 100 meters thick)

5. Derived by summing all the individual thin currents sheets over the entire Ekman Layer

6. Direction is exactly at 90 degrees of the wind direction

7. Oceanographers often treat the Ekman Layer as a slab of water(i.e., the Ekman Layer)that movesin unison at 90 degrees to the right (northern hemisphere) or left (southern hemisphere) of the winddirect

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EkmanLayer

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GeostrophicMotion/Currents inthe Subsurface Layer of the Ocean

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Response of Fluid in the DeepOcean Due to Pressure GradientForce and Coriolis Force

3. The fluid continues to be pushed by Coriolis until the Coriolis force is directed equal and opposite of the pressuregradient force - at which point the two forces cancel and the fluid move at steady speed (no acceleration) - remembertoo that there is not friction to bring this steady motion to a halt!

4. The point at which currents move with steady speed with Coriolis and pressure gradient forces in prefect opposition isreferred to asgeostrophic balanceand the resulting current is referred to as ageostrophic current.

5. Notice that fully developed geostrophic currents move along lines of constant pressure (black lines in the figureabove)

1. Fluid first responds to pressure gradientforce (blue arrow) and moves down thepressure gradient toward lower pressure

2. Coriolis force (red arrow) alwayspushes the fluid to the right ofitspresent direction of travel

Top View of Horizontal Pressure Gradient

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Formation of Subtropical Gyres

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1. Dark lines are contours of constantpressure that decrease away fromthe center of the mound region.

2. Yellow arrows are the direction offluid motion (geostrophic currents)

for a northern hemisphere mound.

3. Light Blue Arrows point out thedirections of the pressure gradientforce - toward lower pressure

4. Red Arrows point out the Coriolisforce and are directed to the rightof fluid motion - and in this case tothe center of the high pressure

If a mound of water builds up at the surface of the ocean, a central high pressure developsunder the mound and circular geostrophic currents develop

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Development of the Subtropical Gyre in Response to Forcing by the Westerlyand Trade Wind Belts

1. Westerly and Trade Winds drive the EkmanLayer to the center of the gyre to create a

mound of surface water

2. A center of high pressure develops under themound of surface water.

3. The fluid initially moves away from the center of

high pressure

4. the push of Coriolis turns the fluid until ittravels along concentric lines of constantpressure - geostrophic balance

5. Note that Coriolis force always points toward thecenter of high pressure and in the northernhemisphere the right hand Coriolis rule then

requires concentric clockwise motion to achieve acenter directed Coriolis force. Counterclockwisemotion is needed in the southern hemisphere withits left hand Coriolis Rule

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1. Currents on western side of all subtropical gyres are calledwestern boundary currents- they areveryswift and narrowjets thatbringwarm water from the tropics to high latitudes

2. Currents on eastern side of all subtropical gyres are called eastern boundarycurrents- they arebroadand slowandbring cold water from the high latitudes toward the tropics.

Western Intensification of Gyre Circulation

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Equatorial & Coastal Upwelling

A Result of Ekman Transport

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Equatorial Upwelling

4. Surface water also piles up on the western side of the ocean basin forcing a downward tilt tothe thermocline

5. Water is drawn up from the base to the shallow surface Ekman Depth. If the thermocline isclose the Ekman Depth, then cold water is drawn to the surface and if the thermocline isdeep only more warm water is drawn the the surface

1. North East TradeWindsdriveEkman Transport of the surface

layer to the north

2. South East Trade WindsdriveEkman Transport of the surface

layer to the south

3. The parting of the surfacewater to the north and southrequires, by continuity, that deep

water rises upward to the surfacealong the Equator.

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Sea Surface Temperature in the Equatorial Pacific

1. Eastern Equatorial Pacific Cold Tongue- due to equatorial upwellingANDthe close proximity of

the thermocline to surface Ekman Depth

2. Western Equatorial Pacific Warm Pool- upwelling still occurs, but the thermocline is much deeperthan the surface Ekman Depth so upwelling just draws more warm water to the surface with noeffective cooling. 54

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Coastal Upwelling Along theWashington Oregon Coast

1. Wind blowing out of thenorthdrives the Ekmanlayer to the right (northernhemisphere) which isoffshore

2. The offshore transport ofthe Ekman surface layer isreplaced byupwellingofdeeper cold water along thecoast

3. Wind blowing out of the southdrives the Ekman Layer again to the right (because northern

hemisphere) which is onshore

4. The onshore transport of the Ekman surface layer is driven downward - a process calleddownwelling

UpwellingFavorable Winds Downwelling Favorable Winds

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Thermohaline Circulation in theDeep Ocean

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Temperature-Salinity Signaturesof the three Major Water Masses

in the Global Ocean

1. NADW: North Atlantic Deep Water

2. AABW: Antarctic Bottom Water

3. AAIW: Antarctic Intermediate Water

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Water Masses of the North Atlantic

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Circulation of DeepWater in the GlobalOcean

3. Deep water in the North Atlantic moves south and mergers with Antarctic water in the Southern Oceanas they both swing around Antarctica and up into the Indian and Pacific Ocean basin.

4. The oldest deep waters (c.a., 2000 years) are in the deep Pacific.

1. Cold dense water sinksin the North Atlanticand around Antarctica to

form deep water

2. North Pacific salinity istoo low to sink eventhough temperatures arejust as cold as the NorthAtlantic

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Locations of Deep Water Formation in the North Atlantic

1) Labrador, 2) Irminger, 3) Greenland and 4) Norwegian Seas

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Locations of Deep WaterFormation Around

Antarctica

1) Ross and 2) Weddell Sea

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Global Conveyor Belt CirculationCombining the Deep Circulation and the Wind-Driven

Surface Circulation

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Conveyor Belt Circulation: A Super Idealized Pattern

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Conveyor Belt Circulation: A Heavily Idealized Pattern

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Schematic of Heat

Transport by the GlobalConveyor Belt Circulation

3. After giving up heat at high latitudes, surface water sinks and enters the thermohaline deep circulationand eventually is returned to the surface to pick up more heat and start the journey all over again.

1. Heat is gained by thesurface ocean at low

latitudes and transportedvia the wind-driven surface

circulation (westernboundary currents)to higherlatitudes.

2. Heat is given up by theocean to the atmosphere athigher latitudes to warm

these regions.

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Most Heat Transport to HighLatitudes is Done by StrongWestern Boundary Currents

1. Western Europe experiencesmild winters, relative toNorthern Canada, because

the Gulf Stream bringswarm ocean waternorthward to warm theatmosphere off the coast.

2. Westerly winds then carry

the warm coastal air overland.

7 Ocean Circulation(1)

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