OCEAN CIRCULATION

I. Ocean Layers and circulation types

1) Ocean Layers

• Ocean is strongly Stratified • Consists of distinct LAYERS –controlled by density

•takes huge amounts of energy to mix up the stable layers!

Temperature slice in the central Atlantic NorthSouth

Thermocline – a sharp transition in temperature and hence DENSITY (marked by many contour intervals)

The “main” thermocline divides WARM SURFACE from COLD DEEP water.

Ocean Layers

SURFACE OCEANTHERMOCLINE

DEEP OCEAN

• Water above the thermocline can be moved by wind (via friction with the atmosphere)

• Water below thermocline- much larger barrier, due to density layers.

What forces can move layers of water?

1) Circulation “types”

two very separate types of ocean circulation –

•1) surface -driven by wind •2) deep driven by density

II. Wind Fields and Main Ocean “Gyres”

Rev: Main-wind beltsRecall: Main WINDBELTS – these drive Surface circulation!

Prevailing Westerlies

Trade Easterlies

Recall again..

If wind drives the circulation….

What major ocean currents would you predict these wind

bands would create?

Prevailing Westerlies

Prevailing Westerlies

Easterly Trade

Easterly Trade

Subtropical gyres and their Main Current Types

Equatorial/tropical current

W E

Eastern Boundary current

L

0°

30°S

30°N

60°N

60°S

L

H

H

L

High latitudecurrent

High latitudecurrent

5 Major subtropical gyres

Notice smaller Sub-polar Gyres as well !

30°N

30°S

H

H

More realistic Global Surface Currents

Kaufman Fig. 4.12

Franklin map

Had Just started:

The strange story of Nansen’s icebergs…

How do we know they exist?Ben Franklin’s map of the Atlantic deduced the Atl. Gyre from the way

ships would drift along their route to and from the New World

• Large ~ circular current flows

• Set in motion by main wind patterns

• Bounded by continents on E and W

• “subtropical” gyres most pronounced

Summary: Ocean Gyres

But What is wrong with this picture

(so far..) ?

II. ECKMAN TRANSPORT(CORIOLIS strikes again)

Nansen contemplating ice

Nansen’s funky icebergsNansen noticed icebergs moving

20˚ - 40˚ to right of wind direction

Ekman SpiralModel of surface response to wind forcing

- When wind blows over the ocean, surface water moves 45o to the right of the wind in the northernhemisphere

- Current rotates and weakens deeper in water column

Ekman TransportNet flow of surface layer when forced by wind

Most important point:

Add up all the arrows, the AVERAGE direction of flow is 90o to the right of the wind!

90o

Ekman TransportNet flow of surface layer when forced by wind

Most important point:

Add up all the arrows, the AVERAGE direction of flow is 90o

to the right of the wind!

Ekman Transport Causes Convergence of Water in

Middle of Gyres

Prevailing Westerlies

Northeasterly Trades

HCONVERGENCE

30°N

HPara ver esta película, debedisponer de QuickTime™ y de

un descompresor .

Convergence Zone

Surface LayerThickens

Gyre Flow out of screen

Gyre Flow into screen

H

Ekman Trans. Ekman Trans.CONVERGENCE

HPara ver esta película, debedisponer de QuickTime™ y de

un descompresor .

X

Geostrophic FlowPRESSUREGRADIENTFORCE

CORIOLIS

•PGF = High to Low Pressure Flow•Plus Coriolis•Results in Flow Clockwise around Gyre (NH)•Strength of Flow related to PG

PRESSUREGRADIENTFORCE

CORIOLISX

HPara ver esta película, debedisponer de QuickTime™ y de

un descompresor .

• Wind fields create areas of lower and higher water: “humps” and “valleys” of water.

• Result: just like in atm = Pressure gradients• And just like in atm, water tries to flow from Hight

to Low pressure zones

Pressure gradients in surface ocean

• Can be used to calculate current strengths.

From Regional Oceanography © 2001 - 2003 M. Tomczak and J. S. Godfrey

“Dynamic Height Maps” Maps of relative height of surface ocean

from the measurements of the ocean density

• 1) Direct Effect of Wind– Confined to upper 50-100 m– Frictional surface layer flow is ~90˚ to right of wind

direction in NH, to left in SH -> Ekman Transport

• 2) Indirect Effects of Coriolis– Eckman Transport sets up Pressure gradients drive

focused currents (jets)– Can be 1 - 2 km below surface

SUMMARY:WHAT DRIVES SURFACE CURRENTS?

• Wind stress drives surface convergences

• Sea surface rises

• Balance between coriolis effect and PG at the gyres: Geostrophic currentmoving in a circular path around the gyre hill

Summary of Main Gyre Circulation.

One more thing:Notice anything funny about this gyre ?

“WESTERN INTENSIFICATION”

&W. Boundary Currents

• Geostrophic current is around the sea-surface high

• BUT Gyres are “squeezed” up against western boundaries of ocean basins

Gyre Circulation & W. Boundaries.

Notice asymmetry in circulation

Reasons for western intensification (asymmetric gyres)

Notice asymmetry in circulation

• 1) Earth’s rotation-coriolis effectasymmetries with latitude: Westerlies more effect than Easterlies

• => higher speed along the western margin of the hill compared to the eastern side.

Western boundary currents• Found in all the ocean basins: NH and SH• Fast (~1 m/s) and narrow (~100 km)

Figure 7-4

Gulf stream• Classic W. Boundary

current.

• “Starts” as the Florida current between Florida and the Bahamas

• Leaves coast at Cape Hatteras, NC, cross the Atlantic.

• Transports HUGE amounts of water and HEAT – keeps Europe warm..

Water transport Water flow rates, units are in Sverdrups (= 1 million cubic meters/second)

100

~ 6

Moves so fast, that creates:

Gulf Stream “rings”Temperature June 11, 1997

cold water surrounded by warm rings:unique physical characteristics and biological habitats

Warm core rings

Currents shape climate! • Warm current - warms air (L) = high water vapor= humid coast

East Coast USA, also E. Coast Asia (Japan in Aug..)• Cool current cools air (H) = low water vapor dry coast

W. Coast USA (Santa Cruz)

Finally:Back to the coasts- How would Eckman transport affect coastal

surface circulation?

UPWELLING(revisited)

Coastal UpwellingExample from Northern Hemisphere

W Emiddle of gyre

Typical Northerly wind

Ekman TransportSea level

1000 m

wind

Coastal UpwellingExample from Southern Hemisphere

W Emiddle of gyre X

southerly wind

Ekman Transport

XSea level

1000 m

Peru and CA-Amazingly productiveUpwelling-drivenEnvironments! -due to the upwelling of nutrients to the surface

Global chl map

Chlorophyl a: productive upwelling areas vs oceanic desserts

Equatorial Divergent Upwellingmap view

2˚N

2˚S

EQ Trade Winds

West East

UPWELLING

Ekman Transport{to right in NH}

Ekman Transport{to left in SH}

DIVERGENCE

summary cartoon

Equatorial Divergent Upwellingcross-section

2˚N2˚S

West East

DIVERGENCE

X XXTrade Winds

UPW

ELLING

Ekman Trans.

{to left in SH}

Ekman Trans.

{to right in NH}

Surface LayerThins

What about in the deep?

Does it circulate? How?

Next time: DEEP OCEAN

CIRCULATION