Outline Further Reading: Chapter 07 of the text book - vertical structure - surface circulation (wind driven circulation) - 3D circulation (thermohaline

Outline

Further Reading: Chapter 07 of the text book

- vertical structure

- surface circulation (wind driven circulation)

- 3D circulation (thermohaline circulation)

Natural Environments: The AtmosphereGE 101 – Spring 2007

Boston University

MyneniLecture 19: Ocean Circulation

Mar-05-07(1 of 15)

Introduction


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Mar-05-07(2 of 15)

• Previously, we learned about the general circulation of the atmosphere– Surface winds and pressure patterns– Role in transporting energy from low latitudes to high latitudes– And you will learn of upper air winds after these classes on ocean circulation

• Today we are going to talk about the general circulation of the oceans– Vertical Structure of the oceans– Surface wind driven circulations: Gyres and western boundary currents– Thermohaline circulations: the movement of water in the deep ocean

Background


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Mar-05-07(3 of 15)

• The oceans are important for:– Maritime v. continental climates– El Nino– Heat Transport (summer storage-winter release)– Source of water to the atmosphere– Monsoon circulations

• Some facts about World’s oceans– Cover approximately 70% of earth’s surface– Average depth is 3800m (or 12,500 feet)– Mainly salt water (96.5% water, 3.5% salts - chloride compounds) -> more dense

(1026 kg/m3) than fresh water (1000 kg/m3)– Density increases with decreasing temperature and with increasing salinity– Ocean water freezes at about -2C compared to 0C for fresh water– Are liquid -> affects heat capacity, speed of currents– Ocean currents may be slow, 1 cm/s (36 m/h) in the ocean interior or swift >1 m/s as

(3.6 km/h) in the Gulf Stream. – Movement of water in the vertical is much smaller, ranging from a maximum of

1mm/s to more characteristic speeds of 0.1 mm/s.

“it’s wet, has low albedo, large heat capacity, and it’s fluid”

Vertical Structure


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Mar-05-07(4 of 15)

Temp

Depth

500m

600m

4000m0 C 18 C

Mixed Layer

Thermocline

Deep Layer

– Three main levels to the ocean– These levels are stratified by temperature with warmer water on top and cooler water at the bottom– Upper mixed layer

• Well mixed by winds at the surface• Therefore it’s fairly homogeneous with respect to temperature• This is where oceans interact with the atmosphere (i.e. like in El Nino)

– Thermocline• Rapid transistion to colder waters• Produces a strong barrier between surface waters and deep waters

– Deep layer• Extends to the bottom of the ocean• Has some variability in it but is also generally homogeneous

– Depth of all these layers varies with respect to latitude and season, for example, the mixed layer is very deep in the winter because storms mix up the ocean very effectively

Surface Circulation-1


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Mar-05-07(5 of 15)

• Two types of circulation: surface circulations (wind driven) and thermohaline circulations– Circulations at the surface generally referred to as “gyres”– These circulations are produced by the frictional drag exerted by the winds

• easterlies produce “westward” currents• westerlies produce “eastward” currents

– They are also affected by the presence of continental barriers which deflects currents north and south

• Note that in the southern hemisphere there are westward currents circling the entire globe


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Mar-05-07(6 of 15)


Western Boundary Currents– Distinct fast-moving (1m/s) warm currents on the western edge of ocean

basins – Relatively narrow - compared to width of basin - poleward current hugging the

western basin boundary, transports warm waters polewards– Examples: Gulf stream, Kuroshio Current, Brazil current– Western boundary current separation region - where the current leaves the

western edge and extends into the basin interior. This is where it meets the sub-polar gyres (Oyashio in the N Pacific, Labrador in the N Atlantic) which brings with it cold water. The currents meander and form rings.

– The waters return equatorwards on the eastern side of the basin (eastern ‘boundary current’). The return flow is wider and slower-moving than the western boundary current, and brings cold water equatorwards.

– Surface circulations important for short-term climate (i.e. weeks to years)• Influence air-sea interactions• Also influence coastal climates (e.g. Europe)



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Mar-05-07(7 of 15)



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Mar-05-07(8 of 15)

The wind driven circulation is confined mostly to the upper kilometer or two of the ocean. The mean surface winds patterns are composed of specific elements. - The trade winds blow westward. - The trade winds of the two hemispheres meet at the intertropical convergence zone (ITCZ), where updrafts of air induce high precipitation. - Poleward of the trades are the westerlies (air flow towards the east).- Still further poleward are the polar easterlies. - The westerlies are associated with the strongest wind, but often very variable, as shaped by storms and atmospheric fronts.

ITCZNE Trades

SE Trades

Stormy Westerlies



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Mar-05-07(9 of 15)

The wind exerts a force or stress on the ocean surface, proportional to the square of the wind speed. This produces not just ocean waves but also injects momentum (“mass in motion”) into the surface layer of the ocean.

The wind makes the surface layer of the ocean move,– it’s not in the direction of the wind stress, but rather at an angle to it - why?

Because of the Coriolis Force. A balance is achieved between the wind stress and the Coriolis Force.

The surface Ekman layer (named after the person who developed the theory in 1908) extends to about 50 to 200 meters depth. The mean transport within the Ekman layer is 90° towards the right of the wind in the northern hemisphere, 90° to the left in the southern hemisphere.

Typically the wind induced surface current is around 2 or 3% of the wind speed. One clear effect of the Ekman transport can be seen in along the coast of California, where cold subsurface water is brought up to the sea surface from a depth of perhaps 100 meters to replace surface water forced offshore by the Ekman transport. These regions are rich in nutrients and support important fisheries.

Win

d

Ekman Transport



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Mar-05-07(10 of 15)

Generally, wind changes in strength and direction from place to place. Thus, in some regions the Ekman transport forces accumulation (convergence) of surface water, In other regions it results in the removal (divergence) of surface water. As surface water is less dense than deeper water this has the effect of - heaping buoyant surface water in the convergence regions and - removing it from the divergence regions. The buoyant surface layer thickens in convergence, thins in divergences regions.

The thickening or thinning of the buoyant surface layer by the Ekman transport produces “hills” and “valleys” of the sea surface. The total relief of the ocean surface (not counting waves or tides) is only about 1-2 meters.



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Mar-05-07(11 of 15)

hill

valley

hill

low

low

The wind [through Ekman transport] redistributes the buoyant surface water inducing a relief to the sea surface [about 2 meters]

Westerlies

TradesET



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Mar-05-07(12 of 15)

How do hills and valley drive a circulation?

The pattern of the induced currents is governed not just by the wind stress and its Ekman transport but also by the earth's rotation, the Coriolis Force. The Coriolis Force acts at right angles (90°) to the ocean current (or wind) direction, to the right in the north hemisphere to the left in the southern hemisphere. The Coriolis Force strength is proportional to the strength of the ocean current.

In short: the wind produces convergences and divergences of surface water, which causes hills and valleys of sea level, which then produce a horizontal gradient of pressure. As the pressure gradients make the water move from high pressure to low pressure, the Coriolis Force starts its action, and eventually a balance is achieved in these two forces, the horizontal pressure gradient equals the magnitude of the Coriolis Force, but is directed in the opposite direction.

This balance is called the geostrophic balance, and a current in such a balance is called a geostrophic current. Ocean currents are very close to being in geostrophic balance


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Mar-05-07(13 of 15)

3-D Circulation-1Now we want to look at the 3-dimensional circulation of the ocean

Cold, salty sinking water

EvaporationIce

Expulsion of Salt

- Cooling of the ocean makes the ocean denser, removing buoyancy- Evaporation makes the ocean saltier and hence denser, again removing buoyancy- Heating and excess precipitation has the opposite effect, they add buoyancy to the ocean. - Surface water that is made denser, through cooling or increase of salinity, sinks to an equilibrium - depth.- To reach a steady state condition the loss of water from the surface layer must be replaced, the loop must be closed: the displaced water must find its way back to the sinking region. - This sets up the thermohaline circulation. It involves overturning of the full depth of the ocean, hence bringing deep water removed from direct contact with the atmosphere for about 1000 years back to sea surface.


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Mar-05-07(14 of 15)

3-D Circulation-2

Thermohaline Circulation


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Mar-05-07(15 of 15)

3-D Circulation-3

• Thermohaline circulation– Circulations generally referred to as “the conveyor belt”– These circulations are produced by changes in density of water due to changes in temperature

and salinity• As water in the north Atlantic cools and freezes, the salt is left behind in the water• Called the North Atlantic Deep Water• This water is very cold and very salty (less buoyant), i.e. very dense, and it begins to sink

– After the water sinks, it flows south to Antarctica and then circles to the east– Eventually it fills the bottom of the Indian and Pacific Oceans and upwells– It then begins flowing back to the Atlantic at the surface and the cycle is repeated

• Along the margins of Antarctica very dense water is formed. This water descends as thin sheets or plumes down the continental slope into the deep ocean forming Antarctic Bottom water that spreads along the sea floor over much of the world ocean.

– In general, it takes approximately 1000 years for water to complete the entire cycle– Because of this, thermohaline circulations are important for long-term climate (i.e. decades to

millenium)• Influences surface circulations• Influences chemical composition of the ocean and atmosphere (when water sinks it takes

whatever dissolved chemicals with it, most importantly CO2)

Documents

Outline Further Reading: Chapter 07 of the text book - vertical structure - surface circulation (wind driven circulation) - 3D circulation (thermohaline