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Environmental Change 1 - Lecture 6

Ocean Circulation

Ron Kahana

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Ocean Circulation

• What drives the Ocean circulation?

• Two kinds of circulation (?)

– Wind-driven (surface) circulation.

– Thermohaline circulation (deep), driven by fluxes of heat and freshwater across the sea surface.

• Role of Oceans in climate

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Atmosphere and Ocean circulations are all about the redistribution of heat on a rotating planet

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Transporting the heat to the polesNorthward heat transport across each latitude

(1PW=1015W)

Figure from: The Earth System. Kump,Casting and Crane, 2004

• A direct effect: The atmosphere is heated from the bottom, air column becomes unstable and rises. • However, the oceans are heated from the top therefore become more stable.

• Atmospheric and oceanic circulation is about transporting heat from the equator to the poles.

• The maximum energy transport is similar. Oceans peak at 200N

So how the energy received from the sun keeps the ocean circulation going?

the Atmosphere holds the key for that.

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What drives the Ocean circulation?R

adia

tion

fro

m t

he

Sun

Wind stress

Heating and cooling

Evaporation and precipitation

momentum

Salinity

density

Wind driven (Surface) circulation

Thermohaline (deep)

circulation

Temp.

Atmosphere Oceans

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Two kinds of circulation

Surface ocean: Wind-driven circulation, at the mixed area between the surface and the thermocline (300-1000 m depth).

Ocean currents also subject to Coriolis force, which deflects them 20-25 degrees from wind direction

Deep ocean: Thermohaline circulation driven by differences in density(temperature and salinity).

The pycnocline separates the surface zone (aka mixed layer) from the deep ocean

Figure from: The Earth System. Kump,Casting and Crane, 2004

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Subtropical Gyre

Equatorial currents

Antarctic Circumpolar

Current (ACC)

The Wind Driven Ocean Circulation

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Ekman Transport – How winds effect currents?

• Considered the effect of wind and friction on multiple levels in the ocean.

• Coriolis effects all layers because they’re moving

• current velocity decreased exponentially with depth

• Surface current moved at 450 to the wind.

• Deviation is increased with depth (Ekman Spiral)

• The mean current moves at right angle to the wind direction (to the right in the NH, and to the left in the SH)

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How to build a subtropical gyre1. Start from the winds:prevailing winds move surface waters toward subtropical regions both from tropical areas and from high-latitudes.

2. Add continent boundariesthe subtropical gyres are zones of convergence where surface water piles up and forming slopes.

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Gyre Dynamics (how to build II)

3. get geostrophic:Water responds to the slopes of the ocean surface as it would on land -- it runs downhill, but because of earth's rotation it is deflect and flows parallel to the slope.

•The balance between gravitational forces and the Coriolis effect, gives rise to geostrophic currents. Their velocity is proportional to the pressure

4. Consider changes in planetary vorticity to get an asymmetric gyre.

(Find more about that in Ch. 5 of ‘The Earth System’, or in section 4.2 of the Open University ‘Ocean Circulation’).

gradient.

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The asymmetrical subtropical gyres are the main mechanism that carry heat poleward and cold water towards the equator.

The subtropical gyres

western boundary currents are narrow, fast and deepGulf Stream (western North Atlantic): 1.5-2.5 m/s(5-9 km/hr)Kuroshio Current (western North Pacific)

eastern boundary currents are wide, shallow, and sluggish.Canary Current (eastern NorthAtlantic), California Current(eastern North Pacific):0.03-0.07 m/s (< 2 km/hr)

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Equatorial Currents•Best developed in the Pacific

•Consists of:• currents flowing east-to-west (e.g. north equatorial current, south equatorial current), directly driven by the prevailing Trade winds.

•The equatorial countercurrentflows down the slope (caused by the trade winds) towards the east, in zone of small westward wind stress (the Doldrums).

Costal upwelling

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The Antarctic Circumpolar Current (ACC)

Figure from: The Earth System. Kump,Casting and Crane, 2004

Drake Passage

• The only current that flows around the globe (~24,000 km).• “mighty Current” – transports more water than any other current, est. at 100-150 Sverdrups (1 Sv=106

m3 s-1).• extend from the sea surface to depths of 2000-4000m deep and can be as wide as 2000 km.• typical velocity ~0.1m/s (Much slower than the GulfStream ~1-2.5 m/s). •The ACC is in approximately geostrophicequilibrium (PGF=CF)

Try to figure out:If the ACC is in geostrophic balance and flows from west to east.

• In which direction the Coriolis force is acting?

• Could you say will the sea surface be higher to the north or south of the ACC? Would you expect the prevailing winds over the Southern Ocean to be westerly (flowing at the same directionas the ACC) or easterly? (see slide #4 in lec. 3)

For more information about the ACC (and other currents) see the ’Ocean Surface Currents’ page from the University of Miami at: http://oceancurrents.rsmas.miami.edu/index.html.

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Thermohaline circulation

Another mechanism to drive ocean circulation is through fluxes of heat and freshwater

• Seawater are made denser by cooling and/or increasing salinity.

•Deep water is formed in localised areas.when the water column then becomes unstable, leading to large-scale deep overturning of the oceans.

It is estimated that the turnover time required to replace the deep water through deep water formation is on the order of 1000 -5000 years.

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Key features of the Thermohaline circulation

1) Deep water formation:in a few localized areas:

North Atlantic: in the Greenland-Iceland-Norwegian (GIN) Seas, the Labrador Sea

Soutern Ocean: in the Weddell Sea, the Ross Sea.

The MediterraneanSea.

3) Upwelling of deep waters not as localized as convection and difficult to observe (ACC?).

4) Near-surface currents: these are required to close the flow( ~20% of the Gulf stream, ~20Sv).

2) Spreading of deep waters

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Spreading of deep waters

Water Masses basics:Most of the heat and salt exchange between the atmosphere and the oceans occurs in the upper 150m . Once a parcel of water is removed from the surface, its properties (T,S) will not change until it rises again, many years later.

Spreading of deep watersNorth Atlantic Deep Water (NADW), Antarctic Bottom Water (AABW)

Movement of water masses is slowly – and it is unrealistic to measure directly. We deduce it by measuring the age of the water (through carbon-14 dating, for example). or from the distribution of the water properties themselves.

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Role of Oceans in Climate variations

On time-scale of months or years Oceans are vast reservoir of heat and will regulate the climate by heating or cooling the atmosphere (hurricanes / El Niño).

On longer time-scale it is the large heat transport (1PW) of the deep circulation that could change the climate.

3-D Schematic of Thermohaline Circulation by William Schmitz

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The THC and Climate changeChange in surface air temperature during years 20-30 after the collapse of the THC. (from Vellinga and Wood, 2002).One way to estimate the

effect of the THC is to switch it off in coupled climate models (by adding a lot of freshwater to the northern Atlantic), and compare the surface climate before and after switching it off.

-60C

Unfortunately, the magnitude and location of the cooling is model dependent.

most tend to affect temperatures over land in north-western Europe (Scandinavia, Britain) by several degrees, others show strong cooling further west [This figure].

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But have the THC ever been shut?

Both Palaeo-data and model simulations suggest that the THC have been weakened or shut during the last glacial.

Schematic of three circulation modes of the glacial Atlantic: the prevailing cold mode (center), the warm mode associated with Dansgaard-Oeschger events (lower) and the ‘off’ mode occurring after Heinrich events (upper globe). From: S. Rahmstorf: Thermohaline Ocean Circulation. In: Encyclopedia of QuaternarySciences, Edited by S. A. Elias. Elsevier, Amsterdam 2006.

•Would future global warming effect the THC?

•Surface warming and surface freshening both reduce density of high latitudes.

•Most models predict a significant weakening of the NADW These are the sort of model experiments that we perform in

The role of weakening the Gulf Stream is debateable, see: realclimate.orghttp://www.realclimate.org/index.php/archives/2005/05/gulf-stream-slowdown/

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Summary• The circulations of the oceans and atmosphere are driven by energy from the sun and modified by the earth rotation. Both help to reduce equator-to-pole temperature gradients.

• Oceans and atmosphere are a single system of two interacting components, coupled by the air-sea interactions. Oceans can absorb heat in one region and restore it to the atmosphere (perhaps decades or centuries later) at a quite different place.

• Thermohaline and wind-driven currents cannot be separated by oceanographic measurements.

• two distinct physical forcing • very different time scales • but not two uniquely separable circulations.

• A THC collapse is seen to have happen in the past. It is discussed as one of "low probability - high impact“ risks associated with global warming.

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Reading ListMost of the material covered here is in:Kump, L.R., Kasting, J.F and Robert, G. Crane: The Earth System (Ch. 5)

Other sources:Open University: Ocean Circulation

Tomczak, M., Godfrey, S.J: Regional Oceanography (Ch. 1-5). This is an online book that can be downloaded from: http://www.es.flinders.edu.au/~mattom/regoc/pdfversion.html

Bigg, GR.: The Oceans and Climate

Dennis Hartmann: Global Physical Climatology

Good online sources for the thermohaline circulation:Rahmstorf, S: A brief fact sheethttp://www.pik-potsdam.de/~stefan/thc_fact_sheet.html

Rahmstorf, S: Thermohaline Ocean Circulation. In: Encyclopedia of Quaternary Sciences,http://www.pik-potsdam.de/~stefan/Publications/Book_chapters/Rahmstorf_EQS_2006.pdf

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Extra Slides

• Why there’s no deep water formation in the Pacific.

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Inferring the age of deep water massesAge of bottom water can be inferred by the rate of decay of carbon-14 to 12C.

youngerolder

~300 yrs ~1000 yrs

Oxygen at 4000 m gets depleted because of biological consumption

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70C 120C

Source: Most Recent Sea Surface Temperature Field(Polar Orbiting Satellite SST Experimental Products)http://coralreefwatch.noaa.gov/satellite/current/key_sst_50km_field.html

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Characteristics of Ocean Gyres

• Downwelling in the center of the gyre (low productivity.

• Bringing warm tropical waters north. Transphers heat.

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Fact Sheet

Typical salinity: 34.7 Practical Salinity Units (PSU)Density of surface seawater: kg m3

Water density anomaly: freshwater max. density is at 40C. lakes and rivers will freeze from top to bottom. For salty water, the density increases right down to the freezing point (~ -1.80C) so the whole surface layer of the ocean (down to 100-150m) has to be cooled to the freezing point before freezing can begin at the surface.