Thermohaline Circulation

Lecture Outline

1) What is thermohaline circulation2) History of understanding 3) Key water masses4) Formation of deep water5) Theory of deep circulation6) Importance of deep circulation to

climate

Chapter 9 – KnaussChapter 14 – Talley et al.

What is thermohaline circulation?

Rennell 1832

Wind-driven surface circulation – confined to upper ~1000mThermohaline circulation in deep ocean

Ekman Layer

“Upper Waters” primarily wind-driven

“Deep Waters” primarily density-driven

Driven by variations in temperature and salinity

History of thermohaline circulation

von Lenz 1845 – observed that thermocline at equator was shallower – suggested vertical motions as cause

Schott 1902 – Valdivia Expedition

Non-symmetric cells

Brennecke 1909 - 3D circulation, return surface flow, across equatorial flow

Merz 1925 – salinity contours with associated velocity vectors – different types of water

Wust 1935 – Meteor Expedition – geostropic currents and salinityDense water at poles sinks – intermediate depths and bottom Moves through basin and extends across equator

Stommel 1957 – deep flow is in narrow currents along ocean margin – deep western boundary currents

The deep circulation is driven by density differences and is often referred to as thermohaline circulation.

The term thermohaline circulation has been replaced by meridional overturning circulation. Also called abyssal circulation.

Gordon 1986 – expand to world ocean

Broecker 1987 – world ocean – cold deep flow and warmsurface return flow

Gordon 1991 – different types of water in different ocean basins Source of deep water in Southern Ocean

Dense water that drives thermohaline circulation is only formed in a few key areas.

Lumpkin 2009 – consider mesoscale circulation contribution

What forces overturning circulation?

• Form dense water at high latitudes• Remove heat – cools water making it more

dense• Increase salt content through ice formation

making water more dense• Increase salt via evaporation – export of water

vapor • Dense water sinks and spreads along surfaces

of constant density

Observations of deep circulation are difficult and limited.

Much of deep circulation understanding comes from water mass analysis (temperature and salinity distribution).

Definitions

• Source/formation regions - area of ocean where water mass acquires its characteristic properties

• Conservation property – water mass characteristic which has no sources or sinks in the ocean interior

• Apply only below ocean surface mixed layer

Away from the surface temperature and salinity are conservative properties

Definitions• Water type – body of water with a common

formation history, having its origin in a particular region, occupy finite volume – point in T,S space – may not exist

• Source water type – water type that corresponds closely to properties of water mass in source region – variable and gives standard deviation of water mass

• Water mass – combination of source water types and associated standard deviations

Temperature-Salinity (T-S) diagrams

Major Water Masses

Water mass formation• Convection – density of water is increased by cooling

or evaporation so it sinks – Deep and Bottom Waters

• Subduction – water is pushed downwards along an isopycnal – Mode and Intermediate Waters

• Subsurface mixing - does not rely on air/sea exchange, two or more water masses are mixed at depth to form a new water mass – Circumpolar Deep Water

• Import from outside – inputs from marginal seas - Mediterranean Sea Water

Water masses are formed in source areas

No deep or bottom water formed in Pacific.

North Atlantic Deep Water (NADW) Formation

Formed by surface cooling in Greenland and Norwegian Seas. Water sinks and accumulates north of Iceland and spills over sills Resulting water is ~ 2-4°C and 34.9 to 35 psu.

Antarctic Bottom Water (AABW) Formation

Cold wind blows ice offshore (polynya) allowing ice to continually form. During freezing, salts are left behind (brine formation) resulting in water that is more saline. Cold dense water collects on the Antarctic shelf and spills over into deep ocean. Resulting water is ~ -0.4 to -1°C and 34.6 to 34.8 psu. There is less entrainment than with NADW so AABW is densest water in ocean.

Mediterranean Intermediate Water (MIW) Formation

MIW is warm (T~5-10°C) and salty (S~35.5-35.9 psu). Forms by evaporation but is not dense enough (too warm) to reach densities greater than NADW.

Average Salinity at 1000 m

Mediterranean Intermediate Water

Idealized representation of deep circulation

Evolution and Decay• Consolidation – mixing within water mass – reduces

standard deviation without changing water mass

• Mixing – between water masses to get new combinations not found in water masses - identify contributing water masses and determine relative amount

• Absorption – water mass disappears by mixing with another water mass – Mediterranean water mixing with NADW

• Transformation – change to another water mass by subsurface mixing – Circumpolar Deep Water (CDW)

Theory for Deep CirculationSource regionSource region

1) The supply of cold dense water at the poles by itself does not drive the deep circulation. Continuity requires that sinking near the poles to be balanced by rising water.

2) The permanent thermocline depth is relatively constant. There is a net input of heat near the equator, so the thermocline can only remain constant if there is a source of cold water from below (entrainment and mixing).

3) Turbulent mixing and not density differences drives the deep circulation. This requires overturning (mixing) hence meridional overturning circulation.

Stommel’s Theory for Deep Circulation

Similar to surface gyres, more intense circulation is predicted on western side of basin.

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In northern hemisphere moving toward equator increases positive vorticity.

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… friction can only balance planetary vorticity of an equatorward flow along the western boundary.

Source Region

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In southern hemisphere moving toward equator increases negative vorticity.

Role of Thermohaline Circulation in Climate

Upper limb of conveyor is warm ~ 10°C, while NADW is cold ~ 3°C.Each cm3 of upper-limb water releases 7 calories of heat when converted to NADW. With an estimated flux of 20 Sv, this totals 4×1021 calories each year, which is 35% of the heat received from the Sun by the Atlantic north 40° latitude. Estimates are that without the conveyor circulation, surface water in North Atlantic would be 5° colder.

No deep water is formed in the North Pacific because the water is too fresh. Even when cooled to the point of freezing, it does not reach a

density to sink all the way to the bottom because it forms at lower latitudes.

NADW formation from mixing of cold, fresh Arctic water with warm, salty North Atlantic water.

NADW formation is sensitive to salinity.

Freshening of Arctic waters will weaken or shut down the overturning circulation.

Feedback to climate – Strong overturning circulation leads to warmer Arctic which melts back the polar ice. This dilutes the water in the North Atlantic preventing formation of NADW and shutting down the conveyor circulation. Without redistribution of heat by the overturning circulation,

polar regions get colder, ice grows, water becomes more salty, which allows NADW to begin forming again.

Next Class

• Oceanic water mass distribution

• Water mass mixing, analysis and T-S diagrams