Thermohaline Thermohaline CirculationCirculation

(THC)(THC)By Ross AlterBy Ross Alter

What is it?What is it? “…that part of the ocean circulation which is

driven by fluxes of heat and freshwater across the sea surface and subsequent interior mixing of heat and salt” (Rahmstorf, 2006)

OneOne driving mechanism driving mechanism of large-scale “deep” of large-scale “deep” ocean circulationocean circulation

Temperature + salinity Temperature + salinity densitydensity ““Thermo-” + “-haline”Thermo-” + “-haline”

Requires Requires turbulent mixingturbulent mixing to complete the to complete the circulationcirculation

PhysicalPhysical concept, not observational concept, not observational

Source: Rahmstorf (2006)

Figure 2: Idealized meridional section representing a zonally averaged picture of the Atlantic Ocean. Straight arrows sketch the MOC. The color shading depicts a zonally averaged density profile derived from observational data [Levitus, 1982].

Source: Kuhlbrodt et al. (2007)

Why is it important?Why is it important? Maintains ocean Maintains ocean stratificationstratification ComparableComparable transport transport to that of the to that of the

surfacesurface Global volume transport ~ 30 SvGlobal volume transport ~ 30 Sv

Climate modulatorClimate modulator Cold water absorbs COCold water absorbs CO22 more efficiently more efficiently THC modulates THC modulates heat transportheat transport

~ 1 PW (10~ 1 PW (101515 W) in N. Atlantic W) in N. Atlantic

Speculation about the Speculation about the future…future…

Global warmingGlobal warming – warming and – warming and fresheningfreshening

Will the THC Will the THC break downbreak down?? Low probabilityLow probability, high impact, high impact Most likely scenario – Most likely scenario – weakeningweakening of of

THCTHC Would take Would take decades todecades to centuriescenturies to to

changechange No “Day After Tomorrow” scenario…No “Day After Tomorrow” scenario…

ReferencesReferences Stewart, R. H., 2002. Stewart, R. H., 2002. Introduction to Physical Introduction to Physical

OceanographyOceanography. . Kuhlbrodt, T., et al., 2007. On the driving Kuhlbrodt, T., et al., 2007. On the driving

processes of the Atlantic meridional processes of the Atlantic meridional overturning circulation, overturning circulation, Rev. Geophys.Rev. Geophys., 45, , 45, RG2001, doi:10.1029/2004RG000166 RG2001, doi:10.1029/2004RG000166

Rahmstorf, S. Thermohaline Ocean Circulation. In: Encyclopedia of Quaternary Sciences, Edited by S. A. Elias. Elsevier, Amsterdam 2006.