- by Arulalan.T 2012 AST 2532 M.Tech, CAS, IIT Delhi Thermohaline Circulation and Climate Change Prof./Dr.Krishna AchutaRao Science of Climate Change ASL 715
by
Arulalan.T2012 AST 2532 M.Tech, CAS, IIT Delhi
Thermohaline Circulationand
Climate Change
Prof./Dr.Krishna AchutaRao Science of Climate Change
ASL 715
• What drives the Ocean circulation?• Two kinds of circulation (?) – Winddriven (surface) circulation. – Thermohaline circulation (deep), driven by fluxes of heat and freshwater across the sea Surface.● Role of Oceans in climate
Ocean Circulation
What drives the Ocean circulation?
Surface ocean: Winddrivencirculation, at the mixedarea between the surface andthe thermocline (3001000 mdepth).
Ocean currents also subject toCoriolis force, which deflectsthem 2025 degrees fromwind direction (how about deep ocean?)
Deep ocean: Thermohalinecirculation driven bydifferences in density(temperature and salinity).
Two kinds of circulation
Figure from: The Earth System. Kump,Casting and Crane, 2004
The pycnocline separates the surface zone (mixed layer) from the deep ocean
It shows the how the temperature and salinity takes place to make the denser water in the ocean depth, which is plotted by using the ARGO measured data.
The isolines replicate the sigmat (density1000) value.
T Vs S Vs Sigmat
Figure Plotted Using CDAT5.2
● Seawater are made denser by cooling and/or increasingsalinity.
• Deep water is formed in localised areas.
Thermohaline circulation
Figure source : S. Rahmstorf, Thermohaline Ocean Circulation. Encyclopedia of Quaternary Sciences, 2006
• The highlatitude cooling● filling box dynamics● Sandström’s theorem – heating at greater depth than
cooling, by turbulent mixing (~1000 years)● Mechanism to drive ocean circulation is through fluxes of
heat and freshwater ?
● Deep water is formed in localised areas.
● when the water column then becomes Unstable, leading to largescale deep overturning of the oceans.
● It is estimated time period 1000 5000 years.
What drives the THC ?
1.Deep water formation:In a few localized areas,
North Atlantic: in the GreenlandIcelandNorwegian (GIN) Seas,the Labrador Sea
Soutern Ocean: in the Weddell Sea, the Ross Sea.
2.Spreading of deep waters
3. Upwelling of deep waters not as localized as convection Sea and difficult to observe. Mainly in the Antarctic Circumpolar Current region (ACC).
4. Nearsurface currents: are required to close the flow. The Gulf Stream is primarilya winddriven current, forming part of the subtropical gyre circulation.(~20% of the Gulf stream,~20Sv contributed to south flow of NADW)
Key features of the THC
Figure source : S. Rahmstorf, Thermohaline Ocean Circulation.Encyclopedia of Quaternary Sciences, 2006
THC Currents
Figure source : Wiki commons
Water Masses basics:Most of the heat and salt exchangebetween the atmosphere and theoceans occurs in the upper 150m .Once a parcel of water is removed fromthe surface, its properties (T,S) will notchange until it rises again, many yearslater.
Spreading of deep watersNorth Atlantic Deep Water (NADW),Antarctic Bottom Water (AABW)
Movement of water massesis slowly – and it is unrealistic to measure directly. Wededuce it by measuring the age of the water (through carbon14 dating) from the distribution of the water properties themselves.
Spreading of deep waters
Figure source : S. Rahmstorf, Thermohaline Ocean Circulation. Encyclopedia of Quaternary Sciences, 2006
History of the THC Three major circulation modes indentified: Modern mode, Glacial mode & Heinrich mode
Modern mode or warm mode similar to thepresentday Atlantits. (Not shown)
The ‘off’ mode occurring after Heinrich events(after major input of freshwater, either from surging glacial ice sheets or in form of meltwater floods Younger Dryas event) (upper globe).
Glacial mode or cold mode with NADW forming south of Iceland in the Irminger Sea (center)
Transitions between warm and cold modes of the Atlantic THC DansgaardOeschger (D/O)(lower) dramatic warm 8 to 16oC within a decade over Greenland
Figure source : S. Rahmstorf, Thermohaline Ocean Circulation. Encyclopedia of Quaternary Sciences, 2006
The future of the THC
Global warming can affect the THC in two ways: surface warming and surface freshening, both reducing the density of highlatitude surface waters & deep water formation.
Most models predict a significant weakening of NADW formation (by 2050%) in response to anthropogenic global warming during the 21st century [IPCC, 2001]. Some also find a reduction in AABW formation.
A major weakening or shutdown of NADW formation could have serious impacts on marine ecosystems, sea level and surface climate, including a shift in ITCZ & tropical rainfall belts.
● Using the climate model HadCM3 by applying the GHG effect as same as today, forecasted the THC shutdown in the 2050s.
● Cooling of the NH of −1.7 C◦
● Most tend to affecttemperatures over landin northwestern Europe(Scandinavia, Britain) byseveral degrees, others showstrong cooling further west.
● SH warming
In 2050s
Figure source : Vellinga and Wood, Impacts of thermohaline circulation shutdown in the twentyfirst century, 2008
Change in surface air temperature during years 2030 after the collapse of the THC.
● The meridional overturning circulation (MOC) refers to the northsouth flow as a function of latitude and depth.
● MOC describing a meridional flow field, a mix of both winddriven and thermohalineflow, often integrated in eastwest direction across an ocean basin or the globe and graphically depicted as a stream function.
● THC associated with zonal overturning cells and it describing the influence of cooling or freshwater forcing on the ocean circulation.
MOC
• Atmospheric and oceanic circulationis about transporting heat from theequator to the poles.
• The maximum energy transport is similar.Oceans peak at 20°N
• A direct effect: The atmosphere isheated from the bottom,air column becomesunstable and rises.
Transporting the heat to the poles
Figure source: The Earth System. Kump,Casting and Crane, 2004
Northward heat transport across each latitude(1PW=10^15W)
● On timescale 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 timescale it is the large heat transport (~1PW) of the deep circulation that could change the climate.
● Oceans can absorb heat in one region and restore it to the atmosphere (perhaps decades or centuries later) at a quite different place.
● A THC collapse is seen to have happen in the past. It is widely discussed as one of a number of "low probability high impact" risks associated with global warming.
Role of Oceans in Climate variations
References
[1] Wallace S. Broecker, James P. Kennett, Benjamin P. Flower, James T. Teller, Sue Trumbore, Georges Bonani, and Willy Wolfli. Routing of meltwater from the laurentide ice sheet during the younger dryas cold episode. Nature, 341(6240):318–321, September 1989.
[2] Peter U. Clark, Nicklas G. Pisias, Thomas F. Stocker, and Andrew J. Weaver. The role of the thermohaline circulation in abrupt climate change. Nature, 415(6874):863–869, February 2002.
[3] J. F. McManus, R. Francois, J. M. Gherardi, L. D. Keigwin, and S. BrownLeger. Collapse and rapid resumption of atlantic meridional circulation linked to deglacial climate changes. Nature, 428(6985):834–837, April 2004.
[4] Michael Vellinga and RichardA Wood. Impacts of thermohaline circulation shutdown in the twentyfirst century. Climatic Change , 91(12):43–63+, 2008.
[5] S. Rahmstorf, S. A. Elias. Elsevier and Amsterdam. Thermohaline Ocean Circulation. Encyclopedia of Quaternary Sciences, 2006.
Thank you