The document discusses the mechanisms and importance of thermohaline circulation (THC) in ocean and climate dynamics, explaining how it is driven by density differences due to temperature and salinity. It highlights the potential impacts of global warming on THC, predicting significant weakening in deep water formation, which could disrupt marine ecosystems and climate patterns by the 2050s. Historical modes of THC are also described, along with its role in climate regulation over various timescales.

1. Thermohaline Circulation
and
Climate Change
Prof./Dr.Krishna AchutaRao
Science of Climate Change ­ by
ASL 715 Arulalan.T
2012 AST 2532
M.Tech, CAS,
IIT Delhi

2. Ocean Circulation
• 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

3. What drives the Ocean circulation?

4. Two kinds of circulation
The pycnocline separates the surface zone (mixed layer)
Surface ocean: Wind­driven from the deep ocean
circulation, at the mixed
area between the surface and
the thermocline (300­1000 m
depth).
Ocean currents also subject to
Coriolis force, which deflects
them 20­25 degrees from
wind direction (how about
deep ocean?)
Deep ocean: Thermohaline
circulation driven by Figure from: The Earth System. Kump,Casting and Crane, 2004
differences in density
(temperature and salinity).

5. T Vs S Vs Sigma­t
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.
Figure Plotted Using CDAT5.2

6. Thermohaline circulation
●
Seawater are made denser by
cooling and/or increasing
salinity.
• Deep water is formed in
localised areas.
Figure source : S. Rahmstorf, Thermohaline Ocean Circulation.
Encyclopedia of Quaternary Sciences, 2006

7. What drives the THC ?
• 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.

8. Key features of the THC
1.Deep water formation:
In a few localized areas,
North Atlantic: in the
Greenland­Iceland­
Norwegian (GIN) Seas,
the Labrador Sea
Soutern Ocean: in the
Weddell Sea,
the Ross Sea.
2.Spreading of deep
waters
Figure source : S. Rahmstorf, Thermohaline Ocean Circulation.
Encyclopedia of Quaternary Sciences, 2006
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 primarily
a wind­driven current, forming part of the subtropical gyre circulation.
(~20% of the Gulf stream,~20Sv contributed to south flow of NADW)

9. THC Currents
Figure source : Wiki commons

10. 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 waters
North Atlantic Deep Water (NADW),
Antarctic Bottom Water (AABW)
Figure source : S. Rahmstorf, Thermohaline Ocean Circulation.
Encyclopedia of Quaternary Sciences, 2006
Movement of water masses
is slowly – and it is unrealistic to measure directly. We
deduce it by measuring the age of the water (through carbon­14 dating) from
the distribution of the water properties themselves.

11. History of the THC
Three major circulation modes ­ indentified:
Modern mode, Glacial mode & Heinrich mode
Modern mode or warm mode similar to the
present­day 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

12. 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.

13. In 2050s
●
Using the climate model HadCM3 by
applying the GHG effect as same as Change in surface air temperature during
today, forecasted the THC shutdown years 20­30 after the collapse of the THC.
in the 2050s.
●
Cooling of the NH of −1.7◦ C
●
Most tend to affect
temperatures over land
in north­western Europe
(Scandinavia, Britain) by
several degrees, others show
strong cooling further west.
●
SH warming
Figure source : Vellinga and Wood, Impacts of thermohaline circulation
shutdown in the twenty­first century, 2008

14. MOC
●
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.

15. Transporting the heat to the poles
Northward heat transport across each latitude
(1PW=10^15W)
• Atmospheric and oceanic circulation
is about transporting heat from the
equator to the poles.
• The maximum energy
transport is similar.
Oceans peak at 20°N
• A direct effect:
The atmosphere is
heated from the bottom,
air column becomes
unstable and rises.
Figure source: The Earth System. Kump,Casting and Crane, 2004

16. Role of Oceans in Climate variations
●
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.

17. 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.

18. Thank you