Today I have presented "The Thermohaline Circulation and Climate Change" as Mini-Project for our Science of Climate Change Course ! We can expect THC shutdown around 2050s... OMG ! Yes, we can expect "The Day After Tomorrow" around 2100... All the images credited to the reference papers except one T-S-Sigmat created by me using CDAT5.2.
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.
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
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.