1. Thermohaline Circulation
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Thermohaline Circulation
SOEE3410: Lecture 12

2. Thermohaline Circulation
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Thermohaline Circulation
Global heat redistribution - oceans’ role
Surface currents
Density-driven deep currents
Thermohaline ‘conveyor belt’
North Atlantic Deep Water (NADW)
Climate implications
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

3. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Revision 60 30 90
Ferrel Cell
Polar Cell
Net Radiation
Heat Transport
Idealized model of atmospheric circulation. N.B. actual circulations are not continuous in space or time.
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

4. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Heat budget equation
Heat fluxes into a region of ocean:
Incoming
Short wave radiation
Qsw
Net
Long-wave radiation
Qlw
Turbulent Fluxes
Atmosphere
-ve
Net latent heat
Qlat
Net sensible heat
Qsen
+ve
-ve
-ve
Air-sea
interface
+ve
+ve
+ve
-ve
Ocean
Qad
Advection
Qnet = Qsw+Qlw+Qlat+Qsen+Qad
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

5. Radiation terms: short-wave & long-wave
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Radiation terms: short-wave & long-wave
Qsw
Qlw
ECMWF 40-year reanalysis.
Units are W/m2.
+ve is into the surface.
From Kallberg et al 2005.
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

6. Latent & sensible heat terms
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Latent & sensible heat terms
Qlat
Qsen
ECMWF 40-year reanalysis.
Units are W/m2.
From Kallberg et al 2005.
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

7. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Net heating at surface
Qsw +Qlw + Qlat + Qsen
ECMWF 40-year reanalysis. Units are W/m2. From Kallberg et al 2005.
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

8. Meridional Heat Transport
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Meridional Heat Transport
Heat transport north
Units of PW = 1015 W
Heat transport south
Northward heat transport for 1988 in each ocean and the total transport summed over all oceans calculated
by the residual method using atmospheric heat transport from ECMWF and top of the atmosphere heat fluxes from the Earth Radiation Budget Experiment satellite. From Houghton et al., (1996: 212), using data from Trenberth and Solomon (1994).
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

9. Global surface current system
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Global surface current system
From: Ocean Circulation - 2nd ed., The Open University
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

10. Deep ocean circulation
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Deep ocean circulation
Deep ocean i.e. the other 80 % of the ocean is driven by:
Poleward transport of heat
Air-sea interaction at the poles
Results in the formation of water masses
i.e. waters with specific core temperature and salinity signatures
Spatial differences in heat/salt (density) drive a much slower process
than the surface wind-driven circulation called:
Thermohaline Circulation
Thermo haline
(heat) (salinity)
Density
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

11. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Annual mean SST
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

12. Annual mean global salinities
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Annual mean global salinities
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

13. Global thermohaline circulation - theory development
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Global thermohaline circulation - theory development
Gulf Stream region - well studied back to 19th century
Deep measurements of T and S -> geostrophic flow calculations
sea-surface
east
Gulf Stream - N Atlantic
i.e. N hemisphere
=> Coriolis force acts
to right of flow
west
Fpg Fc
Calculations: northward (NE) surface flow - Gulf Stream
Also evidence of deep counter-currents i.e. southward (SW) flow
Stommel (1965) developed theory of global thermohaline circulation
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

14. Thermohaline circulation (Meridional overturning)
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Thermohaline circulation (Meridional overturning)
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

15. Thermohaline circulation - North Atlantic
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Thermohaline circulation - North Atlantic
Thermohaline forcings in North Atlantic:
Thermal forcing
High-latitude cooling; low-latitude heating
=> northward surface flow
2. Haline forcing
Net high-latitude freshwater gain at surface from melting ice in summer and precipitation and continental run-off;
low-latitude evaporation increases surface salinity
=> southward surface flow
High latitude increase in density (brine rejection from freezing ice + cooling) causes sinking
=> southward deepwater flow
At present, thermal forcing dominates North Atlantic,
=>flow of upper current is northward
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

16. Circulation in the Nordic Seas
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Circulation in the Nordic Seas
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

17. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
NADW - deep convection
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

18. North Atlantic - Air-Sea interaction
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
North Atlantic - Air-Sea interaction
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

19. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Deep convection
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

20. Mean vertical structure
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Mean vertical structure
At low and mid-latitudes, temperature primarily determines density.
At high latitudes, T < 5oC - salinity is more important.
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

21. ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

22. Summary - thermohaline circulation
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Summary - thermohaline circulation
Generally, at high latitudes, oceans:
- lose heat
- gain fresh water (precipitation and continental runoff),
which have opposite effects on the density of ocean water
Density of high latitude water is influenced by warm, salty water from the low latitudes.
This constitutes the positive feedback maintaining the Atlantic Thermohaline circulation
This intricate balance is influenced by:
- surface heat fluxes, and
- fresh water mass
i.e. precipitation, evaporation, continental runoff, sea-ice formation
Are these processes likely to change in the future.
What are the climate implications?
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

23. Modelling - capturing global heat transport
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Modelling - capturing global heat transport
World Ocean Circulation Experiment (WOCE) – 1990s:
establish role of oceans in global climate system
obtain suitable basline dataset for assessing future climate
develop means to predict climate change i.e. models
From “Ocean Circulation”, OU 2002
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

24. Modelling - capturing global heat transport
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Modelling - capturing global heat transport
Model: net heat transport from Atlantic Ocean into Indian Ocean
Theory: net heat transport into Atlantic Ocean from Indian Ocean
Why the difference?
From “Ocean Circulation”, OU 2002
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

25. Climate implications: Oceans heating up
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Climate implications: Oceans heating up
In the last half of the 20th century, it is clear that the world oceans are heating up.
The oceans have absorbed about 30 times more heat than the atmosphere since 1955
Oceans x 1022 J
Atmosphere x 1021 J
Levitus et al. Science, 2000
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

26. Climate implications: not just about temperature
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Climate implications: not just about temperature
It is expected that the planetary water cycle – Evaporation/Condensation/Precipitation and Freezing/Melting – will be altered as a result of global warming.
There is ample evidence of it already happening…
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

27. Climate implications: salinity levels
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Climate implications: salinity levels
Salinity distributions have been changing in the last few decades
Low latitude surface waters have become markedly more saline
Water masses formed at high latitudes have become fresher
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

28. Climate implications: precipitation
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Climate implications: precipitation
Increased precipitation is perhaps the dominant factor:
- elevating continental run-off into the Arctic basins,
- contributing to salinity decreases in N Atlantic and N Pacific
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

29. Summary: Climate implications
ENVI3410 : Coupled Ocean & Atmosphere Climate Dynamics
Summary: Climate implications
Climate change:
- surface air temperatures
- SST
- surface salinities
- deep water formation
- thermohaline circulation
- feedback on climate
SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics