Ocean Circulation A climate regulator ?

1. Ocean CirculationA climate regulator ? Equator-to–Pole heat flux ~ 2,000 TW (25-50% of total) Response time 100-1,500 years

2. Great Ocean Conveyer Belt • In the tropics heat is absorbed in the surface layers of the ocean. • As the surface water moves poleward it loses heat to the (cooler) atmosphere. • The result is a net poleward transport of heat of 2,000 TW (2x1015 W).

3. Elements of the Ocean Circulation Wind driven surface currents/ western boundary currents. eg. Gulf Stream Intense cooling - ice formation - deep water production. Deep water creeps south at velocities too slow to measure. BUT WHAT DRIVES THE CIRCULATION ?

4. Vertical Circulation in the Ocean • Why doesn’t the ocean fill up with the cold water from convection ? • Either a heat source at the sea bed or heat is mixed down. • Geothermal heating is relatively insignificant. • Mixing is the key ! Ice formation - deep water formation Atlantic Ocean Section – A16 65 deg. S Equator 65 deg N

5. Sandstrom’s theorem • Atmospheric heat engine - the energy source to drive the circulation comes from heating at ground level. – strength of circulation is friction controlled. • Sandstrom’s theorem: A closed steady circulation can be maintained in the ocean only if the heating is situated at a lower level than the cooling source. • BUT the Ocean is predominantly forced by heating/cooling at the surface. • Thus no thermally driven circulation is possible (in considering horizontal gradients, any circulation would be weak and restricted to the surface few 100ms). • Thus the vertical ocean circulation is dependent on a mechanical energy input. • This mechanical energy mixes the water column, resulting in an increase in water column potential energy. • But limits the strength of this circulation ?

6. Vertical Mixing Winter Convection Vertical Circulation Uniform upwelling/ mixing/ diffusion across the entire abyssal ocean. Sinking due to convection in polar regions.

7. Vertical Circulation • Munk (1966) estimated from radiocarbon and density profiles collected in the Central Pacific; • w = 0.7x10-7 ms-1 or Kz ~ 1 cm2s-1 • Is necessary to balance the 25 Sv of deep water production estimated to be taking place in the polar oceans. (1 Sv = 106 m3s-1)

8. Vertical Circulation & Climate • A ocean wide value of Kz~1 cms-1 would require a mechanical energy input of ~ 2.1 TW. • Where does that energy come from ? • Is the energy source part of a climatic feedback back loop ? • Is the strength of the circulation (and hence poleward heat flux) limited by the availability of energy to perform the mixing, or by the rate at which sea ice is formed in the polar oceans ?

9. Vertical Circulation & Climate • Lecture 6: Deep water formation. • Lecture 7: Vertical Mixing in the Ocean. References: R.X. Huang (1999). Mixing and Energetics of the Oceanic Thermohaline Circulation. Jour. Phys. Oceamogr., 29, 727-746. J. Marotzke & J.R. Scott (1999). Convective Mixing and the Thermohaline Circulation. Jour. Phys. Oceanogr., 29, 2962-2969. W. Munk & C. Wuench (1998). Abyssal recipes II: energetics and wind mixing. Deep Sea Research, 45, 1977-2010. C. Wuench (2002). What is the Thermohaline Circulation. Science, 298, 1179-1180

10. Vertical Mixing • Molecular diffusion – a very gradual process (Kz ~ 10-2 cm2s-1). ie. too small to account for oceanic changes ! • Turbulence increases area over which diffusion takes place, thus effectively increases the rate of molecular diffusivity. • Turbulent Mixing. • Mixing efficiency, Rf – links the large scale rate of increase in water column potential energy (break down of stratification) with the rate at which turbulent kinetic energy is made available by large scale energy sources, P.