Thermohaline Circulation

1. Thermohaline Circulation Lecture 16 OEAS-604 November 16, 2011 • Outline: • Overview of deep circulation • Key Water Masses • Formation of Deep Water • Theory of Deep Circulation • Importance of Deep Circulation to Climate

2. So far we have focused on only the surface currents, which are largely driven by wind.

3. Ekman Layer “Upper Waters” primarily wind-driven “Deep Waters” primarily density-driven

4. While the surface circulation is driven by wind, the deep circulation is driven by density differences and is often referred to as thermohaline circulation. The term thermohaline circulation has been dropped in favor of meridional overturning circulation. Also called abyssal circulation.

5. Dense water that drives thermohaline circulation is only formed in a few key areas.

6. Observations of deep circulation are very difficult and as a result, we have much fewer direct observations of deep circulation. Much of our understanding of deep circulation comes from identifying water masses (temperature and salinity distribution).

8. T-S diagrams

9. Away from the surface temperature and salinity are conservative properties.

10. Water masses are formed in source areas. No deep or bottom water in Pacific.

11. Idealized representation of abyssal circulation.

12. Major Water Masses

13. Formation of North Atlantic Deep Water (NADW) Formed by surface cooling in Greenland and Norwegian Seas. Water sinks and accumulates north of Iceland. Pulse of NADW intermittently spill over sills, cascading into the Atlantic basin. During this processes it entrains significant amounts of overlying water. Resulting water is ~ 2-4°C and 34.9 to 35 psu.

15. Formation of Antarctic Bottom Water (AABW) Cold wind blows ice offshore (polyna) allowing ice to continually form. During freezing, salts are left behind (brine formation) resulting in water that is more saline. This cold dense water collects on the Antarctic shelf and spills over into deep ocean. Resulting water is ~ -0.4-1°C and 34.6 to 34.8 psu. There is less entrainment than with NADW so AABW is densest water in ocean.

16. Formation of Mediterranean Intermediate Water (MIW) MIW is warm (T~5-10°C) and very salty (S~35.5-35.9 psu). It forms by evaporation but is not dense enough (too warm) to reach densities greater than NADW.

17. Average Salinity at a Depth of 1000 m. Mediterranean Intermediate Water

18. Theory for Deep Circulation Source region Source region • The supply of cold dense water at the poles by itself does not drive the deep circulation. Continuity requires that sinking near the poles must be balanced by rising water. • It is also commonly observed that the permanent thermocline depth is relatively constant. With the net input of heat near the equator, thermocline can only remain constant if there is a source of cold water from below (entrainment and mixing). • Thus, it is often said that turbulent mixing and not density differences drives the deep circulation. This requires overturning (mixing) hence meridional overturning circulation.

19. Stommel’s Theory for Deep Circulation Just like the surface gyres, more intense circulation is predicted on western side of basin. This is consistent with vorticity arguments from previous lecture.

20. + - Constant In northern hemisphere moving toward equator increases positive vorticity. In southern hemisphere moving toward equator increases negative vorticity. Source Region negative positive Eastern Boundary Eastern Boundary positive negative Western Boundary Western Boundary Source Region … friction can only balance planetary vorticity of an equatorward flow along the western boundary.

21. Role of Thermohaline Circulation in Climate Upper limb of conveyor is warm ~ 10°C, while NADW is cold ~ 3°C. Each cm3 of upper-limb water releases 7 calories of heat when converted to NADW. Given the estimated flux of 20 Sv, this totals 4×1021 calories each year. This is 35% of the heat received from the Sun by the Atlantic north 40° latitude. It is estimated that without the conveyor circulation, surface water in North Atlantic would be 5 degrees colder.

22. No deep water is formed in the North Pacific because the water is too fresh. Even when cooled to the point of freezing, it does not reach a density to sink all the way to the bottom.

23. Strong thermohaline circulation mixes with the relatively fresh Arctic water, keeping the salinity relatively high. This allows for the formation of NADW. However, formation of NADW is very sensitive to salinity. If waters in the Artic get fresher it is possible, that this could weaken or shut down the conveyor circulation. It has been suggested that there may be a feedback between the conveyor circulation and climate. Strong conveyor circulation leads to warmer arctic which melts back the polar ice. This dilutes the water in the North Atlantic preventing formation of NADW and shutting down the conveyor circulation. Without redistribution of heat by the conveyor, polar regions get colder, ice grows, water becomes more salty, which allows NADW to begin forming again. And so on and so on.

24. Next Exam is Monday, November 21st. It will cover all the material in Lectures 8-16, plus chapters 5-8 in Knauss.