Thermohaline Ocean Circulation

1. Thermohaline Ocean Circulation Stefan Rahmstorf

2. What is Thermohaline Circulation? • Part of the ocean circulation which is driven by fluxes of heat and freshwater. • Distinction of thermohaline vs wind-driven - Ocean’s density distribution  pressure gradient (circulation) - Currents and mixing  density distribution • MOC is the north-south flow that includes wind-driven parts whereas THC is also associated with zonal overturning cells.

3. [Key features] • Deep water formation • Spreading of deep waters : NADW, AABW • Upwelling of deep waters : ACC region by Ekman pumping • Near-surface currents to close the flow

4. What Drives the THC? • High-latitude cooling  high density  sinking of water • Ocean basin would be filled with dense water and the circulation would stop. • It requires a mechanism that continually works to make the water in the box less dense, so that the source water remains denser than the water already in the box. • Sandstrom’s theorem : Heating at greater depth than cooling is required. This enables an ongoing replacement by colder water from above.

5. In the Real Ocean? • Heating occurs at the surface due to the sun. • The slow diffusion of heat by turbulent mixing caused by tides and the winds.

6. Nonlinear Behavior of the THC Tropics, subtropics : warm, saltier  low density High latitudes : cold, fresher  high density Thus, THC is thermally driven. Freshwater input  density difference  circulation  salinity difference

7. The Circulation’s Effect on Climate Large heat transport of up to 1 PW in the North Atlantic.  warm by ~5K  sea ice pushed back  albedo feedback THC switched off 1) NH cools / SH warms  shifts thermal equator and ITCZ southward 2) sea level around the northern Atlantic rise by up to 1m

8. THC in Quaternary Climate Changes • Warm mode : similar to present-day • Cold mode : NADW forming south of Iceland • Switched-off mode : occurred after major input of freshwater • Heinrich events – large input of freshwater by iceberg discharge  Stopped NADW formation  Proxy data shows cooling around mid-latitude Atlantic • Younger Dryas – meltwater floods • DO events – north-south shifts in convection location  Dramatic warm events in Greenland and northern Atlantic & Coinciding increase in salinity in the Irminger Sea pushes warm, salty Atlantic waters northward into the Nordic Seas • Glacial conditions – Southward shift of the main deep water formation (Nordic Seas  South of Iceland) reduces ocean heat transport to high latitudes and expands sea ice.

9. The Future of the THC • Global warming : surface warming & surface freshening  Both reduce the density of high-latitude surface waters  Inhibit deep water formation  weakening of NADW formation  serious impact on marine ecosystems, sea level and surface climate including a shift in ITCZ • THC collapse : “low probability – high impact” risks associated with global warming

11. In the Real Ocean? • Heating occurs at the surface due to the sun. • The slow diffusion of heat by turbulent mixing caused by tides and the winds. • Both THC and wind-driven circulation drive the observed MOC in the world oceans. Westerlies over Southern Ocean Divergence of surface currents Upwelling from deep waters due to “Drake Passage Effect” High-latitude North Atlantic with the highest surface densities and stable stratification.