Thermohaline Ocean Circulation Stefan Rahmstorf

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.

[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

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.

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.

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

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

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. THC in Quaternary Climate Changes

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

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.