Thermohaline Ocean Circulation Stefan Rahmstorf What is Thermohaline

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 ~5 K 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 1 m

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 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.

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.