The Circulation of the Deep Oceans Josh Willis a.k.a. abyssal circulation a.k.a. thermohaline circulation a.k.a. meridional overturning circulation a.k.a. global conveyor belt

In 1798, Englishman Count Rumford postulated that currents bring cold water from the polar regions to fill the abyss In 1845, Emil von Lenz, a Russian-German physicist, noticed the shoaling of the thermocline near the equator and proposed two hemispheric cells.

In 1935, Georg Wüst, a German oceanographer, considering salinity contours suggested a more complicated picture.

Prior to the late 1950s, estimates of overturning in the Atlantic based on hydrographic data suggested only 6-8 Sv of overturning, or inter- hemispheric exchange. Stommel, Arons and Faller – series of papers on theory of deep circulation suggest 15 – 25 Sv!!! Stommel H The abyssal circulation. Deep-Sea Research 5 (1): 80–82. Stommel H., A.B. Arons, and A.J. Faller Some examples of stationary flow patterns in bounded basins. Tellus 10 (2): 179–187. Stommel H., and A.B. Arons On the abyssal circulation of the world ocean—II. An idealized model of the circulation pattern and amplitude in oceanic basins. Deep-Sea Research 6: 217–233. References:

Stommel, Arons and Faller – series of papers on theory of deep circulation suggest 15 – 25 Sv!!! Three fundamental assumptions: 1.Deep water supplied by convection in Greenland & Irminger Seas in the North & Weddell Sea in the South. 2.Uniform mixing brings cold water back toward surface 3.Deep circulation is geostrophic in the interior.

Theory of the Deep Circulation Upper Ocean Stommel, H.M., A survey of ocean current theory. Deep-Sea Research 4, 149–184. Each Contour is 10 Sv DeepOcean

Theory of the Deep Circulation Stommel H The abyssal circulation. Deep-Sea Research 5 (1): 80–82. Circulation is poleward in interior with narrow deep boundary current

Theory of the Deep Circulation Stommel H., A.B. Arons, and A.J. Faller Some examples of stationary flow patterns in bounded basins. Tellus 10 (2): 179–187.

Theory of the Deep Circulation Stommel H., A.B. Arons, and A.J. Faller Some examples of stationary flow patterns in bounded basins. Tellus 10 (2): 179–187.

The Deep Western Boundary Current in the Southern Hemisphere Tomczak, Matthias & J Stuart Godfrey: Regional Oceanography: an Introduction 2nd edn (2003), Chapter 13. Potential Temperature at 30  S Salinity at 30  S Deep Western Boundary Current

The Global Overturning Circulation Reviews of Geophysics Volume 45, Issue 2, pages n/a-n/a, 24 APR 2007 DOI: /2004RG Volume 45, Issue 2, From Kuhlbrodt et al., Rev. Geophys., 2007

The Global Overturning Circulation The polar view of the Global Overturning reminds us that the ACC acts as a huge mix-master, mixing deep water masses together and redistributing them to every ocean basin.

Overturning in the North Atlantic The surface (red, orange, yellow) and deep (violet, blue, green) currents in the North Atlantic. The North Atlantic Current brings warm water northward where it cools. Some sinks and returns southward as a cold, deep, western- boundary current. Some returns southward at the surface. From Woods Hole Oceanographic Institution.

1.deep convection: 1000 to 1500 dbar (or more) overturn due to buoyancy loss (mostly cooling that causes densification) 2.brine rejection: salt rejected from sea ice during formation, most effective when mixed into a shallow layer, say, on a continental shelf. B.R. in some special sites makes the densest ocean waters. 3.upwelling and surface transformation: Southern Ocean 4.diffusion: mixing of heat and salt. Diapycnal diffusion is essential for deep waters to warm and upwell diapycnally (balances the other two densification processes). Includes local vigorous mixing e.g. strait overflows, and broad-scale Processes that set abyssal water properties

Deep convection and brine rejection sites X X X Labrador Sea Greenland Sea Mediterranean Red XX Ross Sea Weddell Sea B B B B B B B B B Brine rejection in all sea ice areas X From Descriptive Physical Oceanography: An Introduction, 6th edition, by Talley, Pickard, Emery, and Swift

Abyssal circulation: diapycnal diffusion due to vigorous mixing at strait overflows Example: Mediterranean Sea, also the Nordic Seas Inflow of surface water, densification within sea, outflow of denser water through strait, descent with vigorous mixing and entrainment From Descriptive Physical Oceanography: An Introduction, 6th edition, by Talley, Pickard, Emery, and Swift

Deep and bottom water production sites: North Atlantic Deep Water (densest portion of it) Antarctic Bottom Water Intermediate water production sites: major impacts on salinity Labrador Sea Water (fresh) Mediterranean Water (salty) Red Sea Water (salty) Antarctic Intermediate Water (fresh) North Pacific Intermediate Water (fresh) Source Waters for Abyssal Circulation From Descriptive Physical Oceanography: An Introduction, 6th edition, by Talley, Pickard, Emery, and Swift

North Atlantic Deep Water Saline, high oxygen, low nutrient, water mass around 2000 m depth as it exits the N. Atlantic to the south. Signature found throughout world ocean. Sources: 1.Upper layer water of N. Atlantic, from Gulf Stream through subpolar gyre, including Antarctic Intermediate Water and surface water from the Indian Ocean 2.Nordic Seas Overflow Water: Dense, cold overflows from intermediate-deep convection in Greenland Sea 3.Labrador Sea Water: Intermediate depth convection in (fresher) Labrador Sea 4.Mediterranean Water: Evaporated, saline waters from Mediterranean Sea 5.Antarctic Bottom Water: Very dense, cold water from Antarctic

Atlantic 25W salinity and water mass names MOW LSW AAIW NADW AABW NSOW

T-S Plots from various Basins Abyssal waters are a complex mixture of water from various sources Other important tracers: Oxygen, Silicates, Phosphates, 3 He, 3 H

Why do we care about the overturning? At 24  N: Gulf Stream Carries 40 Sv at ~ 18  C Gulf Stream Carries 40 Sv at ~ 18  C DWBC Returns 14 Sv. At ~ 2  C DWBC Returns 14 Sv. At ~ 2  C (14 x 10 6 m 3 /s * 16  C) x 1030 kg/m 3 x 4000 J/(kg  C) = 0.9 petawatts 1.2 petawatts is the accepted value today! ~ 25% of net northward heat transport Moderates winters in Europe

Why do we care about the overturning?

What’s up with the ice sheet?

Evacuate the Country!!!

How many grad. students will it take to couple my paleoclimate model to an AOGCM?

The Great Ocean Conveyor Belt

20,000 years ago in North America

Ice Discharge from Glaciers

Could this Really Happen?

Both Arctic Sea Ice and Greenland Ice Sheet are shrinking

The Hosing

∆Precip ∆SST Coupled Model: Shutting Down the AMOC Vellinga & Wood, Climatic Change, 2002 Cooling: Hurricane Connection? African Drought Brazilian Rainfall SW Australian Drought US Rainfall

Hu et al., GRL, 2009 BUT! Shutdown unlikely with realistic melt

Probably safe for today….

Knight et al., GRL, 2005 Still, AMOC could be important for regional climate HadCM year unforced, coupled model run

Goldenberg et al., Science, 2001 # of Big Hurricanes AMO Atlantic Hurricanes and the AMO

How to Measure the Overturning?

RAPID Array Temperature and salinity profiles measured near the boundaries using moorings

RAPID Array Transport through the Florida Straights is measured using the voltage across a cable Transport in the Ekman layer is estimated using wind observations

RAPID Array Adding these all together makes it possible to estimate zonally averaged overturning Overturning Streamfunction

RAPID Array Adding these all together makes it possible to estimate zonally averaged overturning

Subsurface Floats From Lozier, Science, 2010 ( Float Deployments have suggested a more complicated picture than Stommel’s Deep Western Boundary Current DWBC is highly variable and interior pathways are also important

Floats Satellite Observations of SSH

Argo Floats Limited by 2000 m isobath Subsurface velocity level of known motion Profile data provide geostrophic shear

Computing subsurface displacements Park et al., JTECH, 2005 xx xx A few hours ~7 days A few hours

Altimeter Data

‘04-’06 mean – 1000 db Velocity SSH Geostrophic Velocity Can we integrate, west to east?

Boundary Current Separated ‘04-’06 mean – 1000 db Velocity Steep Topography Northward flow of surface water NADW Return flow Difference dynamic height at 2000 m isobath

Time Series at 41°N

MOVE Array From Srokoz, BAMS, 2012

Key Points: Abyssal Circulation Sets stratification, modulate climate Mechanism to exchange CO 2 with deep ocean Northward heat transport impacts N. Hemisphere regional climate Deep water formation occurs in only a few locations A wide variety of observational tools needed to understand and measure variability of abyssal circulation