The Circulation The Circulation of the Deep of the Deep
OceansOceans
Josh [email protected]
a.k.a. abyssal circulationa.k.a. abyssal circulationa.k.a. thermohaline circulationa.k.a. thermohaline circulationa.k.a. meridional overturning a.k.a. meridional overturning circulationcirculationa.k.a. global conveyor belta.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. 1958. The abyssal circulation. Deep-Sea Research 5 (1): 80–82.
Stommel H., A.B. Arons, and A.J. Faller. 1958. Some examples of stationary flow patterns in bounded basins. Tellus 10 (2): 179–187.
Stommel H., and A.B. Arons. 1960. 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., 1957. 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. 1958. 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. 1958. 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. 1958. 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 30S
Salinity at 30S
Deep Western Boundary Current
The Global Overturning Circulation
Reviews of GeophysicsVolume 45, Issue 2, pages n/a-n/a, 24 APR 2007 DOI: 10.1029/2004RG000166http://onlinelibrary.wiley.com/doi/10.1029/2004RG000166/full#rog1618-fig-0001
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
http://oceanworld.tamu.edu/resources/ocng_textbook/chapter13/chapter13_01.htm
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 Deep convection and brine rejection sitessites
XX
X
Labrador Sea Greenland Sea Mediterranean Red
XX
Ross Sea Weddell Sea
B
B
B
B B B
B BB
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 Abyssal circulation: diapycnal diffusion due to vigorous mixing at diffusion due to vigorous mixing at
strait overflowsstrait overflows
Example: Mediterranean Sea, also the Nordic SeasInflow of surface water, densification within sea, outflow of denser water through strait, descent with vigorous mixing and entrainmentFrom 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 CirculationSource Waters for Abyssal Circulation
From Descriptive Physical Oceanography: An Introduction, 6th edition, by Talley, Pickard, Emery, and Swift
North Atlantic Deep North Atlantic Deep Water 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 Atlantic 25W salinity and water mass nameswater mass names
MOW
LSW
AAIW
NADW
AABW
NSOW
T-S Plots from various T-S Plots from various BasinsBasins
http://oceanworld.tamu.edu/resources/ocng_textbook/chapter13/chapter13_03.htm
Abyssal waters Abyssal waters are a complex are a complex
mixture of mixture of water from water from
various various sourcessources
Other Other important important tracers: tracers: Oxygen, Oxygen, Silicates, Silicates,
Phosphates, Phosphates, 33He, He, 33HH
Why do we care about the overturning?
At 24At 24NN::•Gulf Stream Gulf Stream Carries 40 Sv at ~ Carries 40 Sv at ~ 1818CC•DWBC Returns DWBC Returns 14 Sv. At ~ 214 Sv. At ~ 2CC
(14 x 10(14 x 1066 m m33/s * 16 /s * 16 C) x 1030 kg/mC) x 1030 kg/m33 x 4000 J/(kg x 4000 J/(kg C) = 0.9 C) = 0.9 petawattspetawatts
1.2 petawatts is the accepted value 1.2 petawatts is the accepted value today!today!
~ 25% of net northward heat transport~ 25% of net northward heat transportModerates winters in Europe 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 The Great Ocean Conveyor BeltBelt
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
0.29
0.19
0.03
1.65
0.54
0.15
Hu et al., GRL, 2009
BUT! Shutdown unlikely with realistic melt
Probably safe Probably safe for today….for today….
Knight et al., GRL, 2005
Still, AMOC could be important for regional climate
HadCM3 1400-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 ArrayTransport 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 (http://www.sciencemag.org/content/328/5985/1507.full.html)
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 x x
A few hours
~7 days
A few hours 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
Time Series at 41°N
MOVE Array
From Srokoz, BAMS, 2012
Key Points: Abyssal Circulation
•Sets stratification, modulate climate•Mechanism to exchange CO2 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