The Circulation The Circulation of the Deep of the Deep

OceansOceans

Josh WillisJoshua.k.willis@jpl.nasa.gov

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