On the role of the Southern Ocean in the global thermohaline circulation

1
On the role of the Southern Ocean in the global
thermohaline circulation
Daniele Iudicone
2
The present perception of Southern Ocean
overturning it makes the global thermohaline
circulation possible
Lumpkin and Speer (2007)
Speer et al., 2000 Sloyan and Rintoul, 2001ab
3
The present perception of Southern Ocean
overturning most questions are open
Import North Atlantic Deep Water
(NADW) Export 1. Antarctic Bottom Water
(AABW) 2. Thermocline Waters Sub-Antarctic Mode
Water (SAMW) Antarctic Intermediate Water (AAIW)
eddy transport
Which surface buoyancy fluxes sustain the
transformations?
What is the role of mixing?
Upwelling of NADW Döös (1995) 9-12 Sv Sloyan
and Rintoul (2001) 0 Sv
Speer et al., 2000 Sloyan and Rintoul, 2001ab
4
Mixing is crucial A scenario with a surface
buoyancy forcing of NADW upwelling is problematic
Warm sources have to be deeper than cold
ones The Sandströms theorem
NADW Formation
NADW Consumption
Mixing
Intense THC
No THC
5
Mixing is crucial Different scenarios for the
NADW upwelling
a) Pushing by deepwater formation
c) Pulling by wind stress surface waves
b) Pulling by deep mixing
understanding the physics related to the
spatial and temporal distribution of mixing is
one of the most important research frontiers in
physical oceanography. Huang (2004)
6
Outline

• The model and its validation
• Quantitative thermodynamics a generalized
  definition of water mass transformations
• The role of solar irradiance
• The non-linearity of the equation of state
• The role of Southern Ocean forcings and mixing in
  the global conveyor
• Thermodynamic balance
• The role of the ML
• The global conveyor from a Southern Ocean
  perspective
• The Southern Ocean overturning
• The NADW fate and origins
• Coupling Lagrangian and Eulerian results
• Conclusions Perspectives

7
The modelling tool
ORCA2-LIM ice-ocean model (Madec et al. 1998)
Ocean ORCA2 global ocean configuration Sea-Ice
model LIM (Louvain la Neuve Ice
Model) Surface Forcing ERS1-2 wind
stress bulk formulae using monthly climatology
penetration of solar radiation CMAP
precipitation Climatological Runoffs GRDC
Baumgartner Reichel (1975) Restoring to
Levitus SSS Bottom Forcing Spacially varying
Geothermal heating Bottom Boundary Layer
Beckmann Döscher 97 Free surface formulation
(Roullet and Madec, 2000)
One simulation 1500y long off-line tracers
(C14, CFCs) simulations
8
CFCs and C14 off-line integrations
The quantitative validation exercise gave quite
good results CFCs The upper ocean ventilation
is in good agremeent with data C14/He Deep
ocean ventilation is not too bad. In fact NADW
carries en excess of C14 into the SO that masks
the weak AABW formation
OMIP intercomparison (Dutay et al. 2002)
The model inventory
CFC data South Atlantic
9
The model global tracer fields
Mean global T anomaly
Mean global S anomaly
Model-Levitus94
Model-Levitus94
Cool anomaly the IW range has been changed
The water masses definition used here Six main
classes
10
A shallow and a deep overturning
Data analysis Talley et al. (2003)
Model net transport at 30S
15 Sv
19.4 Sv
3.5 Sv
10.2 Sv
7.9 Sv
10.1 Sv
Global overturnings compare quite well
11
The model flaws
The model solution is in sufficiently good
agreement with observations.
Nevertheless

• The Autumn ice cover is in excess (30)
• SAMW and AAIW are underestimated at the Drake
  Passage (50-60)
• The deep overturning is weak (only 10 Sv
  dramatic in the Indian Sector)
• The SAMW northward transport in the Indian Ocean
  is twice as large than in observations
• The meridional heat transport in the North
  Atlantic Ocean is weak

12
The thermohaline circulation (THC) is the
ensemble of streamlines that connect the regions
of trasformations
Transformations and pathways are intimately
linked

• To understand the dynamics of the THC we thus
  need to characterize both
• the transformations (here changes of density)
• the associated streamlines

To examine the role of the Southern Ocean in the
global THC we need
To define the role of each physical process in
transforming the water-masses in the Southern
Ocean
To individuate the streamlines (pathways) of the
THC in the Southern Ocean
13
How can we characterize the transformations AND
the associated streamlines? A novel quantitative
approach
Definition A water mass is characterized by the
density value (in primis)
Density variation along the streamlines
Water mass transformation

• To simplify the analysis these terms have to be
  evaluated in a density framework
• Quantitative thermodynamics

• Identification of the streamlines
• Lagrangian approach

14
Water masses transformation and formation

• A generalized definition of water-mass
  transformation/formation
• ?
• A quantitative approach to the ocean circulation
  thermodynamics
• Iudicone et al. JPO 2007a

15
The diapycnal transport in a neutral density
framework
16
Impact on data inversions
Taking into account surface buoyancy fluxes in
data inversions using ? implies the use of ?
Lumpkin and Speer (2007)
17
The Generalized buoyancy function
Marsh et al. (2000)
Total northward transport of waters denser than ?
across the line of latitude ?? (Meridional
streamfunction)
Total diapycnal transport down across ? south
of ?? Clear physical interpretation!
This is, in fact, the generalization of the
Generalized heat function of Greatbatch et al.
(2007)
18
Penetrative solar irradiance was never considered
before
The classical formulation associated the solar
irradiance to the isopycnal outcrop (ie to the
ML a 2D approach)
Isopycnal layer
Our formulation associates the solar irradiance
to the water mass volume (a 3D approach)
19
Sub-surface heating is important
February Clear water solar irradiance at the
base of the ML (W/m2) Climatological data (
LOCEAN ML climatology, DeBoyer-Montegut et al.,
2004)
20
The global ocean estimate of surface forcing
transformations
Our approach
Classic approach
Total
E-P
HEAT
21
The lightest Worlds ocean water masses are
related to the precipitation excess
?0
Lightest waters are formed by precipitations
(and runoff) Eg not as in Large and Nurser (2001)
22
McDougalls neutral density A framework that
covers the entire water column

• Neutral motions occurs on neutral surfaces
• We need a unique framework for the whole depth
  and ?x are useful only for a limited depth range
  gt We cannot compare quantitatively surface
  forcing with deep mixing

g
s0 , s2
A meridional section of the model density field
(Indian sector)
23
The neutral density framework accounts for the
non-linearity of the equation of state of
seawater
The isoneutral diffusion term represents
cabbeling and thermobaricity
Cabbeling The water formed by mixing two
isopycnal waters parcels is denser than its
origins
Thermobaricity Compressibility is function of
temperature
Two water parcels with the same T/S have a
density difference that depends on
depth ie Vertical movements can cause convection
24
Forcings The role of surface fluxes
? Mixing Forcing
Data analyses
Forcing Heat brine rej. E-PR
40
TW
MW
IW
AABW
CDW
-80
27.2
26.0
27.2
26.0
Ice-ocean interactions form AABW
Iudicone et al. JPO 2008b
25
The role of mixing
? Mixing Forcing
TW
MW
IW
AABW
CDW
26
The neutral density framework is necessary for
describing deep waters formation
Our approach
Classical approach
?
?0
The mixing of deep waters is completely
inversed gt The deep cell disappears
27
The role of mixed layer dynamics
Bowl (max volume occupied by the ML)
Deep mixing or subsurface mixing? Most of the
transformations are associated to the ML
dynamics
TW
MW
IW
AABW
CDW
Vertical mixing occurs in the bowl gt Deep
mechanical energy is not required
Interior
The NADW upwells even if there is no outcrop
____ ____ ____ -------
Surface fluxes
Net volume flux
Vertical diffusion
Isop. Diffusion (cabbeling and thermob.)
28
The role of heat in the transformations
29
The role of heat in the transformations
Heat component
Freshwater component
30
The role of Southern Ocean surface forcings and
mixing in the Global Conveyor In an Ice-Ocean
coupled model
BOWL
SAMW large formation forcingmixing
AAIW compensation cabbeling
CDW upwelling via ML processes
AABW ML and Internal mixing
31
The role of mixed layer dynamics
TW
MW
IW
Vertical mixing occurs in the bowl gt Deep
mechanical energy is not required
Interior
The NADW upwells even if there is no outcrop
CDW
Deep mixing or subsurface mixing? Most of the
transformations are associated to the ML
dynamics
AABW
32
The diagnostic strategy to use of a
quantitative Lagrangian Tool

• Bruno Blankes Lagrangian diagnostics ARIANE
• ? water mass pathways, transports
  characteristics derived from multiple particle
  trajectories

The overturning in the Southern Ocean 800,000
water parcels were followed during their 3D
journey from 30S to 30S
30S
33
The mother fountain of all the waters of the
world (in ancient cosmogonies)
BOWL
0 m
Mode water a blend of TW (70) and deep water
upwelling (20) (Trull et al. 2001 CFC SAMW 70
20-30)
AAIW secondary? mostly former SAMW
CDW is almost completely destroyed in the SO
AABW mostly NADW Internal mixing
5000 m
Antarctica
30S
Orsi et al. (2002) CFC analysis
Iudicone et al. JPO (2007, 2008c)
34
Water mass ventilation
Ventilation is computed by tracing water parcels
backward in time from 30S to the ML

• The total transport from the ML to the pycnocline
  is huge (75 Sv)
• agreement with Kartsensen and Quadfasel (2003)
• Subduction is about twice the surface production
  gt obduction is important

Ventilation per water mass (in percent of the
northward flow at 30S)

• The ventilation of AAIW is small
• Small consumption

35
The fate of NADW
NADW is almost completely destroyed (80) NADW
partly upwells (40) and partly -gt AABW (40)
The fate of NADW in the Southern Ocean is very
complex
PAC ATL IND
TW 10 1 1
MW 6 5 12
IW 2 1 2
UCDW 1 11 5
LCDW 11 1 5
AABW 20 4 1
Upper NADW fate ()
NADW gtDenser water
NADW gtUpwelling
PAC ATL IND
TW 7 1 1
MW 2 2 7
IW 1 - 1
UCDW - 2 1
LCDW 4 24 15
AABW 16 11 4
Lower NADW fate ()
36
The geographical distribution of the NADW
first crossing of ? 27.8 (AAIW)
Indian 1.1 Sv Pac 3.3 Sv Atl 2.2 Sv
The main time scale is 130y The flushing time is
300y Homogenization (several trips around
Antarctica slow upward spiraling)
37
The thermodynamics of the NADW first crossing
of ? 27.8 A freshwater gain causes the
conversion into AASW(AAIW)
Buoyancy gain
Seasonality
Total (30S-gt1st Upw)
Normalized seasonal values
1st Upw. (Lagrangian)
Salt Vertical mix. (Eulerian)
A freshwater gain causes the upwelling into
lighter layers A net cooling is also observed
Completely independent estimates give a clear
maximum in Summer
38
The overturning in the Southern Ocean
Combining the analysis of watermass
transformations with the Lagrangian analysis
The net transformation are fed by the THC flow
TW gt SAMW (SF, V.Mix)
SAMW gt AAIW (V.Mix, cabbeling)
Weak AAIW exchanges
CDW gt SAMW/TW/AAIW (sequence of transformations)
CDW gt AABW (coolingdeep mixing)
39
Water mass ventilation and export of the large
freshwater excess
Lagrangian freshwater budget (concentration/dilut
ion) per final density
30
50
20

• The freshwater gain is mostly in the Indian and
  Pacific SAMW formations.
• AAIW is recirculating (in slow spirals in the
  sub-tropical gyres cabbeling) and then requires
• much less freshwater to get the final S minimum
  at 30S.

40
Main conclusions
The Southern Ocean site of important water
masses transformation, occurring mostly in the
boundary layers, and of a significant interbasin
exchange
SAMW Upwelling is important (70 TW) The
intense surface forcing is only partially
compensated by (vertical) mixing gt
Large production
AAIW Surface production compensated by vertical
mixing and cabbeling gt Weak production
(recirculation)
CDW Several destructive processes (NADW)
gt Large consumption (80)
40 into TW/SAMW/AAIW via the downward
propagation of surface freshwater 40 into
AABW via surface forcings and mixing from
non-linearity of state eq(eg thermobaricity)
41
Final remark The novel approach demostrated to
be very powerful
Combining thermodynamics and pathways
We obtain a detailed and, above all, quantitative
view of the ocean thermohaline circulation.
42
Perspectives
Southern Ocean water masses formation analysis
of ARGO data and inspection of the role of
water transparency (biology) from ocean color data
The role of freshwater in the global conveyor
Lagrangian analysis of the fate of the Southern
Ocean excess
Extension of the quantitative approach to active
tracers The role of the physics in the global
bio-geochemical cycle
43
Natural Southern Ocean air-sea CO2 flux
Perspectives Extension of the quantitative
approach to tracers to unravel the role of
physicsin the global bio-geochemical cycle
The Polar Front the boundary between in-gassing
and out-gassing
Main result the THC dominates the CO2 internal
redistribution
Ingassing
Outgassing
mol/m2/yr
44
Thanks to
Gurvan Madec LOCEAN (France) Sabrina Speich,
Bruno Blanke LPO (France)
Keith Rodgers GFDL (USA) Jean-Claude Dutay CEA
(France)
Richard Schopp LPO (France) Trevor J.
McDougall CSIRO (Australia)
And also Peter Killworth, Lynne Talley, Steve
Rintoul and many others
45
Outline

• The model and its validation
• Quantitative thermodynamics a generalized
  definition of water mass transformations
• The role of Southern Ocean forcings and mixing in
  the global conveyor
• The global conveyor from a Southern Ocean
  perspective
• Conclusions Perspectives

46
Outline

• The model and its validation
• Quantitative thermodynamics a generalized
  definition of water mass transformations
• The role of Southern Ocean forcings and mixing in
  the global conveyor
• The global conveyor from a Southern Ocean
  perspective
• Conclusions Perspectives

47
A generalized definition of water-mass
transformation/formation
The right side of the buoyancy equation is binned
in neutral density ? A dianeutral transport
(in Sv) for each physical process
Forcings Penetrative solar Irradiance (R)
Latent Sensible Longwave
geothermal flux E-PR
ice brine rejection
The terms we added are underlined
Mixing Vertical mixing Isoneutral mixing
Double diffusion Entrainment in overflows
Convection
Water mass formation M M d? /d?
48
Outline

• The model and its validation
• Quantitative thermodynamics a generalized
  definition of water mass transformations
• The role of Southern Ocean forcings and mixing in
  the global conveyor
• The global conveyor from a Southern Ocean
  perspective
• Conclusions Perspectives

49
The connection between the Atlantic Ocean and
the Southern Ocean
The discussed origins of waters that feeds the
NADW production
NADW return flow
SAMW recirculates in the Atl. (45) and arrives
from the Indian Ocean (44) AAIW recirculates in
the Atl. (39) and arrives from the Indian Ocean
(59)
50
Outline

• The model and its validation
• Quantitative thermodynamics a generalized
  definition of water mass transformations
• The role of Southern Ocean forcings and mixing in
  the global conveyor
• The global conveyor from a Southern Ocean
  perspective
• Conclusions Perspectives

51
Outline

• The model and its validation
• Quantitative thermodynamics a generalized
  definition of water mass transformations
• The role of Southern Ocean forcings and mixing in
  the global conveyor
• The global conveyor from a Southern Ocean
  perspective
• Conclusions Perspectives

52
Perceptions of the Southern Ocean overturning
circulation The very early works the myth
the mythology conceived Hvergelmer the
Maelstroem of Amlodhi as a vast reservoir, the
mother fountain of all the waters of the world.
In the front rank are mentioned a number of
subterranean rivers which rise in Hvergelmer, and
seek their courses thence in various directions.
But the waters of earth and heaven also come from
this immense fountain, and after completing their
circuits they return thither.gtgt    . Lost in
the depths of the Southern Ocean, they were
capable of giving the Depths of the Sea to our
forefatherst. Fragments from Hamlet's Mill an
Essay on Myth and the Frame of Time. Giorgio de
Santillana and Hertha von Dechend. Boston, 1977.
The "God Boat" on the Arabian celestial globe
made by Tabari. P. Casanova, Bulletin of the
Institut Francais d'Archeologie Orientale 2,
Editions A. J. Picard et Cie., Paris, 1902
53
Perceptions of the Southern Ocean overturning
circulation The early works(30-40) The
mother fountain of all the waters of the world
Wüst (1935), Deacon (1937), Sverdrup et al. (1942)
54
The long sleep (50s-80s) Attention is on the
North Atlantic
55
How we obtained these results?
56
The overview of the thesis work
Quantitative thermodinamics a generalized
definition of water mass transformations The
impact of solar irradiance The impact of
non-linearity of the equation of state The role
of Southern Ocean forcings and mixing in the
global conveyor Thermodynamic balance The role
of the ML and of Ekman dynamics The heat and
freshwater redistribution The role of
eddies The seasonal cycle of mixing The global
conveyor from a Southen Ocean perspective The
interbasin exchange The Southern Ocean
overturning and its relation to the global
conveyor The NADW fate and origins Combined
Lagrangian and Eulerian results Phenomenology
and dynamics of the injection of AAIW into the
South Pacific The fate of the Southern Ocean
freshwater excess
57
SAMW Main source is TW
Surface forcing is only partially compensated by
(vertical) mixing
AAIW The large (freshwater forcing) is
compensated by mixing (mostly cabbeling)
Weak production (recirculation)
CDW Large consumption (80) (NADW)
One half into TW/SAMW/AAIW via the downward
propagation of surface freshwater
One half into AABW via surface forcings and
mixing (eg thermobaricity)
This How we made the analysis possible and how
we made it
58
Why looking for heat sources if water is so
sweet? The thermodynamics of the NADW
Upwelling A Lagrangian description Buoyancy
gain final-initial values (a tricky excercise
because of the non-linearity of equation of state)
____ ___
Heat Freshwater
PDFs of the buoyancy gain
Total (30S-gt30S)
Heat dominates the overturning
59
The origins of waters that feeds the NADW
production is the model correct?
In situ data
CFC-11 concentration age on the AAIW isopycnal
Model
Ages fronts correspond well
60
The global overturning circulation
The present picture
NADW
AABW
Wunsch and Ferrari (2004)
The ocean is NOT a heat engine gt The
thermohaline circulation needs external energy
sources to be sustained
understanding the physics related to the
spatial and temporal distribution of mixing is
one of the most important research frontiers in
physical oceanography. Huang (2004)
61
The Southern Ocean an annular ocean that
connects the worlds ocean basins
The Antarctic Circumpolar Current

• Large volumes (gtgt100 Sv)
• Connects the major ocean basins
• Transports heat and freshwater anomalies around
  the globe
• (At least) Two jets/fronts

62
Overturning Circulation in the Southern Ocean
Southern Ocean water mass transformations
connect the upper and lower limbs of the global
overturning circulation. Zonal and meridional
circulations are intimately linked, eddies play a
crucial role.
Import North Atlantic Deep Water
(NADW) Export 1. Antarctic Bottom Water
(AABW) 2. Thermocline Waters Sub-Antarctic Mode
Water (SAMW) Antarctic Intermediate Water (AAIW)
63
Application to the Med Sea and to the Indonesian
Passage
Med Sea 100 overestimated Bozec et al., JGR, in
press.
Indonesian Passage Tides explain 60 of the wm
transformations Koch-Larrouy et al., in
preparation
64
John Marshalls papers The dynamical approach
(2001-2006)
Ekman circulation
65
The upwelling of the NADW in the Southern Ocean
The time scales
The distribution of time scales per water parcel
Two main time scales 30y (small) and
130y There is a common main time scale for the
upwelling in the different basins gt The long
tail is an exponential exp(-t/?) ?200-300y
(seesaw 1500y) Homogenization (several trips
around Antarctica slow upward spiraling)
66
The neutral density framework
Necessary for describing deep waters formation
67
The generalized approach to the estimate of water
masses transformations/formation

• 3-D approach for surface fluxes
• Use of neutral densities

• 2-D approach for surface fluxes
• Use of sigma0

Density evolution
Dianeutral flux
68
Main conclusions The role of the Southern Ocean
in the global conveyor

• The Southern Ocean is the site of important water
  masses transformation, occurring mostly in the
  boundary layers, and of a significant interbasin
  exchange

• The SAMW is largely ventilated, it is mostly
  cooled and freshened TW, part of the surface
  forcing is compensated by vertical mixing

• The AAIW is weakly ventilated, surface forcings
  are largely compensated by lateral and vertical
  mixing it essentially recirculates with a slow
  spiraling (partly SAMW via vertical mixing and
  cabbeling)

• The NADW is consumed largely (80) the upwelling
  into Indian SAMW and Pacific TW explains 40
  while the rest goes into AABW via surface forcing
  and mixing
• The NADW buoyancy gain occurs in two times a
  freshwater gain at the base of the ML gt the
  maximum occurs in Summer (higher efficiency of
  the mechanical forcing on buoyancy)

69
The fate of the Southern Ocean freshwater excess
Answer Into the Pacific Ocean
Atl Ind Pac gt10 45 45 76
Salinity budget
Lagrangian
The Pacific is the main sink thus
70
Overview on the Southern Ocean
What drives the upper limb?
The deep mixing scenario (eg Wunsch and Munk
1998) is not confirmed by in situ data The
Southern Ocean wind-driven upwelling scenario is
gaining interest (Toggweilers, Gnanadsikans,
Huangs papers etc)
71
Is (South Pacific) AAIW subducted?
In situ data analysis
In grey the max ML depth (DT De Boyer Montegut
et al., 2004) Thick line AAIW winter outcrop
from ARGO data
Iudicone et al JPO 2007 to appear
72
The fate of the Southern Ocean freshwater excess
Question how does it relate to the quasi-global
salinity minimum of Southern Ocean origin (AAIW)?
Global distribution of low-salinity intermediate
water (intermediate salinity minimum). (Hanawa
and Talley 2001)
If AAIW forms an intermediate recirculating
horizon, with weak interactions, where does the
freshwater excess go into?
73
Extension of the quantitative approach to the
analysis of bio-geochemical tracers
Example High-latitude controls of thermocline
nutrients and low latitude biological
productivity
Southern Ocean contribution to the global export
production
74
The export of the large Freshwater excess
Lagrangian freshwater budget (concentration/dilut
ion) per final density
30
50
20

• The freshwater gain is mostly in the Indian and
  Pacific SAMW formations. Not in the AAIW.
• The SAMW origins are salty waters (TW, 70) added
  with fresh AASW (20).
• AAIW is recicrculating (in spirals in the
  sub-tropical gyres) and then requires
• much less freshwater to get the final S minimum
  at 30S.

75
The water-mass transformations in the Southern
Ocean
The role of mixed layer dynamics II
Bowl
Bowl envelope (2 grid box)
Interior
Interior
____ ____ ____ -------
Surface fluxes
Net volume flux
Vertical diffusion
Isop. diffusion
Most of the upwelling is due to sub-surface
mixing just BELOW the ML
76
The water-mass transformations in the Southern
Ocean
Other results
The dense water upwelling is due to a freshwater
gain via vertical mixing It occurs mainly during
the Austral Summer The Ekman layer is the site
of large wm transformations Eddy transport
accounts for 10 Sv Southward diffusion of Heat
in the mixed layer
77
The fate of the Southern Ocean freshwater excess
Question how does it relate to the global
freshwater cycle?
But the waters of earth and heaven also come
from this immense fountain , and after completing
their circuits they return thither
Freshwater transport (Compilation from Weijffel
2001)
Southern Ocean 0.5-0.8 Sv Artic basin 0.4-0.5
Sv The rest is evaporative, especially Indian
and Atlantic
78
The neutral density framework
79
Impact of the correct estimate of solar forcing
The lightest water masses are related to barrier
layers
80
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81
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82
First topic The Deep Overturning
2. Our modelling results

1. NADW upwelling and transformation in the SO
2. AABW, geothermal heating and vertical mixing

83
Second topic The Shallow Overturning

• The SO as producer and exporter of lower
  thermocline waters

1. An overview on the state of the art
84
Second topic The Shallow Overturning

• The SO as producer and exporter of lower
  thermocline waters

2. Our modelling results
85
Second topic The Shallow Overturning
2. Our modelling results

1. Freshwater export
2. The Pacific AAIW

86
SO Thermocline Waters

• SAMW Formed by deep cooled winter mixed layers
  north of ACC.
• Subducted in the Sub Antarctic Front.
• AAIW Recirculates upwelled NADW.
• Subducted in the Antarctic Frontal Zone.

- Link between airsea interaction in Southern
Ocean and thermocline variability further north
(Rintoul and England, 2002) - Determine the
characteristics of the global ocean (500m-1500m)
(Sarmiento et al. 2004)
Freshening of the Mode Waters global
warming? (Wang et al. 1999)
87
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88
What is the role of the Southern Ocean surface
forcing and mixing in the global conveyor?
How does the oceanic circulation redistribute
buoyancy in the ocean thus feeding the mechanical
and thermodynamic processes?
89
The Southern Ocean and the global carbon cycle
(2)
The results on the physics of the THC will used
to understand the role of the Southern Ocean,
recently proposed as the main player in the
global carbon cycle (CO2 sequestration).
MODEL
DIC
A bio-geochemical model is running now off-line
exploiting the same physical output here
presented.
LEVITUS
We started from the mean seasonal cycle (SZN, O.
Aumont (LOCEAN), F. DOrtenzio (LOV))
But the model mixed layer is too shallow More
work is needed
90
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91
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92
Connection with the northern limb of the Conveyor
belt
Atlantic
Indian
Pacific
93
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94
Perspectives
(1)
On the role of the Tropics in the thermohaline
circulation The solar irradiance in the
tropical and equatorial upwellings
Dianeutral velocities in the interior
Vertical mixing
Solar irradiance
We are also creating a new database of spectral
solar irradiance and water transparence (from
Chl) for inspecting their role in the water
masses transformations (F. DOrtenzio, LOV
(France) ISAC (Italy)
95
A new climatology of the upper ocean (ML depth,
T/S properties)
ML depth
Time Distribution
Salinity
Temperature
Recent data Temperature 20 of total Salinity
60 of total
96
Modelling the air-sea interface
The ML dynamics in the ORCA2 ocean model We will
implement new parameterizations
A non-hydrostatic LES model of the ML for
bio-physical studies (Univ. Napoli II, SZN)
?
100m
100m
100m
Convection onset
97
Some reasons to study the Southern Oceans the
global thermohaline circulation
Where do the deep waters upwell and close the
conveyor?
98
Because of the vertical structure of gn densities
at 32S in the model, we have defined the density
boundaries between the four schematic water
masses as follows
This way the AAIW boundaries (27.1 lt gn 27.5
for the Pacific sector) envelope the salinity
minimum layer of each southern basin.The values
in black are the total water mass transport for
the vertical section in the particular direction
indicated by the arrow (northward or southward,
and eastward or westward) The resulting
transport values (given in Sverdrup, 1 Sv 106
m3 s-1) across the different vertical sections
are obtained through the Lagrangian integration
(forward and backward in time) of hundred of
thousands to several millions of individual
trajectories of elementary water particles
(defined by a maximum transport value of 10-4 Sv).
99
The Antarctic Circumpolar Current (ACC)

• The West Wind Drift
• Bottom reaching
• Large volumes (gtgt100 Sv)
• Connects the major ocean basins
• Transports heat, freshwater and climate anomalies
  around the globe
• (At least) Two jets/fronts
• Isolates warmer subpolar surface water from the
  Antarctic

100

• IF AAIW is not exporting freshwater directly,
  what is the link between the salinity surface
  minimum in the AASF and the AAIW?

Salinity N-S section
Antarctica
Tasmania
No advective export
We performed a specific Lagrangian analysis,
using the section Tasmania-Antarctica as a
Poincaré section of the ACC transport. Particles
were released on the section and integrated
backward and forward until they reached the same
section or 30S. The results allowed to trace
the secondary circulation associated to the
salinity minimum in the ACC. The freshwater
export results to be diffusive while the volume
export occurs only via the Ekman surface export.
101
AAIW in a 1500y global ice-ocean simulation
Properties on the model AAIW isonormal surface
(the pathway has been computed with Lagrangian
diagnostics)
New AAIW injection 2.5 Sv
Salinity
Depth

• In particular
• Ison. depth has a plateau
• PV minimum is present even if the model AAIW is
  not ventilated locally (from Lagrangian
  integration)

SAMW
Layer thickness
PV
Iudicone et al. 2005
102
THE AAIW responds to remote large scale
forcings (in 100y-long sensitivity experiments)
Gnanadesikan (1999) AAIW export is the balance
between Ekman export and eddy bolus transport
The Indo. Th. Has been closed
The eddy bolus transport has been reduced by 50
(weaker ACC)
The Southern Ocean winds have been augmented by
50 (Stonger ACC)
Meridional velocity anomalies at 43S the main
response occurs close to the chilean coast and at
intermediate depth (as a baroclinic anomaly)
103
The AAIW injection is ruled by large scale
pressure gradients projected locally by
planetary waves an oceanic teleconnection?
Eq. pathway
Ison. slope anomaly gt Baroclinic response
Anomalies in the ACC pycnocline depth penetrate
into the subtropics via the PV window
Southern Ocean pathway
Iudicone et al. 2005
104
An Exchange Window for the Antarctic Intermediate
Water Injection into the South Pacific
The AAIW in the WOA-NODC 2001 DATASET
There is a large scale northward
flow. (700m/2500m dyn.h. are superimposed) The
pattern is coherent with tracers
The AAIW isoneutral surface 27.4
Ox
S
Si
PV
Iudicone et al. 2005
105
The dynamics of the upwelling of the CDW in the
Southern Ocean
Dianeutral velocities below the ML maps of H(?)
Vertical mixing
Solar irradiance
The solar irradiance can be important The
distribution matches the Lagrangian results
(diapycnal transport)
106
The circulation inferred from the data analysis
Surface circulation Intermediate
circulation Tracers pattern
ACC surface diversion
New pathway
Iudicone et al. 2005
107
The South-African subtropical connection
108
The role of deep diffusivity If most of
transformations occurs at the based of the ML or
along the A. shelf slopes does the deep open
ocean mixing play a role?
The same Eulerian and Lagrangian computations
were repeated with a similar simulation with deep
Kz 10-5 m2/s (instead of 10-4 m2/s) were
performed in order to the impact of deep mixing
on the THC. The main result is that there are
small changes in the transformations in the
Southern Ocean. But Overturning The NADW
upwelling is enhanced (of about 20-30) when Kz
is small. In fact NADW is only weakly
transformed in the path into and along the
Southern Ocean and thus more directly connected
to the surface. Circulation enhanced h.
recirculations while absolute transports are
insensitive to the Kz (deeper sub-tropical
gyres). Meridional heat transport it is almost
insensitive, as proposed recently, to the deep
Kz. In fact the sensitivity is not zero (10)
because the deep temperature anomalies partly
upwells in the (Pacific) Southern Ocean and
dissipates. This effect is not included in the
sector models of previous studies (e.g.,
Marotzke, 2003)
109
Sverdrups tentative conclusions are still open
questions
Review of Wüsts, Deacons and Sverdrups work in
Sverdrup et al. (1942)
What are the physical processes that transform
the water-masses in the Southern Ocean?
How do these physical processes relate to
water-mass pathways to which are intimately
related?
110
The role of deep diffusivity If most of
transformations occurs at the based of the ML or
along the A. shelf slopes does the deep open
ocean mixing play a role?
Meridional streamfunction
Meridional streamfunction anomaly
(reference run)
First new result (from Lagrangian analysis) the
upwelling of NADW increases if Kz decreases gt a
more direct route
111
The role of deep diffusivity In sector
(Atlantic-only) models heat transport is
insensitive to deep mixing (e.g., Scott and
Marotzke, 2002)
Meridional heat transport (PW)
T Anomaly (N-S sections)
Atlantic
Pacific
Indian
The heat anomaly is due to upwelling of
anomalously warm deep layers In the Southern Ocean
112
Impact of Geothermal Heating and Vertical Mixing
on the thermohaline circulation
comparison of simulated natural C14 with GLODAP
data
Dutay et al. 2005
DATA and Control simulation
Geothermal Heating uniform vs realistic
Vertical Mixing vs realistic Geothermal Heating
113
Buoyancy
Large fluxes and in fact positive but the NADW
does not reach the surface
114
The conveyor belt and the Southern Ocean
Models also suggest the NADW overturning cell is
closed by upwelling and water mass transformation
in the SO.
Döös and Coward (1997)
30y simulationgt unsteady
115
Overturning Circulation in the Southern Ocean
It ventilates the major oceanic intermediate and
bottom waters
Import North Atlantic Deep Water Export 1.
Bottom Water 2. Thermocline Waters
116
The Southern Ocean producer of AABW, the
densest world water mass
CFC inventory 8 Sv AABW 21 Sv total input to
deep ocean
Orsi et al., 2002
Orsi et al., 1999
117
AABW and NADW what is their link in terms of
global ocean circulation and climate ?
118
On the role of Southern Ocean in the global
thermohaline circulation in a OGCM
Daniele Iudicone
119
The novelty of the study A 3D formulation that
takes into account the penetration of solar
irradiance
The standard diagnostics
F(x0,y0)
Bf(H, E-P-R)f(x0,y0)
But HI(x,y,z)Q(x,y)
with I(z) I0e-z/k
and k 23m (Clear water)
Bf(x0,y0,z)B1B2

• Not all the heat goes into the isopycnal layer
  that outcrops at (x0,y0)

G(?)-?Ddiff/??B where B includes the
effective solar irradiance
Iudicone and Madec, 2005
120
A new quantitative estimate of water masses
transformation

• The aim is to evaluate the total dianeutral
  transport across isoneutral surfaces due to the
  interior mixing processes and surface buoyancy
  fluxes
• A function H(?) that has the dimension of a
  transport (m3/s) (? is neutral density).
• First, we remind that the evolution equation for
  the locally referenced potential density is
• where ?l is the locally referenced potential
  density, U is the velocity field. The first term
  on the right side represents the vertical mixing,
  the contribution from cabbelling and
  thermobaricity associated to the isoneutral
  diffusion and B is the sum of bottom boundary
  layer processes and surface and bottom fluxes.
• To a good approximation, the normal vector to a
  neutral density surface is parallel to the normal
  to the locally-referenced potential density
  surface (e.g., McDougall, 1987)

Local state variable gt Global state variable
121
A new quantitative estimate of water masses
transformation
We perform the volume integral only on the
specific surface by using a 3D delta function,
thus transforming the equation in density
coordinate, and the resulting expression for H(?)
is considering grid boxes of volume
Vi,j,k. The formula in fact allows to compute
the integral of the dianeutral fluxes across the
isosurface ?, decomposed into the thermodynamical
components and for any sub-domain. Note b is a
complex function of space and time and has no
state equation!
)
122
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123
Southern Ocean
Perceptions of the Southern Ocean overturning
circulation The very early works the myth
ltltIt appears that the mythology conceived
Hvergelmer the Maelstroem of Amlodhi as a vast
reservoir, the mother fountain of all the waters
of the world. In the front rank are mentioned a
number of subterranean rivers which rise in
Hvergelmer, and seek their courses thence in
various directions. But the waters of earth and
heaven also come from this immense fountain, and
after completing their circuits they return
thither.gtgt    . Lost in the depths of the
Southern Ocean, they were capable of giving the
Depths of the Sea to our forefatherst. Fragments
from Hamlet's Mill an Essay on Myth and the
Frame of Time. Giorgio de Santillana and Hertha
von Dechend. Boston, 1977.
The "God Boat" on the Arabian celestial globe
made by Tabari. P. Casanova, Bulletin of the
Institut Francais d'Archeologie Orientale 2,
Editions A. J. Picard et Cie., Paris, 1902
124
Dynamics of the overturning
Dynamics of the global overturning circulation
The present picture
NADW
AABW
Wunsch and Ferrari (2004)
125
Overview on the Southern Ocean
What drives the upper limb?
The deep mixing scenario (eg Wunsch and Munk
1998) is not confirmed by in situ data The
Southern Ocean wind-driven upwelling scenario is
gaining interest (Toggweilers, Gnanadsikans,
Huangs papers etc)
126
Dynamics of the overturning
Warm sources have to be deeper than cold ones The
Sandströms theorem the ocean is NOT a heat
engine
127
Impact of the correct estimate of solar forcing
Overestimation
Wrong sign
Surface Forcing (heat component)
Cross-isopycnal flux
E.g., in the Tropical Atl the atmosphere actually
warms the ocean on the whole density range
New
Mix-?SF
Cross-isopycnal flux
Mixing as a residual can be overestimated (e.g.,
in inverse models of in situ data) is
overstimated by gt100
128
Southern Ocean
Perceptions of the Southern Ocean overturning
circulation The early works(30-40) The
mother fountain of all the waters of the world
Wüst (1935), Deacon (1937), Sverdrup et al. (1942)
129
Overview on the Southern Ocean
The long sleep (50s-80s) Eyes are on the North
Atlantic
130
The water-mass transformations in the Southern
Ocean
The surface forcing
Total
Total
Heat
Heat
E-P
E-P
Density fluxes (10-6 kg m-2s-1)