Southern Ocean watermasses in climatescale models

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Southern Ocean water-masses in climate-scale
models
Matthew England Centre for Environmental
Modelling and Prediction The University of New
South Wales www.maths.unsw.edu.au/matthew M.Engla
nd_at_unsw.edu.au
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OUTLINE

• Southern Ocean water masses
• SOWM in climate models 1980s, 1990s
• Todays climate models
• The future?

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Southern Ocean water-masses
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Rintoul and England (2002) JPO
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Animation of seasonal cycle in MLD
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Why bother capturing Southern Ocean water-masses?
Linked to global ocean THC, wind-driven
circulation, poleward heat transport, Tied to
key climate indices such as oceanic CO2 uptake,
rate of change of SST/TAIR due to atmos CO2 rise,
Well-constrained by observations, unlike
(see above list)
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Why is it so hard to get right?

• Depends on correct representations of
• Air-sea and ice-sea fluxes (heat, freshwater
  and momentum)
• Ocean processes
• Downslope flows, entrainment fluxes
• Open ocean and coastal convection, mixed layer
  physics
• Isopycnal and diapycnal mixing
• Eddy fluxes
• Baroclinic flow
• Ekman pumping

Speer, Rintoul and Sloyan, JPO, 2000
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Back in the 80s .
Annual-mean temperature change predicted for
the year 2050 in the GFDL coupled climate model
experiment (Manabe et al. 1989).
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AAIW
NPIW
AABW
Observed
Pacific Ocean Salinity
Climate model (MS, 1988)
Climate model (MS, 1991)
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AAIW
NPIW
AABW
Observed
Pacific Ocean Salinity
Poor representation of Antarctic sea-ice
Excessive cross-isopycnal mixing
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Model tuning, circa 1990
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NADW
AABW
AAIW
Observed
Atlantic Ocean Salinity
Modelled
60ºN
40ºN
20ºN
EQ
60ºS
40ºS
20ºS
80ºS
Latitude
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Toggweiler and Samuels (1995) JPO
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FW flux (S S)
Griffies (2004)
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FW flux (m yr -1)
75S
45S
60S
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Why is getting the correct T-S not enough?
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Radiocarbon GEOSECS Pacific section
Diagnostic Simulations
Prognostic Simulation
Toggweiler et al. 1989a,b JGR
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Geochemical tracers in ocean models
World Ocean Circulation Experiment
Bomb Radiocarbon
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AABW
CDW
Geochemical tracers in ocean models
Spurious convection at 50-60S
Weak CDW upwelling
Broad, sluggish AABW overturn
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Danabasoglu et al. 1994 England, 1995, 1999
Hirst and McDougall, 1996, 1998,
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Observed and modelled radiocarbon
England and Rahmstorf (1999) JPO
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OCMIP
http//www.ipsl.jussieu.fr/OCMIP/
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Southern Ocean abyssal CFC-11
Doney and Hecht (2002) JPO
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Griffies (2004)
Beckmann and Doscher, 1997, Campin and Goosse,
1999 Gnanadesikan, et al 2000
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Doney and Hecht (2002) JPO
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Todays IPCC class of models
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CSIRO Mk2 climate model
JPO
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Interannual to Centennial Variability of Southern
Ocean Water Masses
with Agus Santoso, Steve R. Rintoul, Tony
Hirst, Siobhan OFarrell
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Rintoul and England 2002, JPO, 1308-1321
Santoso and England (JPO, 2004)
Santoso, England, Hirst (JPO, accepted)
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Russell et al. (2005)
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Russell et al. (2005)
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• Errors in interior model T-S are a result of at
  least one of the following
• erroneous surface T-S
• (2) spurious rates of ocean overturn within the
  surface ML
• (3) incorrect interior ocean circulation
• (4) unrealistic mixing processes in the model.
• Errors in surface T-S may themselves be a result
  of
• incorrect air-sea heat / FW / momentum fluxes,
• errors in surface circulation and mixing.
• ? Diagnosis of subsurface ocean model T-S against
  observations is not unambiguous errors may be
  symptomatic of any number of problems in ocean
  model forcing, circulation and/or physics.

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Parameter sensitivity studies ought to still be
in at the fore of our efforts
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Parameter sensitivity studies ought to still be
in at the fore of our efforts
Smax
Gnanadesikan, Griffies, and Samuels (OM, 2005)
? maximum eddy-induced advective transport of Agm
Smax
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Effects in a climate model of slope tapering in
neutral physics schemes Gnanadesikan, Griffies,
and Samuels (2005)
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Eddy-permitting and eddy-resolving models
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Model assessment metrics

• EKE density
• Baroclinic/barotropic flow
• Poleward HT
• Property transports in density classes
• Variability of the above
• Water masses????

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Webb et al. FRAM
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If water masses are analysed in high-resolution
ocean models over short integration times (10-30
years) their T-S fields are very near initial
conditions below 300-m hence using T-S to
assess skill is meaningless.
b) Model
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• Global eddy resolving models ( 1/10o ) are
  computationally expensive
• Water-mass assessment requires tracers T, S,
  chemical tracers, age of water,.
• Tracer equation can adopt a relatively large
  time step, ?t
• Hence high-resolution models can be assessed for
  their representation of geochemical tracers

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Off-Line Tracer Model
OGCM Horizontal Velocity Fields
Continuity Equation
u , v
w
Source Terms
Mixing Terms
Tracer Conservation Equation
Tracer Concentration T (x, y, z, t)
T, S, CFCs, 14C,.
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Off-Line Tracer Model

• Interannual
• Seasonal
• Intraseasonal

OGCM Horizontal Velocity Fields
Continuity Equation
u , v
w
Source Terms
Mixing Terms
Tracer Conservation Equation

• Water-mass source regions
• CFCs, 14C, 3He,
• Radioactive waste
• ?T, ?S (small D)
• Larvae, etc

• Eddy statistics
• Isopycnal mixing
• GM (1990)
• Convective ML
• Wind Driven ML

Tracer Concentration T (x, y, z, t)
T, S, CFCs, 14C,.
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Applications hypothetical spread of radioactive
traces
Hazell and England, J. Enviro. Rad.
Hazell and England, 2003, J. Enviro. Rad.
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Applications (contd) Moon jellyfish advective
pathways
Dawson, Sen Gupta, and England, 2005 Coupled
biophysical global ocean model and molecular
genetic analyses identify multiple introductions
of cryptogenic species. Proc. Nat. Acad.
Sciences, 102, 11968-11973.
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Geochemical tracer simulations derived from
off-line models using eddy-resolving GCMs

• Preliminary example CFC uptake in the POCM
  simulation
• Model includes POCM advective fields, vertical,
  isopycnal and biharmonic mixing, and a pre-run
  diagnosed convective mixed layer. All parameters
  include a seasonal cycle.
• Run model for 100 200 years simulation time
  (NB multi-1000 yr simulations become feasible
  on modest computing facilities so applications
  to age and 14C are also possible, Sen Gupta and
  England, 2004)

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Global animation of ocean CFC spreading gt2000m
POCM ¼-degree global ocean model
Sen Gupta England, JPO, 2004
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AABW flow pathways
Sen Gupta England, JPO, 2004
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NADW CFC content
Modelled
Observed
Sen Gupta England, JPO, 2004
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AABW CFC content
Sen Gupta England, JPO, 2004
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THC stability and mixing in global ocean models
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THC stability and mixing in global ocean models
Sijp, Bates, England (2005) Journal of Climate
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Sijp, Bates, England (2005)
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Convection dominates
Isopycnal mixing dominates
Isopycnal mixing dominates
Non-perturbed states diagnosis of what
processes remove water from the surface North
Atlantic in ocean models
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GM more stable to FW perturbations despite weaker
initial MOT rates
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FW fluxes out of the surface layer
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Sijp, Bates, England (2005) Journal of Climate
Model isopycnal diffusion rates control the
stability of the oceans THC
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Recommendations

• Water-masses (T-S and chemical tracers) remain
  the key metrics for assessing IPCC-class ocean
  models
• Sensitivity and process studies should inform
  and guide development directions
• e.g. Gnanadesikan et al., US WOCE Process teams
• Key water mass physics issues Vertical
  coordinates (z, hybrid, ), partial-cell
  techniques, BBL schemes, inhomogenous mixing, ice
  shelves, polynyas, eddy-ML interactions, gravity
  currents, convection, tidal effects,
• OM assessment probably has to continue across
  each of the genres (climate models, ICCMs,
  ocean-only models)
• No model can escape water-mass / ventilation
  assessment.