tracers in ocean and climate models* matthew england cemap, school of mathematics
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Tracers in Ocean and Climate Models* Matthew England CEMAP, School of Mathematics The University of New South Wales. * See also www.maths.unsw.edu.au/~matthew/publications.html#MR98. Possible due to: GEOSECS, TTO, SAVE, WOCE, …. Why bother with tracers in models?. - PowerPoint PPT PresentationTRANSCRIPT
Tracers in Ocean Tracers in Ocean and Climate and Climate
Models*Models*
Matthew EnglandMatthew England
CEMAP, School of MathematicsCEMAP, School of Mathematics
The University of New South WalesThe University of New South Wales* See also www.maths.unsw.edu.au/~matthew/publications.html#MR98
Possible due to: GEOSECS, TTO, SAVE, WOCE, …..
Why bother with tracers in models?
•Ocean model “validation”
•Diagnosis of model circulation mechanisms
•Studies of the ocean carbon cycle
•Data assimilation studies
•Paleoceanographic considerations
•Ocean model “validation” (e.g. CFCs, 14C)
•Diagnosis of model circulation mechanisms (e.g. dye/age tracers, 39Ar)
•Studies of the ocean carbon cycle (carbon compounds, oxygen, phosphate, nitrate,…)
•Data assimilation studies (e.g. CFCs, tritium)
•Paleoceanographic considerations (e.g. carbon-13, oxygen-18)
Why bother with tracers in models?
100-1000 year ventilation
10-100 year ventilation
Robust diagnostic:T-S restored to observed in the
interior
Robust diagnosticObserved 14C
Prognostic
Supressed convection and vertical motion
Prognostic experiment:Interior T-S free to evolve
Toggweiler et al. [1989]
Chlorofluorocarbons
Plate 2. Distribution of CFC-12 on isopycnal surfaces corresponding to maximum NADW outflow in 1988 in the Redler and Dengg [1999] simulations. (a) In the 4/3° model, and (b) in the 1/3° model. The color bar indicates CFC concentrations in pmol/kg, with iso pycnal layer depths contoured (meters).
Ajax section in the South Atlantic
CDW
CDW
AAIW
AABW
Forcing functions for tracers ?
sea-ice
CFC
14C
3He
CFC14C
3He
CFC
CFC14C
CFC 14C
14C
CFC
14CCFC
Air-sea gas flux = f (k, ice, )
k = piston velocity ~ wind speed, U2 or U3
= solubility ~ SST, (SSS)
How to compute gas uptake:
•Use model-generated ice, winds, T-S?
•Use observed ice, winds, T-S?
Tracers in coupled climate models:
Both approaches can give an apparently goodtracer simulation but for the wrong reason
(see England and Maier-Reimer 2001 for details)
Case 1:
Spurious convection
Case 2:
Other tracer techniques:
•Age/Dye tracers
•Tracer data assimilation
•Off-line tracer models
(Cox, 1989, England 1995, O’Farrell 2000….)
(Haine 1999, Schlitzer 1996, …)
(Aumont 1998, Sen Gupta & England 2003)
Off-Line Tracer Model
Tracer Conservation Equation
OGCM
Horizontal
Velocity Fields
Source Terms
Mixing Terms
Tracer Concentration
T (x, y, z, t)
Continuity Equation
u , v w
•Interannual
•Seasonal
•Intraseasonal
•Water-mass source regions
•CFCs, 14C, 3He
•Radioactive waste
T, S
•Pollution, etc….
•Eddy statistics
•Isopycnal mixing
•GM (1990)
•Convective ML
•Wind Driven MLT, S, CFCs, 14C,….
PhD project: Alex Sen Gupta
Example: CFC simulations in a ¼ degree model
Integrated CFC content below 2000m
Year = 1980
PhD project: Alex Sen Gupta
Integrated CFC content below 2000m
Year = 2000
PhD project: Alex Sen Gupta
Integrated CFC content below 2000m
Year = 2020
PhD project: Alex Sen Gupta
Integrated CFC content below 2000m
Year = 2040
Advantages: • Independent assessment of model (complements T-S)• Can validate within ocean-only component• Complements GBC and carbon cycle modelling
Drawbacks:•Extra CPU (especially 14C)•Can manipulate outcome to some extent
Uncertainties:•Tracer source functions•Sparse data •How to formalise assessment?
Summary – Tracers in climate models