IAEA: Feb 2004 Coupled modelling of water isotopes in the GISS GCM Gavin Schmidt NASA GISS and Center for Climate Systems Research, Columbia University
Focus of GISS water isotope research Forward modelling of paleo-proxies Uses ocean-atmosphere-sea ice coupled models LGM vs. Present Day Use of isotopes as constraints on Cloud processes (i.e. MC entrainment) Stratosphere-troposphere water vapour exchange Sensitivity to isotope processes Kinetic fractionation, cloud phase, super-saturation... i.e. Cappa et al (2003) Bomb Tritium water delivery to surface, stratospheric turnover time
Modelling water isotopes Follow all water fluxes in model evaporation, cloud water, precipitation sea ice, lakes, ground water, rivers ocean freshwater advection, mixing Fractionate isotopes (with kinetic effects) at changes of phase surface evaporation (MJ79) condensation of vapour in clouds (Jouzel82) re-evaporation of precip/cloud liquid water formation of sea ice Uncertainties in parameterisation of kinetic effects mostly affect deuterium excess (d=δd-8 δ 18 O)
Modelling water isotopes: Issues Fractionation in moist convective plumes Instantaneous removal or equilibrium with layer? Only first method gives resolution-independent result! Water vs. Ice clouds Big differences seen when making changing amount of super-cooled clouds Connection between upper troposphere values in tropics to Antarctic d-excess... Cappa fractionation (more later...)
Spatial patterns... δ 18 O Precipitation δ 18 O Surface Ocean
Matching Station data... δ 18 O δd d-excess
Cappa et al (2003) fractionation... Reworking of Merlivat and Jouzel (1979) experiments Adjustment for evaporative cooling New diffusion coefficients for H 2 18 O and HDO less diffusive H 2 18 O, more diffusive HDO New kinetic fractionation for surface evaporation
Using Cappa et al (2003) fractionation? Implemented in GISS GCM: New kinetic evaporation from ocean New diffusion coefficients Adjustment of ocean skin temperature for heat fluxes Control envelope d-excess Cappa envelope d-excess
Stratospheric water vapour Observed value of water entering through tropical tropopause (McCarthy et al,(in press)): δd=-653 +/- 17 δ 18 O=-128 to -248 Estimates for simple Rayleigh distillation models? -800/-900, -120/-140 Is discrepancy related to STE processes for water?: Ice lofting? Convective overshoot? Gradual ascent? i.e. Sherwood and Dessler (2001;2003), Keith (2000), Webster et al (2003)
Stratospheric water vapour In GISS GCM? 20 layer model: -675 (-108 ) 23 layer model: -770 (-134 10% extra MC entrainment in plumes? + 20-25 Force more ice clouds? + 10-15 Cappa fractionation? <5 ) -640-100 -50 0 +50
Upper tropospheric data? CRYSTAL-FACE in-situ data (Webster and Heymsfield 2003) total water isotopes (including cloud water/ice) Near tropopause, sub tropics A lot of scatter - noise? big differences in Harvard/ALIAS total water instruments, but...
Forward modelling of foram δ 18 O carbonate 6 common species Seasonal weighting of δ 18 O carb (T,δ 18 O w ) values by... Temperature Density structure Mixed layer Pycnocline Schmidt and Mulitza (2002)
Forward modelling of foram δ 18 O carbonate Surface Equilibrium Calcite Best-fit Ecological profiles Using observed d18o seawater
Present day vs. LGM coupled runs Thesis work from Duane Thresher (Columbia) Multi-centennial coupled ocean-atmosphere-sea ice runs for LGM and present day boundary conditions Include water isotope tracers Compare coretop δ 18 O c and LGM δ 18 O c :
LGM Sea Surface Temperature LGM LGM -Present day Tropical cooling ~ 3.5 C
LGM difference in ocean δ 18 O w? δ 18 O w LGM -PD Estimate from modelled changes to coretop carbonate δ 18 O w = δ LGM c - δ PD c cores - (δ LGM c - δ PD c ) model = 1.0 Completely independent of other methodologies!
Other paleo examples Isotope paleothermometry in ice cores - Present day Greenland precip shows spatial gradient of 0.67 / C Borehole temperature estimates implies twice as much temperature change at LGM Previous results implicate changes in seasonality and source region as key factors (Werner et al 2000) GISS CGCM: spatial1.1-1.2 / C, temporal 0.32 / C Isotope variability at D-O oscillations, connection with ocean changes, need calibrating Equator-Pole δ 18 O gradients during Eocene/Cretaceous
Conclusions Water isotopes are extremely useful in GCMs - Needed for paleo data-model comparisons Good check on model hydrological cycle Possible constraints of cloud processes/ste Good match to precip data, surface ocean gradients Clear need for more validating data in upper troposphere Large sensitivity to small changes in MC But, always gives less depleted than Rayleigh distillation... => Impact of ice lofting into stratosphere overestimated? Also need more: Experimental/observational data for validating physical parameterisations Forward models (forams, speleothems, tree cellulose, lake carbonates...)
Thanks to: Duane Thresher Allegra Legrande Georg Hoffmann, Jean Jouzel, CEA France Drew Shindell, David Rind, Jim Hansen, NASA GISS