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Salt rejection, advection, and mixing in the MITgcm coupled ocean and sea-ice model

Salt rejection, advection, and mixing in the MITgcm coupled ocean and sea-ice model. An T. Nguyen, D. Menemenlis, R. Kwok, JPL/Caltech. AOMIP/(C)ARCMIP / SEARCH for DAMOCLES Workshop, Paris Oct 29-31, 2007. Motivation : to realistically model the halocline. Outline:. Halocline:

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Salt rejection, advection, and mixing in the MITgcm coupled ocean and sea-ice model

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  1. Salt rejection, advection, and mixing in the MITgcm coupled ocean and sea-ice model An T. Nguyen, D. Menemenlis, R. Kwok, JPL/Caltech AOMIP/(C)ARCMIP / SEARCH for DAMOCLES Workshop, Paris Oct 29-31, 2007

  2. Motivation: to realistically model thehalocline Outline: • Halocline: • Definition + Importance • in current state-of-the-art models (including MITgcm) • Mixing issues: • salt rejection • oceanic boundary layer • Salt Plume Scheme: • lab experiments • numerical experiments • parametrization, scalings • preliminary results • Summary and Ongoing work

  3. Halocline • A stratified salinity pycnocline at near freezing temperature between 50-200m • Prevents heat in deep water masses from reaching the surface and melt sea ice [Rudels, 2004]

  4. Halocline: in AOMIP participating models [Holloway, 2007]“linear thermal stratification in the upper ocean (spanning approximately 50 to 200 m).”“…observations contradict the models, showing instead the more-nearly isothermal layer characteristic of a cold halocline.” [Holloway, 2007]

  5. MITgcm • Ocean model: • ~ 18km horizontal, 50 vertical levels • volume-conserving, C-grid • Surface BC’s: NCEP-NCAR reanalysis • Initial conditions: WGHC • bathymetry: ETOPO2 • KPP mixing [Large et al., 1994] • Sea ice model: • C-grid, ~ 18km • 2-catergory zero-layer thermodynamics [Hibler, 1980] • Viscous plastic dynamics [Hibler, 1979] • Initial conditions: Polar Science Center • Snow simulation: [Zhang et al., 1998]

  6. Contributing factor: Alaska Greenland Siberia • Brine rejection: • salt in top 18km x 18km cells • excessive convection • deepening KPP boundary layer

  7. Salt plume scheme: Parameters: ro ra dr/dz Fo: initial buoyancy N2: buoyancy frequency b f: Coriolis frequency Scalings: 1/f Rotation unimportant (1/f)/(1/N) = N/f> 0.6 Rotation important N/f< 0.6 z • Laboratory experiments: Morton et al., [1956], Helfrich, [1993], Bush and Woods, [1999]

  8. Salt plume scheme: Neutral buoyacy • Laboratory experimentsMorton et al., [1956],Helfrich, [1993], Bush and Woods, [1999] (ro-ra)/ro ~ 0.04-0.20 f = 0.1-1.65 s-1 N = 0.3-0.9 s-1 Reynolds number R ~300-500 Zm = 0.13-0.20m Horizontal spread, controlled by rotation Initial overshoot

  9. Salt plume scheme: • Numerical experiments in the Arctic: Kozo [1983], Smith et al. [1993, 1998, 2002] 2-D lead: width ~ 250m Halocline: at depth –40m dS/dz ~ 0.005 g/kg/m dr/dz ~ 0.004 kg/m3/m Salt flux = w·S ~ 2 x 10-5 g/kg m/s Time scale ~ 6 hours • Conclusion [Smith and Morison, 1998] : • “lead convection plumes are of insufficient buoyancy to penetrate the halocline.” • Salt rejected: • not mixed uniformly in mixed layer • sinks to base of mixed layer • makes mixed layer shallower and more stratified • Supported by observations during LeadEx in 1992 [Morison and McPhee, 1998]

  10. Salt plume scheme: • Duffy et al. [1997, 1999] in the Southern Ocean

  11. Salt plume: Parametrization in the MITgcm • Sea ice forms • average salt flux, assume “plume” at sub-grid scale of 1-10% of cell’s area: w·S ~ 5x10-4 to 5x10-5 g/kg m/s • Density structure: • dr/dz in mixed layer: 10-6 to 10-4 kg/m3/m • dr/dz @ top of halocline ~ 10-3 to 10-2 kg/m3/m • KPP boundary layer depth: • ~30-40m • well mixed  same order of magnitudes as published numerical experiments

  12. Salt plume scheme:sensitivity experiments salt depth • no plume, rejected salt goes to top layer • |dr/dz| = 0.01 + bottom distribution • |dr/dz| = 0.005 + uniform distribution

  13. Salt plume scheme:sensitivity experiments salt depth No plume (top) bottom of dr/dz = 0.01 Uniform to dr/dz=0.005 Take home message: Oceanic mixed layer and halocline are highly sensitive to salt rejection Calibration: of the parameters such as dr/dz_critical, area/concentration will enable us to fit S/T data

  14. Summary and Ongoing work Issue with representation of vertical mixing of salt rejected during freezing in the current coarse resolution MITgcm model Theory + Lab experiments + hi-res numerical simulations suggest salt should mix down vertically, that sub-grid vertical mixing of salinity can not neglected Antarctic ocean, similar issue was addressed by Duffy et al by mixing salt below the mixed layer Exploring similar schemes in the Arctic Ocean and have shown that solution is extremely sensitive to the way salt is rejected from the ice formation Will explore calibrating free parameters of this salt rejection processes, along with other parameters & IC’s and BC’s using ocean-state estimating

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