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

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 Motivation: to realistically model the halocline

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

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]

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]

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

Salt plume scheme: 2.Laboratory experiments: Morton et al., [1956], Helfrich, [1993], Bush and Woods, [1999] Parameters: F o : initial buoyancy N 2 : buoyancy frequency f: Coriolis frequency b z  a d  /dz oo 1/f Scalings: Rotation unimportant (1/f)/(1/N) = N/f  > 0.6 Rotation important N/f < 0.6

Salt plume scheme: 2.Laboratory experiments Morton et al., [1956], Helfrich, [1993], Bush and Woods, [1999] (  o -  a )/  o ~ f = s -1 N = s -1 Reynolds number R ~ Z m = m Initial overshoot Neutral buoyacy Horizontal spread, controlled by rotation

Salt plume scheme: 3.Numerical experiments in the Arctic: Kozo [1983], Smith et al. [1993, 1998, 2002] Conclusion [Smith and Morison, 1998] : 1)“lead convection plumes are of insufficient buoyancy to penetrate the halocline.” 2)Salt rejected: a) not mixed uniformly in mixed layer b) sinks to base of mixed layer c)makes mixed layer shallower and more stratified 3)Supported by observations during LeadEx in 1992 [ Morison and McPhee, 1998 ] 2-D lead: width ~ 250m Halocline: at depth –40m dS/dz ~ g/kg/m d  /dz ~ kg/m 3 /m Salt flux = w·S ~ 2 x g/kg m/s Time scale ~ 6 hours

Salt plume scheme: 1.Duffy et al. [1997, 1999] in the Southern Ocean

Salt plume: Parametrization in the MITgcm Density structure :  d  /dz in mixed layer: to kg/m 3 /m  d  top of halocline ~ to kg/m 3 /m KPP boundary layer depth:  ~30-40m  well mixed  same order of magnitudes as published numerical experiments 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

Salt plume scheme: sensitivity experiments salt depth 1)no plume, rejected salt goes to top layer 2)|d  /dz| = bottom distribution 3)|d  /dz| = uniform distribution

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

Summary and Ongoing work 1)Issue with representation of vertical mixing of salt rejected during freezing in the current coarse resolution MITgcm model 2)Theory + Lab experiments + hi-res numerical simulations suggest salt should mix down vertically, that sub-grid vertical mixing of salinity can not neglected 3)Antarctic ocean, similar issue was addressed by Duffy et al by mixing salt below the mixed layer 4)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 5)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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