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 50-200m• 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]

MITgcmOcean 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:

a

oaoo ρ

ρρgVF

Fo: initial buoyancy

dz

dgN

o

2 N2: buoyancy frequency

ft f

1~ f: Coriolis frequency

b

z

a

d/dz

o

1/fScalings:

2

14

166.2 NFz oM

MM zb 25.0

Rotation unimportant

(1/f)/(1/N) = N/f > 0.6

41

26.3 fFz oR

41

29.0 fFb oR

Rotation important

N/f < 0.6

Salt plume scheme:

2. Laboratory experimentsMorton et al., [1956],Helfrich, [1993], Bush and Woods, [1999]

(o-a)/o ~ 0.04-0.20f = 0.1-1.65 s-1

N = 0.3-0.9 s-1

Reynolds number R ~300-500Zm = 0.13-0.20m

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 layerb) sinks to base of mixed layerc) makes mixed layer shallower and

more stratified3) Supported by observations during LeadEx

in 1992 [Morison and McPhee, 1998]

2-D lead: width ~ 250mHalocline: at depth –40m dS/dz ~ 0.005 g/kg/m d/dz ~ 0.004 kg/m3/mSalt flux = w·S ~ 2 x 10-5 g/kg m/sTime 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:

10-6 to 10-4 kg/m3/m d/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

•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

dept

h

1) no plume, rejected salt goes to top layer2) |d/dz| = 0.01 + bottom distribution3) |d/dz| = 0.005 + 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 = 0.01 Uniform to dr/dz=0.005

salt

dept

h

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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