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: • Definition + Importance • in current state-of-the-art models (including MITgcm)   Salt Plume Scheme: • lab experiments • numerical experiments • parametrization, scalings • preliminary results  Mixing issues: • salt rejection • oceanic boundary layer Summary and Ongoing work

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]

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 • Snow simulation: [Hibler, 1979] • Initial conditions: Polar Science Center [Zhang et al., 1998]

Contributing factor:

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

Salt plume scheme:

2.

Laboratory experiments: Morton et al., [1956], Helfrich, [1993], Bush and Woods, [1999] Parameters:

F o N



V o

2   

g ρ a g



o



ρ a d



ρ o dz

1

t f

~

f N 2

: buoyancy frequency

f F

:

o

: initial buoyancy Coriolis frequency Scalings: Rotation unimportant

(1/f)/(1/N) = N/f

> 0.6

z M b M

  2 .

66  0 .

25 

F z M o

1 4

N

 1 2 Rotation important

z R b R

 

N/f

3 .

6 0 .

9  < 0.6

F o



F o f f

 2  2  1  1 4 4  a d  /dz b 1/f  o z

Salt plume scheme:

2.

Laboratory experiments Morton et al., [1956], Helfrich, [1993], Bush and Woods, [1999] (  o  a )/  o ~ 0.04-0.20

f = 0.1-1.65 s -1 N = 0.3-0.9 s -1 Reynolds number R ~300-500 Z m = 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] 2-D lead: width ~ 250m Halocline: at depth –40m dS/dz ~ 0.005 g/kg/m d  /dz ~ 0.004 kg/m 3 /m Salt flux =

w·

S ~ 2 x 10 -5 g/kg m/s Time scale ~ 6 hours 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 ]

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/m 3 /m d  /dz @ top of halocline ~ 10 -3 to 10 -2 kg/m 3 /m • KPP boundary layer depth:  ~30-40m  well mixed •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  same order of magnitudes as published numerical experiments

Salt plume scheme: sensitivity experiments 1) 2) 3) no plume, rejected salt goes to top layer |d  /dz| = 0.01 + bottom distribution |d  /dz| = 0.005 + uniform distribution salt

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 salt

No plume (top) bottom of dr/dz = 0.01

Uniform to dr/dz=0.005

1) Summary and Ongoing work 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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