# Presentation: 724768

## 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**

- 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]

## 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:

- Brine rejection:
-  salt in top 18km x 18km cells
-  excessive convection
-  deepening KPP boundary layer

## Salt plume scheme:

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

## Salt plume scheme:

- Laboratory experimentsMorton 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
- Zm = 0.13-0.20m

- Initial overshoot

- Neutral buoyacy

- Horizontal spread, controlled by rotation

## Salt plume scheme:

- Numerical experiments in the Arctic: Kozo [1983], Smith et al. [1993, 1998, 2002]

- _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]

- 2-D lead: width ~ 250m
- Halocline: at depth –40m                  dS/dz ~ 0.005 g/kg/m
-                   d/dz ~ 0.004 kg/m3/m
- Salt flux = *w·*S ~ 2 x 10-5 g/kg m/s
- Time scale ~ 6 hours

## Salt plume scheme:

- 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

## Salt plume scheme: sensitivity experiments

- no plume, rejected salt goes to top layer
- |d/dz| = 0.01   + bottom distribution
- |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

## 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

## BLANK