Mixed Layer lateral eddy fluxes mediated by air-sea interaction

The published study

The modulation of air-sea heat fluxes by geostrophic eddies due to the stirring of temperature at the sea surface is discussed and quantified. It is argued that the damping of eddy temperature variance by such air-sea fluxes enhances the dissipation of surface temperature fields. Depending on the timescale of damping relative to that of the eddying motions, surface eddy diffusivities can be significantly enhanced over interior values. The issues are explored and quantified in a controlled setting by driving a tracer field, a proxy for sea surface temperature, with surface altimetric observations in the Antarctic Circumpolar Current (ACC) of the Southern Ocean. A new, tracer-based diagnostic of eddy diffusivity is introduced which is related to the Nakamura effective diffusivity. Using this, the surface lateral eddy diffusivities associated with (i) eddy stirring and small-scale mixing and (ii) surface damping by air-sea interaction is quantified. In the ACC, a diffusivity associated with surface damping of a comparable magnitude to that associated with eddy stirring (~500m2/s) is found. In frontal regions prevalent in the ACC, an augmentation of surface lateral eddy diffusivities of this magnitude is equivalent to an air-sea flux of 100W/m2 acting over a mixed layer depth of 100m – a very significant effect. Finally, the implications of our results for other tracer fields such as salinity, dissolved gases and chlorophyll are discussed. Different tracers are found to have surface eddy diffusivities that differ significantly in magnitude.

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Datas

To perform this analysis I used the cycle1 of a 1/8 global MITgcm simulation:

2000-2005 integration,

1/8-deg horizontal grid spacing,

50 vertical levels,

forced by NCEP reanalysis-1 surface atmospheric state and bulk formulae,

surface relaxation to montly Levitus SSS with a relaxation time constant of 44.5 days,

with full dynamic thermodynamic sea-ice model

Its sweet name is: lle.nb.01 and datas are available at MIT/CSAIL here:

/net/ds-05/scratch-0/cnh/bulk_eight_001

More informations can be found here: http://data.guillaumemaze.org/ecco2-mitgcm

Binary list: http://data.guillaumemaze.org/ecco2-mitgcm#TOC-Cycle-1:-ll2.nb.01-:-2000--2005

Netcdf list: http://data.guillaumemaze.org/ecco2-mitgcm#TOC-Cycle-1:-ll2.nb.01-:-2000--2005-bro

How to compute stuff for Eddy diffusivity with 1/8 simulations ?

How to get basic input fields ?

It is assumed that all fields are already in netcdf, one file per time step, one folder per time step

work/bin2netcdf/cycle1/write_cdf2cdf_getdailymean_v2.m

work/bin2netcdf/cycle1/compute_monthlymean_crossfields.m

and

work/bin2netcdf/cycle1/compute_monthlymean.m

RQ: The following script allows to extract 6-hourly netcdf files from original binaries on the csail network and to send them to Beagle

write_bin2cdf_subdomain_2D_and_send_to_beagle.m

How to get eddy fields ?

/work/ecco2/cycle1/Eddy_diffusivity/1_compute_timeavr_2D_all.m

/work/ecco2/cycle1/Eddy_diffusivity/2_compute_eof_2D.m

/work/ecco2/cycle1/Eddy_diffusivity/3_compute_SeasCycbar_2D_18.m

/work/ecco2/cycle1/Eddy_diffusivity/4_compute_Eddycomponents.m

/work/ecco2/cycle1/Eddy_diffusivity/make_ALLsurfaceVARI.m

How to get air-sea induced eddy diffusivity ?

/work/ecco2/cycle1/Eddy_diffusivity/5_compute_timeavr_3D_v2.m

/work/ecco2/cycle1/Eddy_diffusivity/6_compute_gradTbar_3D_v2.m

/work/ecco2/cycle1/Eddy_diffusivity/7_compute_gradTbar_MLD.m

/work/ecco2/cycle1/Eddy_diffusivity/8_compute_Kairsea_2D.m