HYCOM GOFS 3.0 Global Analysis Ocean Mixed Layer Thickness

[Shelby Bloom]

Hello,

My name is Shelby Bloom and I am a Master's student at Scripps Institution of Oceanography. I am interested in downloading and incorporating ocean mixed layer thickness data from HYCOM for my research. More specifically, I am interested in downloading ocean mixed layer thickness data from the HYCOM + NCODA Global 1/12° Analysis (GLBa0.08) data server. I am reaching out to you in the hopes that you could explain what the difference is between the two different ocean mixed layer thickness variables provided in this dataset, namely mld (ILT) and mlp (MLT)? Please let me know. Thank you.

Best,

Shelby Bloom

[Alan Wallcraft]

Where (which expt and which files)  are you seeing mld and mlp?

Alan.

[Shelby Bloom]

Hi Alan,

I am seeing mld and mlp for ocean mixed layer thickness in GLBa0.08/expt_91.2 in the 'Data at 00Z (mlt)' files for all 3 years available (2016, 2017, 2018).

I have attached pictures below for reference and to help show which files I used. I accessed these files using the NetcdfSubset option.  Let me know if this is enough information.

Best,

Shelby Bloom

[Alan Wallcraft]

ilt is the mixed layer depth based on the temperature difference w.r.t. the surface value.

ilt:long_name = " ILT (0.20 degC)   [91.2H]" ;

mlt is the mixed layer depth based on the sigma2 potential density difference w.r.t. the surface value.

mlt:long_name = " MLT (0.03 kg/m3)  [91.2H]" ;

Alan.

[Shelby Bloom]

Hi Alan,

Thanks for the information.

Just to make sure I understand correctly,

The mld variable is the ocean mixed layer thickness based on the temperature difference with respect to the surface value.

The mlp variable is the ocean mixed layer thickness based on the sigma2 potential density difference with respect to the surface value.

Let me know if my interpretation is correct. Thank you!

-Shelby

[Alan Wallcraft]

Correct.  Alan,.

[Shelby Bloom]

Great! Thank you so much!!!

-Shelby

[Shelby Bloom]

Hi Alan,

Circling back to this email chain because I have a clarification question.

You said that the variable mlt is the mixed layer thickness based on the sigma2 potential density difference with respect to the surface value. Could you clarify what sigma2 means in this context? Does it refer to the potential density reference pressure (2000dbar) used for the vertical coordinates, as discussed in this paper and this presentation? Or does it refer to the reference pressued used for potential density when calculating differences with the surface value (although I don't know why it would be if we are calculating surface mixed layer depths)? 

I look forward to hearing from you for my own understanding. Thank you for your time.

Best,

Shelby Bloom

[Alan Wallcraft]

The model's potential density is sigma2, and it is this field that is used in the MLT calculation (it is the only potential density we have).  So the density difference in question is sigma2 at a given depth minus sigma2 at the surface.

Both sigma0 and sigma2 potential densities are functions of potential temperature (w.r.t the surface) and salinity only, but the sigma2 value will give a larger jump for the same change in potential temperature so a density jump of 0.03 might lead to a shallower MLT  in sigma2 than in sigma0.

Alan.

[Shelby Bloom]

Hi Alan,

Thank you for clarifying that for me! I appreciate the thorough response and explanation. 

Best,

Shelby Bloom