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Naval Research Laboratory (NRL)
Mixed Layer Depth (NMLD) Climatology

U.S. Naval Research Laboratory, Stennis Space Center

Monthly climatologies of isothermal layer depth (ILD) and mixed layer depth (MLD) have been constructed at NRL. These monthly ILD and MLD fields have been used for validating the mixed layer in the early development of the global NLOM with an embedded mixed layer. They have also been used to study barrier layer formation in the North Pacific Ocean and to examine global ocean mixed layer depth characteristics.

These ILD and MLD fields are being made available to the scientific community as the NRL Mixed Layer Depth (NMLD) climatology.

NMLD Monthly Climatology

The NMLD climatology uses an optimal definition for determining ILD from temperature profiles and MLD from density profiles that can be applied in all regions of the world's oceans. This method can accommodate not only in situ data but also climatological data sets that typically have much lower vertical resolution.

The optimal MLD definition is able to accommodate the wide variety of temperature profiles that occur within the global ocean. This includes temperature inversions that occur at high latitudes, a subsurface mixed layer underlying a surface thermal inversion, multiple fossil layers beneath the surface mixed layer, a dicothermal layer (i.e. a layer of cold water sandwiched between the warmer surface and deeper layers), as well as the typical temperature profiles with strong and weak thermoclines found in the middle and low latitudes. The MLD definition accurately represents the depth to which turbulent mixing has penetrated and would be a useful aid for validation of one-dimensional bulk mixed layer models and ocean general circulation models with an embedded mixed layer.

In the NMLD climatology the ILD is defined based solely only on temperature while the MLD is defined using a density-based criterion that also accounts for variable salinity. These fields are constructed from the 1°monthly-mean temperature and salinity climatologies of the World Ocean Atlas 1994 (Levitus and Boyer 1994; Levitus et al. 1994) using a method for determining layer depth that can accommodate the wide variety of temperature and density profiles that occur within the global ocean. In brief, the ILD is the depth at which the temperature (Tb) has changed by an absolute temperature difference of D T=0.8° C from the temperature at 10 meters beneath the ocean surface (Tref), i.e. Tb = |Tref - D T|. The MLD is constructed using a density criterion based on a density variation (Dst) determined from the corresponding temperature change D T in the equation of state

Dst = st (T + D T, S, P) - st (T,S,P) ,

(Note: if a greek character does not appear here (\alpha) then your web browser does not support greek symbols. For a correct appearance of this page, try using another web browser or download the technical report below.)

where T and S are surface, or near surface, temperature and salinity values and P is set to zero. This latter criterion has been shown to provide an optimal representation of the depth of the mixed layer and is recommend as the most appropriate for model validation purposes because the optimal MLD definition takes full account of density changes due to temperature and salinity variations with location. The implementation of the criteria used requires a multiple-step procedure that is separately applied when determining an ILD or MLD. Further details on the method used to construct these ILD and MLD can be found in Kara et al. (2000a,b).

Pictures of the monthly ILD and MLD fields for latitudes 65° N to 72° S of the global ocean are available via the URLs below. Note that the optimal layer depth (i.e., mixed layer depth) is obtained using a density criterion based on a density variation (Dst) with D T=0.8° C

Technical Report

Further details on the NMLD climatologies can be found in the NRL technical report available below. This technical report also includes monthly mean climatological ILD and MLD fields (January through December) for D T values of 0.2, 0.5, 0.8, 1.0, 1.3, and 1.5° C, separately, over the global ocean. Because of the many color graphics, the report has been separated into 6 parts (a-f) to facilitate downloading over the web and printing on color printers.

Kara, A. B., P. A. Rochford, and H. E. Hurlburt, 2002: Naval Research Laboratory Mixed Layer Depth (NMLD) Climatologies. NRL Report No. NRL/FR/7330--02-9995, 26 pp. [Available from the Naval Research Laboratory, Code 7323, Stennis Space Center, MS 39529-5004, USA.]

FORTRAN 90 MLD Program

A sample FORTRAN 90 program implementing the optimal MLD algorithm with input and output data is available below. The input data is a 1992 time series obtained by the TAO array mooring located at (0°, 156°E) in the western Equatorial Pacific. Both ILD and MLD can be determined for any user supplied temperature difference criteria. The FORTRAN 90 application is modularized so the end user can easily modify it to meet their particular needs.


The NMLD climatologies are publicly available as formatted ASCII and network Common Data Format (netCDF) data sets. They can be downloaded via the links below or at (ftp address).


Kara, A. B., P. A. Rochford, and H. E. Hurlburt, 2003: Mixed layer depth variability over the global ocean. J. Geophys. Res., 108, doi:10.1029/2000C000736.

Kara, A. B., P. A. Rochford, and H. E. Hurlburt, 2000a: An optimal definition for ocean mixed layer depth. J. Geophys. Res., 105, 16 803-16 821.

Kara, A. B., P. A. Rochford, and H. E. Hurlburt, 2000b: Mixed layer depth variability and barrier layer formation over the North Pacific Ocean.J. Geophys. Res., 105, 16 783-16 801.

Levitus, S., R. Burgett, and T. P. Boyer, World Ocean Atlas 1994, vol. 3, Salinity, NOAA Atlas NESDIS 3, 99 pp., U.S. Govt. Print. Off., Washington, D.C., 1994.

Levitus, S., and T. P. Boyer, World Ocean Atlas 1994, vol. 4, Temperature, NOAA Atlas NESDIS 4, 117 pp., U.S. Govt. Print. Off., Washington, D.C., 1994.

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Last revised: Thu Feb 23 14:34:35 CST 2006