MLD dataset information page

Short Description

Mixed layer schematic.jpg
Schematic overview of an ocean mixed layer

Statement: Ocean mixed layer depth climatology

Abstract: The global Mixed Layer Depth (MLD) Climatologies available here are computed from more than 5 million individual profiles obtained from the National Oceanographic Data Center (NODC), from the World Ocean Circulation Experiment (WOCE) database, and from the ARGO program. Those are all the high vertical resolution data available since 1941 until 2008, including mechanical bathythermograph (MBT), expendable bathythermograph (XBT), conductivity-temperature-depth probes (CTD), and profiling floats (PFL).

The MLDs are estimated directly on individual profiles with data at observed levels. The MLD is defined through the threshold method with a finite difference criterion from a near-surface reference value. A linear interpolation between levels is then used to estimate the exact depth at wich the difference criterion is reached. The reference depth is set at 10 m to avoid a large part of the strong diurnal cycle in the top few meters of the ocean. The optimal temperature criterion is found to be 0.2 °C absolute difference from surface. The optimal one in density is 0.03 kg/m3 difference from surface.

Reduction of the data is done on a regular 2° by 2° grid for every month, by taking the median of all MLDs in each grid mesh. A slight smoothing is then applied to take account of the noisy nature of ship observations. The last step consisted into an optimal prediction of the missing data using ordinary kriging method. This interpolation was limited to a 1000 km radius disk containing at least 5 grid point values, leaving regions without values instead of filled by a doubtful interpolation. The advantage of kriging is that it is an exact interpolator, and an estimation error in the form of the kriging standard deviation, an analogy to the statistical standard deviation, is also provided.

Data format

Data type: Oceanography
Data origin: Measured
Data format:
Other format: NetCDF
Data resolution: 2° by 2° grid for every month

Data Coverage

Spatial data coverage: Global ocean
Temporal data coverage: Time series
Time period covered: 1941-2008 or 1961 - 2008 depending on what data layer you want


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Data source:


  • de Boyer Montégut, C., G. Madec, A. S. Fischer, A. Lazar, and D. Iudicone (2004), Mixed layer depth over the global ocean: an examination of profile data and a profile-based climatology, J. Geophys. Res., 109, C12003, doi:10.1029/2004JC002378.
  • d'Ortenzio, F., D. Iudicone, C. de Boyer Montégut, P. Testor, D. Antoine, S. Marullo, R. Santoleri, and G. Madec (2005) Seasonal variability of the mixed layer depth in the Mediterranean sea as derived from in situ profiles, Geophys. Res. Lett., 32, L12605, doi:10.1029/2005GL022463