DOI: 10.17882/98226
title: Mixed layer depth over the global ocean : a climatology computed with a density threshold criterion of 0.03 kg/m3 from the value at the reference depth of 5 m

description: The dataset made available here is the monthly climatology of an estimate of the ocean surface Mixed Layer Depth (MLD) over the global ocean, at 1 degree by 1 degree spatial resolution. This climatology is based on about 7.3 million casts/profiles of temperature and salinity measurements made at sea between January 1970 and December 2021. Those profiles data come from the ARGO program and from the NCEI-NOAA World Ocean Database (WOD, Boyer et al. 2018), including e.g. data from marine mammals or ice-tethered profilers for the high latitudes.
Here, the MLD is computed on each individual cast/profile using the threshold criterion method. The depth of the mixed layer is defined as the shallowest depth where the surface potential density of the profile is superior to a reference value (usually taken close to the surface) added with the chosen threshold. In this work, we take a fixed threshold value for the density of 0.03 kg/m3, and a surface reference depth at 5 m. This surface mixed layer is by definition homogeneous in density (up to 0.03 kg/m3 variations) and can also be called an isopycnal layer. The latter value of 5 m for the reference depth differs from the rather usual 10 m value used in de Boyer Montégut et al., 2004 (see dataset at https://www.seanoe.org/data/00806/91774/). It is important to note that this shallower value has been chosen for a specific study dedicated to the Arctic Ocean (Allende et al., submitted to Geoscientific Model Development), where both the presence of ice and a very weak diurnal cycle may lead to MLD shallower than 10 m and possibly lasting for several days or more (which should be rare outside polar latitudes). Differences with the MLD climatology using a 10 m reference depth, are located mostly in the polar high latitudes (poleward of 60 degree) and somehow in the north atlantic in winter. They are rather weak in other places of the global ocean (about less than 10m, MLD with ref depth at 5 m being shallower).
This product is rather intended for studies in the polar latitudes (e.g. Arctic ocean with the presence of sea ice), and/or for validation of MLD fields in such areas from Ocean models like Ocean General Circulation Models (e.g. see Allende et al. submitted to Geoscientific Model Development). In any case, we recall that it is important to estimate, as much as possible, the MLD from the model with the same method as in the chosen observed climatology i.e. here with the same criterion values (0.03 kg/m3 threshold, and 5 m reference depth).
This dataset (netcdf file) and its more detailed product documentation (pdf file) are produced at LOPS laboratory (associated research unit with following french research institute and university : IFREMER, University of Brest, IUEM, CNRS, and IRD). Those files are available by clicking the download buttons hereunder on this page. Some links to other related climatologies of ocean surface variables (e.g. MLD with other criteria, barrier layer thickness...) can be found at the LOPS-IFREMER MLD webpages : https://cerweb.ifremer.fr/mld, or similarly at : https://www.umr-lops.fr/en/Data/MLD.