Mixed Layer Depth (MLD)
OceanGraph derives a mixed layer depth exploration indicator from potential temperature, absolute salinity, and potential density anomaly referenced to 0 dbar (σ0).
Use MLD
MLD is available to signed-in users.
- Search for profiles.
- Open Data layer in the map command bar and choose MLD. On narrower layouts, open Map tools first.
- Compare marker colors and tooltip values.
MLD, SOM, and mode water coloring are mutually exclusive. Profiles without a valid MLD are gray. When Trajectory View and a section are open, valid MLD values are also shown as points. Lines connect consecutive profile cycles only; they do not bridge missing cycles or imply interpolation through time.
Calculation
OceanGraph calculates three threshold crossings relative to the profile value at 10 dbar and uses the shallowest crossing.
| Criterion | Threshold |
|---|---|
| Potential temperature | 0.5 °C |
| Absolute salinity | 0.05 g/kg |
| σ0 | 0.125 kg/m³ |
Each crossing is linearly interpolated between the pressure rows that bracket its threshold and recorded to 0.01 dbar. Absolute salinity and potential temperature are derived with TEOS-10; Conservative Temperature is used only as an intermediate value for the density calculation.
The calculation requires at least three finite temperature-salinity rows, including two at 50 dbar or shallower. The shallowest paired row must be at 30 dbar or shallower and the profile must reach 10 dbar. If the first paired row is between 10 and 30 dbar, it supplies the reference values. If no threshold is crossed, MLD is undefined.
Map tooltips and section data layers use pressure in dbar. The color scale is a relative visual aid, and the result is an automatic threshold-based indicator rather than a guaranteed physical boundary. Because SOM selection begins below MLD, an MLD change can also change the reported SOM.