Surface Mixed Layer Depth (MLD)

Schematic of processes operating at the air-sea interface and in the upper ocean mixed layer. (Illustration from Jayne Doucette, Woods Hole Oceanographic Institution)

The term "mixed" refers to a given physical parameter of the ocean state (e.g. temperature, density...) that is assumed to be mixed and homogeneous to a certain level (e.g. regarding some space/time scales), from the surface down to the considered MLD. Here we want to estimate mixed layer over at least a daily cycle, and no more than a few days. This represents the depth over which surface fluxes have been recently mixed and integrated and is a caracteristic timescale of air-sea interactions.

The surface MLDs are estimated here directly on individual profiles with data at observed levels. 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 chosen temperature criterion is 0.2°C absolute difference from surface (DT). The fixed criterion in density is 0.03 kg/m3 difference from surface (DR with R for Rho i.e. density). The variable criterion in density corresponds to a 0.2°C variation in local temperature/salinity conditions, it is obviously linked to the temperature criterion and enables the definition of the BLT. (See de Boyer Montégut et al. JGR 2004, [1] for further details about the choice of the criterion).

NB: such a definition of the MLD with a reference depth at 10 m for the threshold criterion, gives mld fields necessarily deeper than 10 m. The basic hypothesis being that the surface layer is always mixed on at least 10 m depth within one or two days. Be carefull that in region with strong upwelling (e.g. in eastern equatorial Pacific ocean) a minimum of 10 m might already be to deep as there does not exist a vertical uniformely mixed layer at the surface due to permanent upwelling.

 1. MLD in temperature with a fixed threshold criterion (0.2°C) 

This mixed layer depth is a Temperature-Mixed Layer Depth, or Isothermal Layer Depth. It is reminded that T°C inversions are NOT contained into this isothermal layer!
We estimate this MLD, named MLD_DT02, from a fixed threshold on temperature profiles. The criterion is the following :

MLD_DT02 = depth where (θ = θ_10m ± 0.2 °C)

We give hereunder the datasets that we have constructed so far for this MLD_DT02 temperature-mixed layer depth.

 2. MLD in density with a fixed threshold criterion (0.03kg/m3) 

This mixed layer depth is a Density-Mixed Layer Depth, or Isopycnal Layer Depth.
We estimate this MLD, named MLD_DR003, from a fixed threshold on density profiles. The criterion is the following :

MLD_DR003 = depth where(σ₀ = σ_010m+ 0.03 kg.m^-3)

 3. MLD in density with a variable threshold criterion (equivalent to a 0.2°C decrease) 

This mixed layer depth is a Density-Mixed Layer Depth, or Isopycnal Layer Depth.
We estimate this MLD, named MLD_DReqDTm02, from a variable threshold on density profiles. The variable criterion in density corresponds to a 0.2°C variation in local temperature/salinity conditions. The definition of this depth is obviously linked to MLD_DT02 presented above. Their comparison enables to estimate salinity effects on stratification and gives a way to compute Barrier Layer Thickness (BLT). The criterion is the following :

In other words, it is the depth where the density INCREASE compared to density at 10 m depth corresponds to a temperature decrease of 0.2°C in local surface conditions (θ_10m, S_10m, P₀= 0 db, surface pressure). For example:
- with S_10m = 35 psu, θ_10m = 25 °C : Δσ₀ = 0.060 kg/m3
- with S_10m = 35 psu, θ_10m = 15 °C : Δσ₀ = 0.044 kg/m3
- with S_10m = 35 psu, θ_10m = 5 °C : Δσ₀ = 0.023 kg/m3
NB: the fixed threshold criterion in density with 0.03 kg/m3 (cf previous section) corresponds to a 0.2 °C variation in temperature only in conditions close to: S_10m = 35 psu, θ_10m = 8 °C (Δσ₀ = 0.03 kg/m3)