Surface Mixed Layer Depth (MLD)
Schematic of processes operating at the airsea 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 airsea 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 nearsurface 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 TemperatureMixed 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 temperaturemixed layer
depth.
1.1. Global Scale, Annual Climatology (12 month) 
Space/Time Resolution  Basic T/S Profiles Used  Data File  Plots  Reference(s) 


≃ 5 million temperature profiles More info...

mld_DT02_c1m_reg2.0_Global.nc [4.7 Mo] (updated Nov. 2008) 

de Boyer Montégut et al., 2004 [1] 
1.2. Mediterranean Sea, Annual climatology (12 month) 
Space/Time Resolution  Basic T/S Profiles Used  Data File  Plots  Reference(s) 


≃ 240,000 temperature profiles More info...

Data files available on request to Fabrizio d'Ortenzio (updated 2005) 

d'Ortenzio et al., 2005 [3] 
1.3. Indian Ocean, Interannual climatology (41 years) 
Space/Time Resolution  Basic T/S Profiles Used  Data File  Plots  Reference(s) 


≃ 565,000 temperature profiles More info...

mld_DT02_i1m_reg2.0_IndianOcean.nc [33 Mo] (updated Mar. 2011) 

Keerthi et al., 2012 [2] 
2. MLD in density with a fixed threshold criterion (0.03kg/m3)
This mixed layer depth is a DensityMixed 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(σ_{0} = σ_{010m}+ 0.03 kg.m^{3})
DATA2.1. Global Scale, Annual Climatology (12 month) 
Space/Time Resolution  Basic T/S Profiles Used  Data File  Plots  Reference(s) 


≃ 880,000 density profiles More info...

mld_DR003_c1m_reg2.0_Global.nc [4.7 Mo] (updated Nov. 2008) 

de Boyer Montégut et al., 2004 [1] 

≃ 4.5 million density profiles 
mld_dR003_ref10 from SEANOE (updated Nov. 2022) 
see on SEANOE 
SEANOE de Boyer Montégut et al., 2004 [1] 
3. MLD in density with a variable threshold criterion (equivalent to a 0.2°C decrease)
This mixed layer depth is a DensityMixed 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 :
MLD_DReqDTm02 = depth where ( σ_{0} =
σ_{010m} +
Δσ_{0} ) with Δσ_{0} = σ_{0}(θ_{10m}  0.2°C, S_{10m}, P_{0})  σ_{0}(θ_{10m}, S_{10m}, P_{0}) 
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},
P_{0}= 0 db, surface pressure). For example:

with S_{10m} = 35 psu, θ_{10m} = 25
°C : Δσ_{0} = 0.060 kg/m3
 with
S_{10m} = 35 psu, θ_{10m} = 15 °C
: Δσ_{0} = 0.044 kg/m3
 with
S_{10m} = 35 psu, θ_{10m} = 5 °C :
Δσ_{0} = 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
(Δσ_{0} = 0.03 kg/m3)
3.1. Global Scale, Annual Climatology (12 month) 
Space/Time Resolution  Basic T/S Profiles Used  Data File  Plots  Reference(s) 


≃ 780,000 density profiles More info...

mld_DReqDTm02_c1m_reg2.0_Global.nc [4.7 Mo] (updated Nov. 2008) 

de Boyer Montégut et al. 2004 [1] de Boyer Montégut et al. 2007 [4] Mignot et al. 2007 [5] 