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Surface Mixed Layer Depth (MLD)

mixed layer schematic

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 = θ10m ± 0.2 °C)

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

 DATA 
1.1. Global Scale, Annual Climatology (12 month)
 
Space/Time Resolution Basic T/S Profiles Used Data File Plots Reference(s)

  • regular 2°C×2°C grid
  • monthly data

≃ 5 million temperature profiles
More info...

mld_DT02_c1m_reg2.0_Global.nc
[4.7 Mo]

(updated Nov. 2008)

pdf

de Boyer Montégut et al., 2004 [1]


 DATA 
1.2. Mediterranean Sea, Annual climatology (12 month)
 
Space/Time Resolution Basic T/S Profiles Used Data File Plots Reference(s)

  • regular 0.5°C×0.5°Cgrid (no interpolation!)
  • monthly data

≃ 240,000 temperature profiles
More info...

Data files available on request to Fabrizio d'Ortenzio

(updated 2005)

pdf

d'Ortenzio et al., 2005 [3]


 DATA 
1.3. Indian Ocean, Interannual climatology (41 years)
 
Space/Time Resolution Basic T/S Profiles Used Data File Plots Reference(s)

  • regular 2°C×2°C grid (20E-140E, 40S-30N)
  • monthly data (1969 to 2009)

≃ 565,000 temperature profiles
More info...

mld_DT02_i1m_reg2.0_IndianOcean.nc
[33 Mo]

(updated Mar. 2011)

pdf

Keerthi et al., 2012 [2]






 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 = σ010m+ 0.03 kg.m-3)

 DATA 
2.1. Global Scale, Annual Climatology (12 month)
 
Space/Time Resolution Basic T/S Profiles Used Data File Plots Reference(s)

  • regular 2°C×2°C grid
  • monthly data

≃ 880,000 density profiles
More info...

mld_DR003_c1m_reg2.0_Global.nc
[4.7 Mo]

(updated Nov. 2008)

pdf

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 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 :

MLD_DReqDTm02 = depth where ( σ0 = σ010m + Δσ0 )
with Δσ0 = σ010m - 0.2°C, S10m, P0) - σ010m, S10m, P0)

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 (θ10m, S10m, P0= 0 db, surface pressure). For example:
- with S10m = 35 psu, θ10m = 25 °C : Δσ0 = 0.060 kg/m3
- with S10m = 35 psu, θ10m = 15 °C : Δσ0 = 0.044 kg/m3
- with S10m = 35 psu, θ10m = 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: S10m = 35 psu, θ10m = 8 °C (Δσ0 = 0.03 kg/m3)

 DATA 
3.1. Global Scale, Annual Climatology (12 month)
 
Space/Time Resolution Basic T/S Profiles Used Data File Plots Reference(s)

  • regular 2°×2° grid
  • monthly data

≃ 780,000 density profiles
More info...

mld_DReqDTm02_c1m_reg2.0_Global.nc
[4.7 Mo]

(updated Nov. 2008)

pdf

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