roms_tools.setup.utils.compute_mld

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roms_tools.setup.utils.compute_mld#

roms_tools.setup.utils.compute_mld(sigma0: DataArray, depth: DataArray, depth_dim: str, reference_depth: float = 10.0, threshold: float = 0.03) → DataArray#

Compute the mixed layer depth (MLD) from a potential-density field.

Density-threshold criterion (de Boyer Montégut et al. 2004): the MLD is the (positive) depth at which sigma-0 first exceeds its value at reference_depth by threshold kg/m³, found by linear interpolation to the crossing. Fully mixed columns (no crossing) return the full water-column depth; all-NaN (land) columns return NaN. Works per horizontal column, so both 1D and spatially varying (3D) depth coordinates and either vertical orientation (surface-first or surface-last) are supported.

The crossing is found with xgcm.transform — the same engine as xroms’ mld/isoslice (so results are consistent with xroms; pass reference_depth=0 for the surface-referenced xroms convention). Like all xgcm linear transforms it assumes a monotonic profile and does an endpoint flip rather than a full sort, so for a non-monotonic upper-ocean profile (a density inversion / barrier layer) it returns the linear-search crossing rather than guaranteeing the shallowest one.

Parameters:

  • sigma0 (xr.DataArray) – Potential density anomaly (e.g. from compute_potential_density()) with vertical dimension depth_dim.

  • depth (xr.DataArray) – Positive-down depth (m) aligned with sigma0 along depth_dim. May be 1D (broadcast over the horizontal) or share sigma0’s horizontal dims.

  • depth_dim (str) – Name of the vertical dimension.

  • reference_depth (float, optional) – Depth (m) at which the reference density is taken. Default 10 m (de Boyer Montégut). Columns shallower than reference_depth fall back to the shallowest level. Pass 0 for the surface-referenced xroms convention.

  • threshold (float, optional) – Density excess (kg/m³) defining the base of the mixed layer. Default 0.03.

Returns:

xr.DataArray – MLD as positive depth (m), with depth_dim removed.

References

de Boyer Montégut, C., Madec, G., Fischer, A. S., Lazar, A., & Iudicone, D. (2004). Mixed layer depth over the global ocean. J. Geophys. Res. Oceans, 109(C12). See also xroms roms_seawater.mld and the NCL mixed_layer_depth routine.

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