{
  "key": "mld_sigma_theta_002",
  "slug": "mld_sigma_theta_002",
  "label": "Mixed-layer depth: where sigma-theta is 0.02 kg/m3 greater than at 10 m (CalCOFI legacy definition)",
  "short_label": "Mixed-layer depth (Δσθ 0.02)",
  "description": "Mixed-layer depth: where sigma-theta is 0.02 kg/m3 greater than at 10 m (CalCOFI legacy definition)",
  "units": "m",
  "units_nerc_p06": "http://vocab.nerc.ac.uk/collection/P06/current/ULAA/",
  "nerc_p01": null,
  "category": {
    "key": null,
    "name": "Physical Oceanography",
    "icon": "cat-physical",
    "realm": "env",
    "order": 1
  },
  "is_unified": false,
  "derivation": "calcofi4db::ctd_mld(criterion = 'sigma_theta', threshold = 0.02, ref_depth = 10) on the 1 m bins, interpolated linearly between the bracketing bins; the provider's definition (Rasmus Swalethorp 2026-09-23, questions Q01): 'the depth at which sigma-theta is 0.02 kg/m3 greater [than at] a reference depth of 10 m'; absent where the cast never crosses or does not reach 10 m",
  "climatology": true,
  "bounds": {
    "valid_min": 0,
    "valid_max": 6500,
    "declared_by": [
      "mld_sigma_theta_002"
    ]
  },
  "totals": {
    "n_values": 9360,
    "n_samples": 9360,
    "n_roots": 9360,
    "n_datasets": 1,
    "year_min": 1993,
    "year_max": 2026,
    "depth_min_m": null,
    "depth_max_m": null,
    "qual_ok_n": 9360,
    "n_flagged": 0
  },
  "series": [
    {
      "measurement_type": "mld_sigma_theta_002",
      "dataset_key": "calcofi_ctd-derived",
      "source_column": "sigma_theta_ave",
      "qual_column": null,
      "description": "Mixed-layer depth: where sigma-theta is 0.02 kg/m3 greater than at 10 m (CalCOFI legacy definition)",
      "units": "m",
      "nerc_p01": null,
      "is_canonical": true,
      "n_values": 9360,
      "n_samples": 9360,
      "n_roots": 9360,
      "n_cells": 113,
      "n_cruises": 138,
      "year_min": 1993,
      "year_max": 2026,
      "depth_min_m": null,
      "depth_max_m": null,
      "years": {
        "1993": 130,
        "1994": 262,
        "1995": 260,
        "1996": 313,
        "1997": 281,
        "1998": 413,
        "1999": 260,
        "2000": 257,
        "2001": 252,
        "2002": 258,
        "2003": 312,
        "2004": 331,
        "2005": 291,
        "2006": 317,
        "2007": 284,
        "2008": 285,
        "2009": 312,
        "2010": 299,
        "2011": 313,
        "2012": 273,
        "2013": 324,
        "2014": 248,
        "2015": 304,
        "2016": 341,
        "2017": 323,
        "2018": 291,
        "2019": 231,
        "2020": 233,
        "2021": 259,
        "2022": 217,
        "2023": 317,
        "2024": 265,
        "2025": 232,
        "2026": 72
      },
      "months": [
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        115,
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        1550,
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        117,
        955,
        1166,
        25
      ],
      "depth_bands": {
        "0-10": 0,
        "10-50": 0,
        "50-100": 0,
        "100-200": 0,
        "200-500": 0,
        "500-1000": 0,
        "1000-2000": 0,
        "2000+": 0
      },
      "qual": {
        "none": 9360
      },
      "qual_ok_n": 9360,
      "n_flagged": 0,
      "observed": {
        "min": 10.0246578720256,
        "p05": 10.2932443900186,
        "p50": 18.8447080291978,
        "p95": 60.7277491033155,
        "max": 133.108108108108
      },
      "out_of_bounds": {
        "n": 0,
        "min": null,
        "max": null
      },
      "flags": [
        "no_flag_at_grain",
        "no_p01"
      ],
      "grain": "sample"
    }
  ],
  "related": [],
  "flags": [
    "no_label"
  ],
  "face": {
    "kind": "scale",
    "face_of": null,
    "stands_in_note": null,
    "source": "no NERC P01 on this key; shows its own familiar-scale figure only (D3)"
  },
  "method": [
    {
      "dataset_key": "calcofi_ctd-derived",
      "measurement_type": "mld_sigma_theta_002",
      "platform": "ctd",
      "instrument": "Computed from the CTD profile (1 m bins)",
      "nerc_l22": null,
      "principle": "The mixed-layer depth, headline: the depth at which sigma-theta is 0.02 kg/m3 greater than at a reference depth of 10 m, the CalCOFI legacy definition (CTD team: \"MLD is the depth at which sigma-theta is 0.02 kg/m3 greater [than at] a reference depth of 10 m\"); taken on the 1 m bins and interpolated linearly between the bracketing bins; absent where the cast never crosses or does not reach 10 m.",
      "steps": [
        "take sigma-theta at 10 m",
        "find the first depth 0.02 kg/m3 denser"
      ],
      "wavelength_nm": null,
      "precision": null,
      "bibkeys": [],
      "calcofi_org_url": null,
      "text_fragment": null,
      "source": "the record's own derivation (measurement_type.csv); questions Q01 (Rasmus Swalethorp, 2026-09-23)",
      "source_url": null
    }
  ],
  "why": [
    {
      "rank": 1,
      "kind": "authored",
      "text": "The mixed layer is the wind-stirred skin of the ocean above the density step that keeps deeper water from rising into it; how deep it is decides how much light the plankton in it get and whether deep nutrients can reach them, and off California warmer, more layered years have gone with fewer zooplankton.",
      "bibkeys": [
        "roemmich1995"
      ],
      "source_url": null,
      "eov": null,
      "goos_doc": null
    },
    {
      "rank": 2,
      "kind": "authored",
      "text": "This is the headline mixed-layer depth, the CalCOFI legacy definition the CTD team asked for: the depth at which sigma-theta is 0.02 kg/m3 greater than at a reference depth of 10 m (Rasmus Swalethorp, SIO, 2026-09-23). There is no single mixed-layer depth: each criterion draws the line at a different density or temperature step from a 10 m reference, so the depths published here differ by design; compare a depth only with one made by the same criterion.",
      "bibkeys": [],
      "source_url": null,
      "eov": null,
      "goos_doc": null
    },
    {
      "rank": 3,
      "kind": "wikipedia",
      "text": "Mixed layer",
      "bibkeys": [],
      "source_url": "https://en.wikipedia.org/wiki/Mixed_layer",
      "eov": null,
      "goos_doc": null
    },
    {
      "rank": 4,
      "kind": "wikipedia",
      "text": "Pycnocline",
      "bibkeys": [],
      "source_url": "https://en.wikipedia.org/wiki/Pycnocline",
      "eov": null,
      "goos_doc": null
    }
  ],
  "grain": "sample"
}