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Subsurface Oxygen Maximum (SOM)

OceanGraph calculates the subsurface oxygen maximum (SOM) for individual Argo float profiles using dissolved oxygen and pressure (or depth) data. This metric is widely used in oceanography to characterize the vertical structure of oxygen, especially in subtropical and tropical regions, where a local maximum often appears just below the surface mixed layer.

Subsurface Oxygen Maximum Example

Accessing SOM

SOM display is available to signed-in users only and requires dissolved oxygen data.

  1. Sign in and search for profiles
  2. Use Only profiles with BGC if you want to focus on profiles with biogeochemical measurements
  3. Open MLD / SOM in the map controls and choose SOM
  4. Use the marker colors and tooltips to compare SOM depth and oxygen values

MLD, SOM, and mode water map overlays are mutually exclusive. Selecting SOM turns off MLD and mode water coloring. Profiles without dissolved oxygen data or a valid SOM result are rendered as no-data markers.

When Trajectory View and an oxygen section chart are open, selecting SOM also overlays an SOM line on the vertical section.

Calculation Details

  1. Definition and Search Range

    • When MLD is available, the SOM is defined as the local maximum of dissolved oxygen concentration found within the subsurface layer, between the mixed layer depth + 5 dbar and 300 dbar.
    • When MLD is not available, the search starts at 30 dbar and extends to 300 dbar.
    • If MLD is 300 dbar or deeper, SOM is not calculated for that profile.
    • The very shallow layers are excluded to avoid the influence of transient surface processes and ensure the detected maximum is truly subsurface.

  2. Identification of Local Maximum

    • Within the specified pressure range, the oxygen profile is scanned for local maxima, defined as points where the dissolved oxygen concentration is greater than at both adjacent pressure levels.
    • If multiple local maxima are present, the one with the highest oxygen concentration is selected as the SOM.

  3. Fallback if No Local Maximum Exists

    • If no local maximum exists within the subsurface layer (e.g., if the profile is monotonic), the single highest dissolved oxygen concentration within this range is selected as the SOM.

  4. Output

    • The pressure (or depth) and the corresponding dissolved oxygen concentration of the SOM are recorded for each profile.
    • If no valid SOM can be identified (e.g., due to insufficient data points), the SOM is considered undefined for that observation.