The mixed layer depth is found using a finite difference method that analyzes changes in ocean temperature and potential density (σθ) relative to a specific reference depth. This method identifies the depth at which the water properties significantly diverge from those near the surface, indicating the bottom of the well-mixed layer.
Understanding Mixed Layer Depth Determination
Mixed layer depth (MLD) is a crucial parameter in oceanography, defining the upper layer of the ocean where properties like temperature and salinity are relatively uniform due to mixing processes.
The Finite Difference Method
The specific method described relies on comparing measurements of temperature and potential density (σθ) at various depths to a reference point.
Reference Point
The calculations for mixed layer depth are based on measurements taken at 10 dbar (deci-bars), which is approximately 10 meters depth. This serves as the baseline for determining changes in the water column.
Key Criteria for Determination
Two distinct pairs of criteria are used to define the mixed layer depth based on the differences (Δ) in potential density (Δσθ) and temperature (ΔT) from the 10 dbar reference. The mixed layer depth is estimated by identifying the minimum thickness (shallowest depth) where either the Δσθ or ΔT criterion is met.
Here are the two sets of criteria:
| Criterion Pair | Δσθ (kg/m³) | ΔT (°C) |
|---|---|---|
| Pair 1 | 0.03 | 0.2 |
| Pair 2 | 0.125 | 0.5 |
- Potential Density (σθ): This takes into account temperature, salinity, and pressure to provide a more accurate measure of density, accounting for the compressibility of seawater. A significant change in σθ indicates a stable stratification, marking the bottom of the mixed layer.
- Temperature (ΔT): A notable drop in temperature compared to the reference depth is also a strong indicator of the base of the mixed layer, as the uniform temperature layer ends.
Estimating the Depth
To estimate the mixed layer depth using this method:
- Select a Criterion Pair: Choose either Pair 1 or Pair 2 based on the desired sensitivity for defining the mixed layer.
- Establish Reference Values: Obtain the temperature and potential density values at the 10 dbar depth (T_10dbar, σθ_10dbar).
- Calculate Differences: For each subsequent depth, calculate the difference in temperature (ΔT = T_depth - T_10dbar) and potential density (Δσθ = σθ_depth - σθ_10dbar).
- Identify Threshold Depths:
- Find the shallowest depth where the absolute value of ΔT exceeds the chosen temperature criterion (e.g., 0.2°C for Pair 1).
- Find the shallowest depth where Δσθ exceeds the chosen potential density criterion (e.g., 0.03 kg/m³ for Pair 1).
- Determine Mixed Layer Depth: The mixed layer depth is then the shallower of the two depths identified in step 4. This ensures that the mixed layer is defined by the first significant change in either temperature or density.
