Observations of the transition layer

The transition layer between the surface mixed layer and the stratified interior is generally a region of elevated spice variance, current shear, and vertical stratification compared to the interior. A database of 89,649 km or 24,677 vertical profiles of temperature and salinity from SeaSoar, a towed vehicle, is used to define the transition layer thickness. Acoustic Doppler current measurements are also used, when available. Statistics of the transition layer thickness are compared for 223 straight SeaSoar sections, which range in distance from 65-1129 km with typical horizontal resolution of about 0.7-4 km and vertical resolution of 8 m. The transition layer is defined in three sets of broad definitions based on 1) the transition from a turbulent mixed layer to a weakly turbulent interior, 2) the transition from a mixed layer to interior dynamics, and 3) vertical displacements of the mixed layer base and vertical displacements of interior isopycnals into the mixed layer. 1) Peaks in stratification, shear, and potential vorticity are found beneath the mixed layer base and transition layer vertical thicknesses are calculated in two ways: one is the depth difference of the peak from the mixed layer base, while the other is the full width at half maximum of the peak. 2) Transition layer thicknesses based on the transition from mixed layer to interior dynamics are obtained from: the depth difference between the mixed layer base to the i) closest extremum of the median density ratio, ii) maximum of the median isopycnal slope, and iii) maximum of spice variance. 3) Transition layer thicknesses based on vertical displacements include: standard deviation of a highpass-filtered mixed layer depth and the standard deviation of displacements of the deepest isopycnal from its mean depth which reaches the mean depth of the mixed layer base. From each SeaSoar section, we compile either a single value or a median value for each of the above measures. From the various definitions above, median transition layer thicknesses range from 8-23 m below the mixed layer base. The maximum of the median isopycnal slope is found about 20 m above the mixed layer base. The full width at half maximum of the potential vorticity peak displays the best correlations with other measures of the transition layer thickness and peak values of stratification, shear, potential vorticity, and density ratio in the transition layer. Some transition layer thicknesses are correlated with the mean depth of the mixed layer. Estimates from displacements and the full width at half maximum of the shear and potential vorticity peak give transition layer thicknesses from 0.11-0.22 times the mean depth of the mixed layer. A typical horizontal scale of 5-10 km for the transition layer comes from the product of isopycnal slope and a transition layer thickness suggesting the importance of submesoscale processes in forming the transition layer.