Maximum angle method for determining mixed layer depth from seaglider data

A new maximum angle method has been developed to determine surface mixed-layer (a general name for isothermal/constant-density layer) depth from profile data. It has three steps: (1) fitting the profile data with a first vector (pointing downward) from an upper level to a depth and a second vector (pointing downward) from that depth to a deeper level; (2) identifying the angle (varying with depth) between the two vectors; (3) after fitting and calculating angle all depths, and then selecting the depth with maximum angle as the mixed layer depth (MLD). Temperature and potential density profiles collected from two seagliders in the Gulf Stream near the Florida coast during 14 November–5 December 2007 were used to demonstrate the method’s capability. The quality index (1.0 for perfect identification of the MLD) of the maximum angle method is about 0.96. The isothermal layer depth is generally larger than the constant-density layer depth, i.e., the barrier layer occurs during the study period. Comparison with the existing difference, gradient, and curvature criteria shows the advantage of using the maximum angle method. Uncertainty in determining MLD because of varying threshold using the difference method is also presented.