Maximum Angle Method for Determining Mixed Layer Depth from Seaglider Data

摘要

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 a depth to an upper level 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) finding the depth (i.e., the mixed layer depth) with maximum angle between the two vectors. Temperature and potential density profiles collected from two seagliders in the Gulf Stream near Florida coast during 14 November - 5 December 2007 were used to demonstrate its capability. The quality index (1.0 for perfect identification) 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 due to varying threshold using the difference method is also presented.