Maximum angle method for determining mixed layer depth from seaglider data

Abstract

A new maximum angle method has been developed to determine surface mixed-layer (a 19 general name for isothermal/constant-density layer) depth from profile data. It has three steps 20 (1) fitting the profile data with a first vector (pointing downward) from a depth to an upper level 21 and a second vector (pointing downward) from that depth to a deeper level, (2) identifying the 22 angle (varying with depth) between the two vectors, (3) finding the depth (i.e., the mixed layer 23 depth) with maximum angle between the two vectors. Temperature and potential density profiles 24 collected from two seagliders in the Gulf Stream near Florida coast during 14 November ? 5 25 December 2007 were used to demonstrate its capability. The quality index (1.0 for perfect 26 identification) of the maximum angle method is about 0.96. The isothermal layer depth is 27 generally larger than the constant-density layer depth, i.e., the barrier layer occurs during the 28 study period. Comparison with the existing difference, gradient, and curvature criteria shows the 29 advantage of using the maximum angle method. Uncertainty due to varying threshold using the 30 difference method is also presented.