Mean Structure of the North Atlantic Subtropical Permanent Pycnocline from In Situ Observations

Abstract A new objective algorithm for the characterization of the permanent pycnocline (OAC-P) in subtropical gyres is proposed. OAC-P is based on a pragmatic analysis of vertical density gradient features to identify the permanent pycnocline: OAC-P identifies the permanent pycnocline as the stratified layer found below surface mode waters. OAC-P provides the permanent pycnocline depth, unequivocally associated with a local maximum in the stratification, and top and bottom thicknesses, associated with upward and downward decreases in stratification, respectively. OAC-P uses half Gaussian curves as asymmetric nonlinear analytical models of the stratification peak. It is the first time that an algorithm is proposed to characterize objectively the permanent pycnocline for a region where handling the stronger stratification peak of the seasonal pycnocline is complex. A guideline for how to implement the OAC-P is given, with application to the North Atlantic Ocean Argo data as an example. OAC-P provides a detailed description of the mean structure of the North Atlantic subtropical permanent pycnocline. OAC-P detects a permanent pycnocline throughout the subtropical gyre north of the North Equatorial Current. The large-scale description of the permanent pycnocline depth structure as a classic bowl shape is captured however with much more detail. New regional information is provided. In particular, (i) there is only one region—the southern recirculation gyre of the Gulf Stream extension—where the permanent pycnocline is along an isopycnal surface and (ii) vertical asymmetries clearly discriminate one region from another.