New Methodology and Analysis of Determinants for Mixed Layer Depth -- a Case Study in the Salish Sea

Abstract

A new method, based on the minimum of the first derivative of Brunt–Väisälä frequency and two Quality indices, is established to find Mixed Layer Depth (MLD) from ocean profiles. Unlike previous methods that analyzes the oceanic profiles with a single proxy parameter (temperature, salinity, density etc.), the new method uses a conceptual metric that physically describe stability dynamics of MLD. The algorithm also ensures MLD quality by directly using a quality index (QI) and introducing a second quality index (QI2). The new method is evaluated in the Salish Sea, using CTD profiles from the Pelagic Ecosystem Function (PEF) program and Salish Cruise. By visual inspection comparing with other methods, the new method can identify an accurate Mixed Layer Depth in 90.4% of PEF profiles and xxx% Salish Cruise Profiles. To study the dominant factors of Mixed Layer Depth in Salish Sea, oceanographic variables were calculated from the same CTD profiles, and atmospheric variables were obtained from ERA5 reanalysis product. The correlation coefficient and p-value between the variables and Mixed Layer Depths show that Salinity and Wind are the most controlling factors in San Juan Archipelago with PEF profiles; Wind and salinity are still the two most dominant determinants in each sub-region, with temperature playing a relatively more important role than in the PEF study area. However, different wind components and salinity components are dominant in different sub-region as a result of the highly dynamic and diverse system in Salish Sea.