Technical note: Determining Arctic Ocean halocline and cold halostad depths based on vertical stability

Abstract. The Arctic Ocean halocline separates the cold surface mixed layer from the underlying warm Atlantic Water (AW), and thus provides a precondition for sea ice formation. Here, we introduce a new method in which the halocline base depth is determined from vertical stability and compare it to two existing methods. We also propose a novel method for detecting the cold halostad, a layer characterized by a small vertical salinity gradient, which is formed by the Pacific Winter Water in the Canada Basin or by meltwater off the eastern coast of Greenland and off Svalbard. Our main motivation for determining the halocline base depth depending on vertical stability was that vertical stability is closely related to vertical mixing and heat exchange. Vertical stability is a crucial parameter for determining whether the halocline can prevent vertical heat exchange and protect sea ice from warm AW. When applied to measurements from ice-tethered profilers, ships, and moorings, the new method for estimating the halocline base depth provides robust results with few artifacts. Analyzing a case in which water previously homogenized by winter convection was capped by fresh water at the surface suggests that the new method captured the beginning of new halocline formation in the Eurasian Basin. Comparatively large differences between the methods for detecting the halocline base depth were found in warm AW inflow regions for which climate models predict halocline thinning and increased net surface energy fluxes from the ocean to the atmosphere.