Abstract
Thermohaline staircases are stepped structures of alternating thick mixed layers and thin high gradient interfaces. These structures can be up to several tens of metres thick and are associated with double-diffusive mixing. Thermohaline staircases occur across broad swathes of the Arctic and tropical/subtropical oceans and can increase rates of diapycnal mixing by up to five times the background rate, driving substantial nutrient fluxes to the upper ocean. In this study, we present an improved classification algorithm to detect thermohaline staircases in ocean glider profiles. We use a dataset of 1162 glider profiles from the tropical North Atlantic collected in early 2020 at the edge of a known thermohaline staircase region. The algorithm identifies thermohaline staircases in 97.7 % of profiles that extend deeper than 300 m. We validate our algorithm against previous results obtained from algorithmic classification of Argo float profiles. Using fine resolution temperature data from a fast-response thermistor on one of the gliders, we explore the effect of varying vertical bin sizes on detected thermohaline staircases. Our algorithm builds on previous work with improved flexibility and the ability to classify staircases from profiles with poor salinity data. Using our results, we propose that the incidence of thermohaline staircases is limited by strong background vertical gradients in conservative temperature and absolute salinity.
Highlights
Thermohaline staircases consist of subsurface layers of near homogeneous salinity and temperature, separated by thin layers with large temperature and salinity gradients
We propose that the incidence of thermohaline staircases is limited by strong background vertical gradients in conservative temperature and absolute salinity
In this paper we suggest that strong vertical gradients in salinity and temperature constrain the formation of thermohaline staircases
Summary
Thermohaline staircases consist of subsurface layers of near homogeneous salinity and temperature (referred to here as mixed layers), separated by thin layers with large temperature and salinity gradients (gradient layers). Staircases formation and growth are driven by double-diffusive processes that arise from the difference in molecular diffusivities of heat and salt. Heat diffuses 100 times faster than salt (Stern, 1960). Thermohaline staircases form where the vertical gradients of temperature and salinity have the same sign. These conditions most commonly occur in the Arctic, where cool fresh waters overlie warm salty waters, and tropical/subtropical regions where warm salty surface waters overlie cool, fresh waters. The effects of thermal expansion and haline contraction on seawater are critical to the formation of thermohaline staircases
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