Thermohaline Staircases in the Ocean: Initiation, Evolution, and Structure

Thermohaline staircases are stepped structures—well-mixed layers separated by thin, sharp interfaces—commonly observed in vertical temperature and salinity profiles in the ocean. They occur widely in subtropical thermoclines, marginal seas, and much of the Arctic Ocean beneath sea ice, and they strongly modulate vertical heat and salt transport, with potential consequences for large-scale climate dynamics. Since the last comprehensive review, theoretical, numerical, and observational advances have substantially refined our understanding of these structures and their relation to double-diffusive convection. In this review, we argue that salt-fingering (SF) and diffusive-convection (DC) staircases, although superficially similar, are dynamically distinct and should be treated separately. SF staircases are closely tied to the primary SF instability, whereas DC staircases appear to require additional physics beyond the primary DC instability. We synthesize recent progress on staircase formation, interface structure, and flux parameterizations and discuss the implications of staircase-induced mixing for climate dynamics.