In the Canada Basin of the Arctic Ocean, warm and salty Atlantic-origin Water (AW) lies in the intermediate layer (250 m–800 m) below a colder and fresher surface layer. It results in a depth range where vertical thermohaline gradients are propitious to double-diffusion. Indeed, thermohaline staircases are commonly observed and associated with double-diffusive processes. Using observations from the Beaufort Gyre Exploration Project large database and Ice-Tethered Profilers, we document the presence of density staircases in the 300 m–700 m depth range with a striking strong spatial and temporal coherence. However, since 2007, a progressive smoothing of these staircases has occurred, beginning from the western half of the basin. Quantifying this evolution, we find that a general pattern is a clear evolution over time from numerous thick steps ( ≃ 40 m) with sharp interfaces to fewer and thinner steps ( ≃ 30 m) with smoother interfaces. After 2014, marked density staircases have almost disappeared in most of the Canada Basin. The vanishing of staircases occurs over a few years and coincides with modifications of the large scale circulation and thermohaline large scale horizontal gradients. As the small scale thermohaline structures are thought to play an important role for the vertical and horizontal exchanges of heat within the Canada Basin, the disappearance of the steps may impact the heat distribution at depth, with potential consequences for the evolution of the sea ice cover.

Key Points

Density staircases are detected over the past two decades in in situ observations in the Beaufort Gyre

Density staircases have been gradually smoothed, with a transition that begins in the western part of the Canada Basin

Smoothing of staircases occurs simultaneously with changes in the large scale dynamics

Plain Language Summary

In the western side of the Arctic Ocean, Atlantic-origin water lies around 400m deep, and is warmer and saltier than the surface layer, resulting in a peculiar stratification compared to the other oceans. Moreover, the interior of this ocean is thought to be quieter than the open ocean. As a result of this original setting, specific mechanisms based on the difference in temperature and salinity capacity to diffuse can play a role in the formation of step-like structures often observed in vertical profiles.<br />Using observations of temperature and salinity in the western Arctic, we show that the steps are numerous, thick (∼40m) and coherent over long time and space in 2005-2010. Yet, in recent years, the steps have started to gradually weaken and even to disappear in the western part of the Canada Basin. As the steps are thought to play an important role for the vertical and horizontal exchanges of heat within the Canada Basin, their disappearance may impact the heat distribution at depth, with potential consequences for the evolution of the sea ice cover.