Large-scale destratification in the Eurasian Basin thermocline driving Atlantic Water shoaling

The stratification of the Arctic Ocean plays a central role in regulating the impact of climate change on the Arctic. Though the stratification in the eastern Eurasian Basin halocline is known to have weakened since the 2000s, the variability over the full AW depth range in the whole Eurasian Basin has been little explored.Our analysis aims to combine available in-situ observations to characterize the regional changes in stratification in the Eurasian Arctic Ocean over the past four decades. We find that, in both the Nansen and Amundsen basins, the variability of the temperature and salinity is most pronounced in the thermocline that separates the Atlantic Water (AW) core from the stratified halocline. This variability is affected by both warm and salty pulses entering through the Fram Strait, and by long-term trends. Positive temperature and salinity anomalies in the thermocline are associated with a destratification of the thermocline down to the AW core. In these layers, the stratification is estimated to have decreased by up to 50% across the Eurasian Arctic over the past 40 years, implying the possibility of enhanced vertical salt and heat fluxes up to the base of the halocline. In contrast, the stratification of the halocline has remained approximately constant or increased. Using a conceptual advective-diffusive model which takes into account the impact of stratification changes on vertical diffusion, we further show that the observed structure of changes is well reproduced by vertical diffusion of anomalies travelling from the Fram Strait around the Eurasian Basin. Our approach, using clustering techniques to divide the Eurasian Basin into several regions with coherent temperature, salinity and stratification profiles, provides new insights on the regional evolution of the Eurasian Arctic stratification, in particular in regions where few long-term studies are available like the Amundsen Basin.