Divergence in Climate Model Projections of Future Arctic Atlantification

The Arctic Ocean is strongly stratified by salinity in the uppermost layers. This stratification is a key attri-bute of the region as it acts as an effective barrier for the vertical exchanges of Atlantic Water heat, nutrients, and CO2 be-tween intermediate depths and the surface of the Eurasian and Amerasian basins (EB and AB, respectively). Observations show that from 1970 to 2017, the stratification in the AB has strengthened, whereas, in parts of the EB, the stratification has weakened. The strengthening in the AB is linked to freshening and deepening of the halocline. In the EB, the weak-ened stratification is associated with salinification and shoaling of the halocline (Atlantification). Simulations from a suite of CMIP6 models project that, under a strong greenhouse gas forcing scenario (ssp585), the overall surface freshening and warming continue in both basins, but there is a divergence in hydrographic trends in certain regions. Within the AB, there is agreement among the models that the upper layers will become more stratified. However, within the EB, models diverge regarding future stratification. This is due to different balances between trends at the surface and trends at depth, related to Fram Strait fluxes. The divergence affects projections of the future state of Arctic sea ice, as models with the strongest Atlantification project the strongest decline in sea ice volume in the EB. From these simulations, one could conclude that Atlantification will not spread eastward into the AB; however, models must be improved to simulate changes in a more in-tricately stratified EB correctly.

Automated summary

The Arctic Ocean is stratified by salinity, acting as a barrier for vertical exchanges between depths and the surface. Observations show strengthening stratification in the Amerasian basin and weakening stratification in parts of the Eurasian basin. Simulations project continued freshening and warming in both basins, but with diverging trends in certain regions. Models differ in their projections of future stratification in the Eurasian basin due to varying surface and depth trends. This divergence affects sea ice projections, with stronger Atlantification models projecting the greatest decline in ice volume.