Persistent Freshening of the Arctic Ocean and Changes in the North Atlantic Salinity Caused by Arctic Sea Ice Decline
Arctic sea ice has been declining over past several decades with the largest ice loss occurring in summer. This implies a strengthening of the sea ice seasonal cycle. We examine global ocean salinity response to such changes of Arctic sea ice using simulations wherein we impose a radiative heat imbalance at the sea ice surface, inducing a sea ice decline comparable to the observed. The imposed perturbation leads to enhanced seasonal melting and a rapid retreat of Arctic sea ice within the first 5–10 years. We then observe a gradual freshening of the upper Arctic Ocean that continues for about a century. The freshening is most pronounced within the central Arctic, including the Beaufort gyre, and is attributed to excess surface freshwater associated with the stronger seasonal sea ice melting, as well as a greater upper-ocean freshwater storage due to changes in ocean circulation. The freshening of the Nordic Seas can also occur via a distillation-like process in which denser saline waters with increased salinity are exported to the subtropical/tropical North Atlantic by meridional overturning circulation.
A critical component of the Earth’s climate, the Arctic Ocean responds quickly to global warming. Arctic sea ice has been declining at an unprecedented rate over the past three decades. Meanwhile, recent observations show a gradual increase in the Arctic Ocean's freshwater content. In this modeling study, we find that the enhanced sea ice seasonal cycle associated with Arctic sea ice decline, changes in ocean circulation and the associated increase of freshwater storage, and changes in ocean convection over the Nordic Seas can lead to a persistent freshening of the subpolar region. The Arctic Ocean is expected to freshen in the twenty-first century, and the persistent Arctic freshening effect described in our study should be a major part of the projected freshening along with other factors, including the melting of the Greenland ice sheet and projected increase in precipitation and river run-off in high latitudes. Thus, this study confirms that the future Arctic will become significantly fresher in years to come.
In this modeling study, we find that the enhanced sea ice seasonal cycle associated with Arctic sea ice decline results in more sea ice melted locally in the Arctic during summer and less transported southward. The excess surface freshwater gradually accumulates as the ocean circulation adjusts towards a new balance for about 100 years, during which the Beaufort gyre also strengthens, increasing Arctic freshwater storage. At the same time, the reduction of sea ice supply and stronger brine rejection makes some of the areas at the margins of sea ice and along the continental shelves more saline, especially during the first several decades. These areas serve to transport the anomalously saline water down to oceanic deeper layers. The excess salt is subsequently transported away by the southward branch of the AMOC. The AMOC slow down and the associated reduction of northward salt transport lead to a further freshening of the North Atlantic subpolar region. Thus, enhanced seasonal sea ice melting in a warmer climate can lead to a persistent Arctic freshening with large impacts on the global salinity distribution.