Deepening of the Winter Mixed Layer in the Canada Basin, Arctic Ocean Over 2006-2017

The Arctic Ocean mixed layer interacts with the ice cover above and warmer, nutrient-rich waters below. Ice-Tethered Profiler observations in the Canada Basin of the Arctic Ocean over 2006-2017 are used to investigate changes in mixed layer properties. In contrast to decades of shoaling since at least the 1980s, the mixed layer deepened by 9 m from 2006-2012 to 2013-2017. Deepening resulted from an increase in mixed layer salinity that also weakened stratification at the base of the mixed layer. Vertical mixing alone can explain less than half of the observed change in mixed layer salinity, and so the observed increase in salinity is inferred to result from changes in freshwater accumulation via changes to ice-ocean circulation or ice melt/growth and river runoff. Even though salinity increased, the shallowest density surfaces deepened by 5 m on average suggesting that Ekman pumping over this time period remained downward. A deeper mixed layer with weaker stratification has implications for the accessibility of heat and nutrients stored in the upper halocline. The extent to which the mixed layer will continue to deepen appears to depend primarily on the complex set of processes influencing freshwater accumulation. Plain Language Summary The upper tens of meters of the Arctic Ocean, termed the mixed layer, separate the floating sea ice above from warmer and nutrient-rich waters below. Observations in winter are used to investigate changes to the mixed layer over 2006-2017. The mixed layer has recently gotten thicker (deeper) by 9 m, in contrast to past decades where it thinned (shoaled). Deepening resulted from changes to ocean salinity in the mixed layer that cannot be explained by mixing with saltier water below. Thus, the increase in mixed layer salinity is inferred to result from interannual changes to the ice-ocean circulation, ice melt/growth, or river runoff. Changes to water properties also occurred in the region immediately beneath the mixed layer, making it energetically easier for heat to be combined into the mixed layer. The extent to which the mixed layer will continue to deepen appears to depend primarily on the complex set of processes that can influence mixed layer salinity.