Formation of winter water on the Canadian Beaufort shelf: New insight from observations during 2009-2011

The Arctic halocline forms a cold stratified barrier between the seasonally modified near‐surface layers and deeper Atlantic‐derived waters. Its low temperature is maintained by intrusions of cold water formed over Arctic shelves in winter. Surprisingly, cold salty (33) water capable of halocline ventilation (Beaufort Sea Winter Water: BSWW) has been observed in the Beaufort Sea during some winters despite the low salinity (20–25) of shelf waters there in summer. This study uses year‐round data from moored instruments on the Beaufort shelf and slope during 2009–2011 to investigate the mechanisms involved. Our analysis reveals that four air‐sea interaction processes contribute to the formation of BSWW—flushing of the low‐salinity surface water from the shelf via Ekman transport in late summer and early fall, compensatory upwelling of more saline halocline water onto the shelf, net seaward ice drift that promotes ice production by maintaining a flaw lead, and entrainment of dense upwelled water into the freezing surface layer on the inner shelf. This work moves beyond earlier studies in revealing that while weather conditions were more favorable to BSWW formation in the winter of 2010–2011 than in 2009–2010, the difference was more strongly influenced by Ekman transport (offshore at the surface, onshore at the seabed) than by differences in cumulative brine injection from ice growth. The strength of the Ekman circulation over the Canadian Beaufort shelf in winter and its interannual variation have significance for surface nutrient renewal and for the cross‐shelf transport of pollutants at the surface and the seabed. Key Points: The properties of Beaufort Sea Winter Water (BSWW) vary each year During some years, BSWW is the source of Cold Shelf‐water Intrusions (CSI) Dense BSWW formation is linked to strong upwelling