Water column structure and circulation under the North Water during spring transition: April–July 1998

The North Water is a large recurring polynya in northern Baffin Bay. Over 400 hydrographic CTD profiles taken over a grid of stations between April and July 1998 were analysed to understand the water-column structures underlying the polynya during spring transition. In the North Water, imported and dissimilar water masses converge over a complex bathymetry: Northern assembly (NA) waters are derived from inflows through Nares Strait and Jones Sound; Southern assembly (SA) waters are derived from a northern-moving branch of the West Greenland Current (WGC). NA and SA waters mix primarily over the north–south Smith Sound Canyon, between 76.5°N and 77°N latitude, to form a distinct, relatively thick halocline layer (termed ‘North Water assembly’, NWA), characterised by density-compensating thermohaline intrusions. In the southern portion of the North Water, a sharp front between NWA waters and SA waters roughly aligns with the 500-m isobath. Flow patterns, deduced from potential temperature vs. salinity (θ−S) diagrams and from minima in stability frequency (N) profiles, indicate that this front results from the convergence of NWA waters and a branch of the WGC that is cyclonically re-directed as it approaches Smith Sound Canyon. Not all SA waters are immediately re-directed south towards the front, as a second branch follows the Greenland coast toward Smith Sound. While θ−S signatures show some evidence of upwelling along the Greenland coast (e.g. outcropping of isopycnal surfaces), no surfacing of warm Atlantic-derived water was observed during the sampling period (1998), suggesting that mechanical removal of ice by winds and currents is the primary mechanism by which the North Water forms. Some sensible heat exchange associated with brine rejection and penetrative convection may occur, but only during periods of active ice formation.