Storm-driven hydrography of western Hudson Bay

Abstract

Hudson Bay (Canada) is the world's largest inland sea, which receives upward of ~700 km3 of river discharge annually. Cyclonic water circulation transports this riverine water along the coast toward Hudson Strait and into the Labrador Sea. Yearlong observations of the current velocity profile, collected from an array of oceanographic moorings deployed in western Hudson Bay from September 2016 to September–October 2017, show that cyclonic wind forcing amplifies cyclonic water circulation in Hudson Bay and favours cross-shelf exchange. Cyclonic storms generate synoptic variability of salinity in the coastal regions, which remains poorly understood. Here we use temperature, salinity and Colored Dissolved Organic Matter (CDOM) fluorescence data from the same mooring array to examine the role of atmospheric forcing on variability of temperature and salinity in western Hudson Bay. We find that in terms of the storm-driven variability, the Nelson River estuary is impacted by river runoff year round because the Nelson River flow is regulated. Our data show that cyclonic storms intensify outflow from the Nelson River estuary. This water is also high in CDOM originating from terrestrial organic matter. Thus, the storm-driven cross-shelf displacement of the estuarial water generates a negative correlation between salinity and CDOM. The upstream Hudson Bay area ~250 km northwest of the Nelson River estuary is found to be marine dominated. In this area, cyclonic storms force the surface marine water onshore, generating storm surge along the coast. During summer, the offshore water is saline and higher in CDOM than the inshore water presumably because of a marine DOM component. Thus, the storm-driven onshore displacement of this saline (marine dominated) water generates positive anomalies of salinity and CDOM, resulting in positive correlation between salinity and CDOM during summer-fall. In contrast, during winter the onshore water becomes saltier than the offshore water due to the southward alongshore transport of saline water generated in the upstream Northwest Hudson Bay polynya. The onshore displacement of water from the Hudson Bay interior during winter cyclonic storms generates negative salinity anomalies recorded by our mooring. Since there is no marine primary production during winter, the CDOM is not impacted significantly by winter cyclonic wind forcing.