Spacebased Sea Surface Salinity Depicts Freshwater Changes in the Hudson Bay

Abstract

This study investigates the sea surface salinity (SSS) observed by the Soil Moisture Active Passive (SMAP) mission (2015-present) and Soil Moisture Ocean Salinity (SMOS) mission (2011-present) in the open water season in the Hudson Bay and its association with the freshwater flux. A unique large inland sea in northern Canada located in the southern margin of the Arctic Circle, the Hudson Bay is covered by ice and snow in winter. Around 1300 km3 low salinity liquid water trapped in ice is completely melt and released in summer. The length of the open water season has been increasing in recent years, allowing more days during which the SSS can be retrieved from satellite microwave radiometer data (from SMAP and SMOS). The SSS reflects and links the impact of freshwater inputs from river discharge, sea ice changes, surface forcing (P-E), and exchange with northern Atlantic Ocean through the Hudson Strait and Arctic Ocean through the Foxe Basin. Both SMAP (JPL V4) and SMOS (BEC) SSS shows a general pattern of freshening in the James and southern Hudson bay, salty in the northern part, and saltier in the Hudson Strait and the Foxe Basin, which are consistent for three years during (2015-2017). However, the SMAP SSS shows some patches of extreme low SSS with large inter-annual variations, particularly early in the melt season; which is missing in SMOS. Dividing the Hudson Bay system into sub-regions, we examine local freshwater inputs in these regions using data of river discharge, sea ice concentration, surface precipitation and evaporation data. We found river discharges and seasonal sea ice changes are the two dominant processes affecting SSS inter-annual variability, but at different time and places; while the surface forcing (P-E) may play a secondary role. Daily discharge rate from the rivers surrounding the Hudson Bay (Canadian EC) was analyzed. We identified that the low SSS in the eastern Hudson and James Bay in August 2015 could be linked with anomalous positive river discharge through the James Bay, which is 20% more than the three years average. We also derived the local freshwater flux from sea ice change , Ilocal, from the daily sea ice concentration (NSIDC) and ice thickness. Due to a lack of ice thickness observation coincident with SSS data, we used a uniform ice thickness of 1 m (~previous knowledge of average thickness cross the bay) in calculating Ilocal. We found that the low SSS in the northern Hudson Bay interior observed in June 2017 was associated with the early onset of ice melt, which quickly released huge amount of freshwater; while the Bay was mostly frozen in the June of 2015 and 2016. Monthly surface freshwater flux from P (CMAP) minus E (OAflux) shows magnitude much smaller than sea ice melt contribution than Ilocal.