Revisiting the Circulation of Hudson Bay: Evidence for a Seasonal Pattern.

The Hudson Bay Complex is the outlet for many Canadian rivers, receiving roughly 900 km3/year of river runoff. Historically, studies found a consistent cyclonic flow year‐round in Hudson Bay, due to the geostrophic boundary current induced by river discharge and cyclonic wind forcing that was supported by available observations at that time. Using a high‐resolution ocean general circulation model, we show that in summer, the mean circulation is not cyclonic but consists of multiple small cyclonic and anticyclonic features, with the mean flow directed through the center of the bay. Absolute Dynamic Topography and velocity observations also show this seasonal flow pattern. We find that this summer circulation is driven by geostrophic currents, generated by steric height gradients, which are induced by increased river discharge during the spring freshet, and reinforced by anticyclonic seasonal wind patterns. Plain Language Summary: Knowing the direction of the currents, as well as speed, in Hudson Bay is important for understanding regional ocean flow patterns. Currents in Hudson Bay were historically thought to flow counterclockwise. We use a state‐of‐the‐art computer simulation to see if this flow pattern changes during the year. We found that in May and June, the flow pattern in eastern Hudson Bay reverses and the currents flow in a clockwise direction, while currents in western Hudson Bay still flow counterclockwise. Satellite measurements of ocean currents also show this switch in the current direction in eastern Hudson Bay. This reversal of the current direction occurs because of a change in the wind direction as well as the amount of river water entering the bay during spring (May–June). In spring, river water flowing into the bay increases because of snow melt. More river water enters southeastern Hudson Bay, in and around James Bay, causing higher sea levels in the east compared to the west. This water will flow from high to low (east to west), causing the clockwise flow pattern in summer. Key Points: The spring freshet increases the halosteric height in eastern Hudson Bay, generating a seasonal flowThis anticyclonic seasonal flow pattern is present in observations and the ocean model simulationGeostrophic flow that is reinforced by atmospheric forcing drives this springtime flow pattern [ABSTRACT FROM AUTHOR]