Cumulative Effects of Winds and Tides on Seasonal-Mean Dissolved Oxygen Over Southwestern Scotian Shelf

Abstract

ABSTRACT Dissolved oxygen (DO) and hydrodynamics over the southwestern Scotian Shelf (swScS) were found to have significant temporal and spatial variability. A coupled circulation-oxygen model (L3COM) is used to examine the cumulative effects of winds and tides on the seasonal mean DO over the region. The performance of L3COM is validated using the available in-situ observations and global ocean reanalysis. Analysis of model results demonstrates that both winds and tides enhance the vertical mixing over the swScS, resulting in considerable temporal and spatial variability of DO over this region. Winds significantly affect the inventory and seasonal variability of upper-column DO over the swScS through their effects on the air–sea oxygen exchange, surface heat fluxes, and vertical stratification. In winter, downward air–sea oxygen fluxes and strong wind-induced mixing lead to relatively high (but undersaturated) and nearly uniform DO in the top ∼35 m over the central swScS. This relatively high DO is transported downward through vertical mixing and winter convection. In summer, upward air–sea oxygen fluxes, strong vertical stratification due to sea surface warming, and weak winds result in relatively low (but supersaturated) DO in the surface mixed layer (top ∼10 m) and relatively high DO trapped in the sub-surface layer between 10 and 40 m over the central swScS. The seasonal mean DO over the swScS is also affected by additional vertical mixing and modulation of circulation induced by tides. Over offshore waters to the southwest of Cape Sable Island, tide-induced topographic upwelling and strong vertical mixing significantly enhance the downward DO transfer. Tides also play an important role for reducing the vertical stratification underneath the surface mixed layer over the swScS.