Slope and shelf processes associated with upwelling in the northern California Current system

Abstract

The northern California Current system (CCS) experiences relatively weak upwelling-favorable wind stress during summer months, but high nutrient concentrations and hypoxic conditions occur seasonally. This study examines 1) the relative contribution of physical and biological processes to seasonal dissolved oxygen (DO) declines, 2) the role of coastal trapped waves in setting up the California Undercurrent (CUC) and a poleward alongshelf pressure-gradient force over the slope, 3) the role of submarine canyons in the upwelling of water from the slope onto the shelf. The physical processes of upwelling and changes in source water contribute to interannual variability in hypoxic conditions, but cannot solely account for seasonal oxygen decreases. Mass balances of DO and nitrate show that biochemical processes in the water column and sediments each contribute ~50% to the total consumption of DO in near-bottom water. Alongshelf advection of sharp gradients induced by localized respiration leads to variability at event (2-10 d) time scales on the inner shelf. The dynamics of the CUC and alongshore pressure gradient over the slope are explored using the Navy Coastal Ocean Model and a linear coastal trapped wave model. In the linear model, alongshore winds drive poleward flow events over the slope, but a persistent undercurrent in the northern CCS is dependent on the use of San Diego sea level at the southern boundary. In the northern CCS, alongshore density gradients over the slope lead to a poleward pressure-gradient force within the undercurrent that often opposes an equatorward force associated with coastal sea level. To examine how water is transported from the slope to the shelf, a regional model hindcast is used to isolate the dynamics and impacts of canyon upwelling. Canyon upwelling occurs during periods of southward flow over the shelf which occur throughout the upwelling season. Water is transported from depths of ~150-250 m over the slope to the near-bottom water at mid shelf. Canyon-enhanced fluxes in the bottom portion of the water column are large enough to influence changes in salinity over the shelf at seasonal time scales.