Effects of stratification and shelf slope on nutrient supply in coastal upwelling regions

An idealized, two-dimensional numerical modeling study is presented to investigate the effects of variable shelf slope and stratification on surface mixed layer (SML) nutrient supply during upwelling. As reported previously, the physical flow regime is governed by a topographic Burger number. Gradual shelf slope and weak stratification concentrate onshore transport in the bottom boundary layer (BBL) while steep slope and strong stratification increase the relative interior transport between the SML and BBL. In 20 day model simulations initialized with a linear nitrate profile, BBL nitrate flux decreases with increasing Burger number. The opposite is true for interior nitrate flux. Upwelling source depth is also investigated and increases more rapidly with weak stratification and steep slope. Both nitrate flux and source depth are well represented by an empirical model approaching an asymptotic value with time. Model experiments representing specific locations in major upwelling systems are analyzed to determine the impact of global variability in physical parameters on event-scale nitrate supply. After 5 days, nitrate flux into the SML is similar to 45 mmol s(-1) m(-1) of coastline at a Peru site, similar to 30 mmol s(-1) m(-1) at northern California and northwest Africa sites, and <2 mmol s(-1) m(-1) off Newport, Oregon. BBL flow dominates onshore transport in northwest Africa and northern California runs, while the interior contributes significantly at our Peru and Oregon sites. Nitrate flux estimates based on constant upwelling source depth are strongly dependent on source depth choice at our selected California Current sites and less so at selected Peru and Canary Current sites.