How the Source Depth of Coastal Upwelling Relates to Stratification and Wind

Wind-driven coastal upwelling is an important process that transports nutrients from the deep ocean to the surface, fueling biological productivity. To better understand what affects the upward transport of nutrients (and many other properties such as temperature, salinity, oxygen, and carbon), it is necessary to know the depth of source waters (i.e., “source depth”) or the density of source waters (“source density”). Here, we focus on the upwelling driven by offshore Ekman transport and present a scaling relation for the source depth and density by considering a balance between the wind-driven upwelling and eddy-driven restratification processes. The scaling suggests that the source depth varies as (τ/N)1/2, while the source density goes as (τ1/2N3/2), where τ is the wind stress and N is the stratification. We test these relations using numerical simulations of an idealized coastal upwelling front with varying constant wind forcing and initial stratification, and we find good agreement between the theory and numerical experiments. This work highlights the importance of considering stratification in wind-driven upwelling dynamics, especially when thinking about how nutrient transport and primary production of coastal upwelling regions might change with increased ocean warming and stratification. Plain Language Summary Coastal upwelling is a phenomenon where wind blowing parallel to a coast causes water from depths of hundreds of meters to be brought up to the surface. Many properties in the ocean—such as temperature, nutrients, oxygen, and carbon—exhibit a strong contrast between the surface and the deep ocean, so coastal upwelling is an important process that redistributes these properties in the ocean. For example, nutrient concentration generally increases with depth and density, and coastal upwelling helps to supply high-nutrient water from the deeper ocean to the sunlit surface, which enables the growth of marine plants. Just how much nutrient reaches the surface depends on the strength of upwelling and on the source depth or source density from which water is upwelled, which determines its nutrient concentration. We develop and test a theory for predicting the source depth and density of coastal upwelling based on the strength of the wind and density stratification, that is, the density contrast between the surface and depth. This theory can help predict how effects of coastal upwelling could be altered in the future from changes in wind and increased stratification due to ocean warming.