Coastal Upwelling Revisited: Ekman, Bakun, and Improved Upwelling Indices for the US West Coast

Coastal upwelling is responsible for thriving marine ecosystems and fisheries that are disproportionately productive relative to their surface area, particularly in the world's major eastern boundary upwelling systems. Along oceanic eastern boundaries, equatorward wind stress and the Earth's rotation combine to drive a near-surface layer of water offshore, a process called Ekman transport. Similarly, positive wind stress curl drives divergence in the surface Ekman layer and consequently upwelling from below, a process known as Ekman suction. In both cases, displaced water is replaced by upwelling of relatively nutrient-rich water from below, which stimulates the growth of microscopic phytoplankton that form the base of the marine food web. Ekman theory is foundational and underlies the calculation of upwelling indices such as the Bakun Index that are ubiquitous in eastern boundary upwelling system studies. While generally valuable first-order descriptions, these indices and their underlying theory provide an incomplete picture of coastal upwelling. Here we review the relevant dynamics and limitations of classical upwelling indices, particularly related to representation of the surface wind stress, the influence of geostrophic currents, and the properties of upwelled water. To address these shortcomings, we present two new upwelling indices for the U.S. West Coast (31-47 degrees N), which are available from 1988 to present. The Coastal Upwelling Transport Index and the Biologically Effective Upwelling Transport Index provide improved estimates of vertical transport and vertical nitrate flux, respectively, by leveraging technological and scientific advances realized since the introduction of the Bakun Index nearly a half century ago. The California Current System, running along the North American West Coast, hosts a rich and diverse marine ecosystem that provides considerable socioeconomic benefit. The process underlying this exceptional biological productivity is wind-driven coastal upwelling, which delivers deep, nutrient-rich water to the sunlit surface layer and stimulates growth of phytoplankton that form the base of the marine food web. Given the ecological importance of upwelling, indices designed to monitor its intensity (e.g., the Bakun Index) were introduced nearly 50years ago. While these indices have proved extremely useful, they have a number of limitations as they are derived from relatively coarse resolution atmospheric pressure fields. In particular, uncertainties arise in the estimation of wind stress and from the omission of the influence of ocean circulation. Furthermore, historical indices estimate only the amount of water upwelled, not the nutrient content of that water. Here we present new indices that leverage ocean models, satellite data, and in situ observations to more accurately estimate upwelling strength as well as the amount of nitrate being upwelled. The new indices are publicly available, extend from 1988 to present, and will be valuable for monitoring upwelling in near real time and for understanding its impacts on the marine ecosystem.