Cross‐Shore Flow and Implications for Carbon Export in the California Current Ecosystem: A Lagrangian Analysis

Abstract

In Eastern boundary upwelling systems (EBUS), winds blow alongshore during the upwelling season, causing nutrient-replete deep waters to come to the euphotic zone through divergence of Ekman transport at the coast, stimulating elevated net primary production (NPP) and inorganic carbon uptake. This primary production induces secondary production, the recycling of nutrients, and export of organic carbon into the aphotic zone. EBUSs are strongly advective systems: Ekman currents, as well as intense geostrophic mesoscale eddies and filaments, transport nearshore surface water offshore. The transport of organic matter (OM) from formation sites to export sites, occurs in three-dimensional space: the location of carbon export may be offset spatially from the region of elevated PP (Gruber et al., 2011; Olivieri & Chavez, 2000; Plattner et al., 2005). Accurately representing the processes and spatial locations of carbon production and export is Abstract Eastern Boundary Current Upwelling Systems are regions of elevated primary production and carbon export and thus play a central role in the global carbon cycle. In these regions, nutrient upwelling occurs in a narrow region close to the coast, but primary production and carbon export are typically observed across a broader region. The fact that productive waters reach the open ocean has important consequences for the biological carbon pump, because such transport connects nutrient sources close to the coast to the deep carbon sinks of the offshore ocean. However, many aspects of this offshore transport are still not known. Here we address seasonal and interannual variability of upwelling-related cross-shore flows in the California current ecosystem (CCE) by employing Lagrangian diagnostics of horizontal transport inferred from satellite data. We define an advective age as the time a water parcel flowed offshore of the 500 m isobath. We find that the offshore extension of high Chl-a waters covaries with the age of a coastal water parcel, and is consistent with mesoscale circulation. Interannual variability in the offshore extent of older waters is primarily driven by mesoscale variability and covaries with large scale forcing by both ENSO and the NPGO. The measured ratio of in-situ new production: carbon export also covaries with water age, and tends to be ≫ 1 in younger and more balanced in older waters. Our results may help to parameterize the role of the finescale on the export of carbon in upwelling regions for climate resolving models.