Emergence of Anthropogenic Trends in California Current Upwelling in the Presence of Natural Climate Variability

Upwelling in the California Current sustains a productive ecosystem and is mediated by alongshore winds that are associated with a steep atmospheric pressure gradient between the North Pacific High (NPH) and a low-pressure system over the North American continent. Recent studies have projected a poleward displacement of the NPH, which would influence the magnitude, seasonality, and location of upwelling in the California Current. While previous studies have investigated this hypothesis, such attempts have been limited in their ability to distinguish anthropogenic forcing from natural climate variability due to their utilization of multi-model ensembles. In multi-model ensembles, model divergences are the result of both natural variability and inter-model variability. To address this concern, we utilized the output of the CESM1 Large Ensemble in which divergences among runs can be attributed entirely to the model’s natural variability. We compared the influence of anthropogenic activity and natural climate variability on future upwelling in the California Current. Our results demonstrate that projected anthropogenic trends in coastal upwelling rate differ with latitude, with areas of future summertime enhancement of upwelling found to the north of Cape Blanco, Oregon, and areas of weakened coastal upwelling found to the south. These changes were accompanied by a weakening of curl-driven downwelling off the coast of California. We believe that the overall response is related to a poleward migration of the NPH. Here, we discuss future California upwelling from the top down—from the position of the NPH to the magnitude of vertical velocity in the ocean. We approach this probabilistically, considering the emergence of the anthropogenic signal amidst the noise of natural climate variability.

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