The Influence of Natural Variability on Future California Current Upwelling

Abstract:

Upwelling in the California Current sustains a productive ecosystem and is largely mediated by the cross-shore, atmospheric pressure gradient that develops during the boreal summer. This pressure gradient may be intensified through increased warming of the continent relative to the Pacific Ocean, resulting in accelerated alongshore winds and amplified coastal upwelling (a concept known as the Bakun Hypothesis). Past attempts to investigate the Bakun Hypothesis have utilized multi-model “ensembles-of-opportunity” to examine the response of upwelling to climate change. However, attempts to distinguish anthropogenic changes relative to natural climate variability are limited, largely because model divergences present in an ensemble-of-opportunity include both natural variability and inter-model variability (i.e., the differing response of each model to identical radiative forcing). To address this concern, modeling centers are developing “large ensembles” consisting of numerous, independent model runs separated only by round-off differences in their initial atmospheric state. Differences between these runs are void of inter-model variability, allowing disparities between runs to be attributed entirely to natural variability within the system. Using output from the CESM1 Large Ensemble, we consider the influence of anthropogenic activity—relative to natural climate variability—on the future seasonality, extent, and amplitude of upwelling in the California Current. Here we show that minor perturbations in the initial atmosphere significantly alter the influence of natural variability on the system. Furthermore, we expect change to vary both seasonally and meridionally, following trends of enhanced and weakened upwelling conditions. Finally, upwelling is expected to follow a complex path, rather than increasing linearly. Our results demonstrate the importance of considering upwelling conditions over varying scales of space and time, as quite different patterns emerged over differing seasons and portions of the coastline. More strikingly, natural variability seems to play a larger role than anticipated in impacting the direction and intensity of change in the system.