Winds over troubled waters : temporal changes in the onset and intensity of coastal alongshore winds in the Pacific Northwest

The timing, duration, and intensity of wind-driven upwelling and downwelling along the north Eastern Pacific coast play an integral role in coastal circulation and basin-wide ecosystem composition. It has been suggested that global warming will cause changes in these winds. Here we develop objective criteria to determine the onset, duration and intensity of upwelling and downwelling seasons due to local wind forcing. We use these criteria to examine and better characterize temporal trends in wind-driven coastal currents by allowing a flexible onset to each season, and relate them to global warming and large-scale climate oscillations in the coastal ocean between northern California and Vancouver Island (39° and 51°N). We find an exceptionally variable onset of upwelling at all locations. No significant trends in the timing and duration were found. Positive phases of the Pacific Decadal Oscillation are correlated to later and shorter upwelling seasons, but not necessarily weaker upwelling. Warm phases of the El Niño Southern Oscillation are correlated with an earlier onset of summer upwelling, and with more intense downwelling throughout the study area. Our analysis identifies strong interannual to interdecadal variability, and the importance of time series length when isolating physical temporal trends influenced by large-scale oscillatory behaviour of the climate.