Seasonality of coastal upwelling off central and northern California: New insights, including temporal and spatial variability

The coastal ocean environment off California is largely determined by wind-driven coastal upwelling, with an ecosystem that is tightly coupled to seasonality in this upwelling. Three decades of data measured over the California shelf at NOAA buoys are used to describe the seasonal variability of the winds that force upwelling and the response of the coastal ocean in terms of sea temperature. Moreover, seasonal patterns in surface chlorophyll and alongshore currents are determined from one decade of data. In addition to clear seasonality, all these data exhibit distinct spatial and non-seasonal temporal variability in upwelling. Based on alongshore wind stress characteristics in central and north California, three seasons are defined: Upwelling Season (April-June) with strong upwelling-favorable winds and large standard deviation due to frequent reversals; Relaxation Season ( July-September) with weak equatorward winds and low variability; and Storm Season ( December-February) characterized by weak mean wind stress but large variability. The remaining months are transitional, falling into one or other season in different years. In addition to large-scale latitudinal differences in wind stress, spatial differences are associated with coastal topography - specifically the acceleration of wind downstream of capes. Latitudinal differences in sea surface temperature depend on wind stress, both local and large-scale, but also on surface heating and offshore influences. Intra-annual and inter-annual anomalies in wind and sea surface temperature are associated with variability in coastal winds, large-scale winds, and offshore basin-scale ocean conditions. Satellite chlorophyll concentration shows an optimal window relation with upwelling forcing, allowing maximum concentrations during moderate winds and minimal during poor or strong winds.