North Pacific Gyre Oscillation modulates seasonal timing and ecosystem functioning in the California Current upwelling system

Abstract

International audienceOn interannual and longer time scales, dynamical and biogeochemical fluctuations in the North Pacific are dominated by two modes of variability, namely the Pacific Decadal Oscillation and the North Pacific Gyre Oscillation (NPGO). In this study the regional expression of the NPGO in the California Current System (CCS) is detailed. The statistical relationship between the NPGO index and nearshore wind variability (mostly upwelling favorable) along the U.S. West coast is strongest in the wintertime (December to March) off Central California. Most importantly, NPGO fluctuations are associated with a seasonal shift of 1-2 months in the onset of the upwelling season. Regional numerical simulations show that an early (late) onset of upwelling during the positive (negative) phase of the NPGO leads to a more (less) productive planktonic ecosystem throughout spring and summer, i.e., several months after the direct NPGO effects on the system have ceased. These results bring new light on the California ecosystem variability as observed in atypical years such as 2005 and 2007

California Coastal Upwelling Onset Variability: Cross-Shore and Bottom-Up Propagation in the Planktonic Ecosystem

“…At the spring transition – the onset of upwelling – winds become predominantly upwelling-favorable, usually sometime between January and April, with some latitudinal and interannual variability. Chenillat et al [8] showed that a positive phase of the NPGO is characterized by strong alongshore winter winds leading to an early spring transition, with the opposite patterns during a negative NPGO. These changes in the onset of upwelling change the nutrient input and consequent phytoplankton bloom, which in turn influence the rest of the coastal trophic web [9], [10], [11], [12].…”

“…A bottom-up process can certainly be anticipated, but its expression in time and space and the way it affects trophic links are unknown. Building on new findings about the seasonal expression of the NPGO and its effect on the onset of upwelling [8], we investigate the influence of changes in the timing of the spring transition on the structure and functioning of the California Current planktonic ecosystem. In particular, we examine the cross-shore dependence of the ecosystem response to a time lag in the onset of wind-driven coastal upwelling.…”

“…We use a regional hydrodynamic model forced by two synthetic wind climatologies that only differ in winter, corresponding to early or late upwelling onset, such as those typically arising in NPGO+ or NPGO- conditions [8]. This model is coupled to an ecosystem model composed of several phytoplankton and zooplankton size classes.…”

Synthesis of Pacific Ocean Climate and Ecosystem Dynamics

“…An important fraction of these changes in the southern CCS was linked to the NPGO (Chenillat et al, 2012). In the PCCS, changes in seasonal timing are still not well understood but are likely connected to changes in the seasonal cycle of the ENSO system.…”

Seasonal distribution of late larval and juvenile rockfish (Sebastes spp.) and associated environmental conditions off Oregon and Washington: new insights based on genetics

“…Large-scale indices (PDO and NPGO) displayed values considered more favorable for productivity (more negative PDO and more positive NPGO; Chenillat et al, 2012;Sydeman et al, 2014) . Highest densities of late-larval and juvenile rockfish (identified and unidentified) were sampled in August and September of all years.…”