A 31 year sequence of historical analyses of the California Current System (CCS) is used to describe the central CCS (35-43˚N) coastal upwelling response to El Niño-Southern Oscillation (ENSO) variability. The analysis period captures 10 El Niño and 10 La Niña events, including the extreme El Niños of 1982-1983 and 1997-1998. Data-assimilative model runs and backward trajectory calculations of passive tracers are used to elucidate physical conditions and source water characteristics during the upwelling season of each year. In general, El Niño events produce anomalously weak upwelling and source waters that are unusually shallow, warm, and fresh, while La Niña conditions produce the opposite. Maximum vertical transport anomalies in the CCS occur $1 month after El Niño peaks in midwinter, and before the onset of the upwelling season. Source density anomalies peak later than transport anomalies and persist more strongly through the spring and summer, causing the former to impact the upwelling season more directly. As nitrate concentration covaries with density in the central CCS, El Niño may exert more influence over the nitrate concentration of upwelled waters than it does over vertical transport, although both factors are expected to reduce nitrate supply during El Niño events. Interannual comparison of individual diagnostics highlights their relative impacts during different ENSO events, as well as years deviating from the canonical response to ENSO variability. The net impact of ENSO on upwelling is explored through an ''Upwelling Efficacy Index'', which may be a useful indicator of bottom-up control on primary productivity.

The Regional Ocean Modeling System (ROMS) 4-dimensional variational (4D-Var) data 1 assimilation tool has been used to compute two sequences of circulation analyses for the 2 U.S. west coast. One sequence of analyses spans the period 1980-2010 and is subject to 3 surface forcing derived from relatively low resolution atmospheric products from the Cross-4 Calibrated Multi-Platform wind product (CCMP) and the European Centre for Medium 5 Range Weather Forecasts (ECMWF) reanalysis project. The second sequence spans the 6 shorter period 1999-2012 and is subject to forcing derived from a high resolution product from 7 the Naval Research Laboratory Coupled Ocean Atmosphere Mesoscale Prediction System 8 (COAMPS). The two analyses periods are divided into eight day windows, and all available 9 satellite observations of sea surface temperature and sea surface height, as well as in situ 10 hydrographic profiles are assimilated into ROMS using 4D-Var. The performance of the 11 system is monitored in terms of the cost function and the statistics of the innovations, and 12 the impact of data assimilated on the circulation is assessed by comparing the posterior 13 circulation estimates with the prior circulation and the circulation from a run of the model 14 without data assimilation, with particular emphasis on eddy kinetic energy. This is part I 15 of a two part series, and the circulation variability of the 4D-Var analyses is documented in 16 part II.17

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A historical analysis of California Current System (CCS) circulation, performed using the Regional Ocean Modeling System with four‐dimensional variational data assimilation, was used to study upwelling variability during the 1988–2010 period. We examined upwelling directly from the vertical velocity field, which elucidates important temporal and spatial variability not captured by traditional coastal upwelling indices. Through much of the CCS, upwelling within 50 km of the coast has increased, as reported elsewhere. However, from 50 to 200 km offshore, upwelling trends are negative and interannual variability is 180° out of phase with the nearshore signal. This cross‐shore pattern shows up as the primary mode of variability in central and northern CCS vertical velocity anomalies, accounting for ∼40% of the total variance. Corresponding time series of the dominant modes in the central and northern CCS are strongly correlated with large‐scale climate indices, suggesting that climate fluctuations may alternately favor different biological communities.

Given the ecological and economic importance of eastern boundary upwelling systems like the California Current System (CCS), their evolution under climate change is of considerable interest for resource management. However, the spatial resolution of global earth system models (ESMs) is typically too coarse to properly resolve coastal winds and upwelling dynamics that are key to structuring these ecosystems. Here we use a high-resolution (0.1°) regional ocean circulation model coupled with a biogeochemical model to dynamically downscale ESMs and produce climate projections for the CCS under the high emission scenario, Representative Concentration Pathway 8.5. To capture model uncertainty in the projections, we downscale three ESMs: GFDL-ESM2M, HadGEM2-ES, and IPSL-CM5A-MR, which span the CMIP5 range for future changes in both the mean and variance of physical and biogeochemical CCS properties. The forcing of the regional ocean model is constructed with a “time-varying delta” method, which removes the mean bias of the ESM forcing and resolves the full transient ocean response from 1980 to 2100. We found that all models agree in the direction of the future change in offshore waters: an intensification of upwelling favorable winds in the northern CCS, an overall surface warming, and an enrichment of nitrate and corresponding decrease in dissolved oxygen below the surface mixed layer. However, differences in projections of these properties arise in the coastal region, producing different responses of the future biogeochemical variables. Two of the models display an increase of surface chlorophyll in the northern CCS, consistent with a combination of higher nitrate content in source waters and an intensification of upwelling favorable winds. All three models display a decrease of chlorophyll in the southern CCS, which appears to be driven by decreased upwelling favorable winds and enhanced stratification, and, for the HadGEM2-ES forced run, decreased nitrate content in upwelling source waters in nearshore regions. While trends in the downscaled models reflect those in the ESMs that force them, the ESM and downscaled solutions differ more for biogeochemical than for physical variables.

Long‐term timeseries of upper ocean salinity and nutrients collected in the Alaskan Gyre along Line P exhibit significant decadal variations that are shown to be in phase with variations recorded in the Southern California Current System by the California Cooperative Oceanic Fisheries Investigation (CalCOFI). We present evidence that these variations are linked to the North Pacific Gyre Oscillation (NPGO)—a climate mode of variability that tracks changes in strength of the central and eastern branches of the North Pacific gyres and of the Kuroshio‐Oyashio Extension (KOE). The NPGO emerges as the leading mode of low‐frequency variability for salinity and nutrients. We reconstruct the spatial expressions of the salinity and nutrient modes over the northeast Pacific using a regional ocean model hindcast from 1963‐2004. These modes exhibit a large‐scale coherent pattern that predicts the in‐phase relationship between the Alaskan Gyre and California Current timeseries. The fact that large‐amplitude, low‐frequency fluctuations in salinity and nutrients are spatially phase‐locked and correlated with a measurable climate index (the NPGO) open new avenues for exploring and predicting the effects of long‐term climate change on marine ecosystem dynamics.

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A physical‐biogeochemical model is used to produce a retrospective analysis at 3‐km resolution of alongshore phytoplankton variability in the California Current during 1988–2010. The simulation benefits from downscaling a regional circulation reanalysis, which provides improved physical ocean state estimates in the high‐resolution domain. The emerging pattern is one of local upwelling intensification in response to increased alongshore wind stress in the lee of capes, modulated by alongshore meanders in the geostrophic circulation. While stronger upwelling occurs near most major topographic features, substantial increases in phytoplankton biomass only ensue where local circulation patterns are conducive to on‐shelf retention of upwelled nutrients. Locations of peak nutrient delivery and chlorophyll accumulation also exhibit interannual variability and trends noticeably larger than the surrounding shelf regions, thereby suggesting that long‐term planktonic ecosystem response in the California Current exhibits a significant local scale (O(100 km)) alongshore component.