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Much of the observed interannual variability in the physical and biogeochemical state of the California Current System (CalCS) is associated with El Nino Southern Oscillation. Yet it is unclear whether this is primarily a result of atmospheric teleconnections forcing the ocean locally through changes in wind and fluxes of heat and freshwater, or whether this is a consequence of oceanic interior processes that transport tropical variability through, e.g., coastally trapped waves to the region. Here we investigate the relative contribution of these two mechanisms in the CalCS using a novel setup of the Regional Oceanic Modeling System coupled to a biogeochemical/ecological model. We conducted a hindcast simulation over the period 1979–2013 and contrast the results with those from sensitivity simulations with climatological atmospheric boundary conditions either for the U.S. West Coast or the rest of the Pacific. We find that remote forcing dominates the variability of the physical state in the nearshore region of the CalCS, explaining up to 80% of monthly mean sea-surface height and temperature variability. In contrast, local processes tend to drive variations in the biogeochemical/ecological state, particularly along central and northern California, explaining up to 50% of the observed surface variability. Most of the remote forcing is a consequence of coastally trapped waves that travel northward at speeds of approximately 230 km d−1, and thereby alter sea-level height, thermocline structure, and upwelling along California. Biogeochemically active tracers respond to this remote forcing as well, especially at depth, but are more strongly modulated by local atmospheric forcing, especially variations in upwelling-favorable winds.

Ocean deoxygenation has led to a measurable shoaling of subsurface hypoxic waters ([O-2] < 60 mmol m(-3)) in the tropical Pacific and elsewhere. This shoaling compresses the vertical habitat of heterotrophic organisms between the well oxygenated surface and the hypoxic interface. Superimposed on this long-term trend, natural variability can cause shorter-term, extreme habitat compression events, which we term Transient Habitat Reduction Extreme Events (THREEs). Here, we investigate THREEs in the Eastern Pacific (EP) between 1979 and 2016 using output from a high-resolution hindcast simulation with the coupled physical-biogeochemical ocean model ROMS-BEC. We identify THREEs on the basis of the hypoxic interface being shallower than the 99th percentile of its variability and requiring a minimum duration of five days. We find that THREEs compress the aerobic habitat by up to 50%-70% (locally exceeding 80%) in the subtropical and tropical EP and reach horizontal scales exceeding 300 km in the tropical EP. There, THREEs are pre-conditioned by La Nina conditions and occur primarily in boreal winter and spring. In the subtropical EP, THREEs are associated with mesoscale eddies and occur independently of the season. 71% (32%) of all THREEs are characterized by unusually high acidity (cold) conditions (i.e., below the first percentile), thus constituting compound events. The strong habitat compression during THREEs is bound to lead to substantial changes in water column biogeochemistry, in food-web interactions, and in ecosystem structure. Yet, so far, very little is known about the impact of these transient phenomena., Journal of Geophysical Research: Oceans, 127 (6), ISSN:0148-0227, ISSN:2169-9275