Patrick Marchesiello, James C.
McWilliams, Keith Stolzenbach,
Nicolas Gruber

University of California at Los Angeles, Physical
Oceanography, IGPP, UCLA, 405 Hilgard Avenue, Los
Angeles, CA 90095-1567, USA, Email:
patrickm@atmos.ucla.edu

Objectives

Oceanic scientists have collected information about the CCS for nearly a century. The last thirty years has seen an acceleration in these investigations, albeit with inherently fragmentary observational sampling. Some of these studies have constructed models of the physical, biological, and biogeochemical processes occurring in this coastal region of the eastern Pacific. There is now a critical need to extend and work towards unifying these model investigations with the goal of developing a coherent and comprehensive picture of the oceanic processes for the upwelling systems.

The need for coastwide unification arises because the evidence for coherent variability in water properties, currents, zooplankton biomass, and sea-level height at coast-wide scales is now clear. If one wants to simulate phenomena confidently out to seasonal scales and longer, then far-field influences will have to be accounted for through lateral boundary conditions for the coastal region. An excellent example occurred during the 1997-98 winter when the entire coast, from 30N - 60N, experienced anomalously high sea-level. In this case the large El Nino event caused the coherent, anomalous sea-level height fluctuation. But, in addition, studies over the last twenty years have clearly demonstrated the prominence and ubiquity of energetic, mesoscale features — squirts, jets, and filaments. The many excellent observational programs mounted on this coast, some of which date back to the 1970s, are characterized by spotty spatial coverage and intermittent sampling. They cannot distinguish between phenomena forced by coast-wide forcing and those driven by local features. Modeling studies have accompanied many of these same observational programs. Unfortunately, they have suffered from the same limitations in spatial coverage, duration, and scientific scope. To strengthen multi-disciplinary modeling and data synthesis, and to satisfy the critical need for coastwide unification, we have started a few years ago a computational research program developing the Regional Oceanic Modeling System to simulate phenomena over the entire CCS, comparing the results with the extensive data sets available on the west coast. We do this modeling to address some primary scientific questions, one of them is: What is the relative importance of oceanically intrinsic vs. atmospherically forced variability? Our present solutions for the equilibrium, seasonal-cycle California Current System, are of interest both for the mean-seasonal circulation itself and as a clean demonstration of intrinsic circulation variability (mostly mesoscale) that arises from instabilities of the persistent currents in the absence of added forced variability by synoptic and inter-annual atmospheric fluctuations.

Results

We present simulations of the regional California Current System off the U.S. West Coast using ROMS. Under the influences of mean-seasonal atmospheric forcing and subtropical-gyre open boundary conditions, a robust equilibrium state is established on a time scale of a few years. It has mean alongshore and cross-shore currents similar to those estimated from hydrographic climatologies, and it also has vigorous, deep, standing-eddy patterns associated with capes and subsurface ridges along the coast. Its large-scale and mean-seasonal circulation and sea-level structure are relatively insensitive to resolution refinements below a horizontal grid scale of 20 km. However, the more sensitive standing eddies and transient eddies do modulate the classical Sverdrup balance within the coastal transition zone. The CCS has large seasonal anomalies (also as observed), driven by seasonal wind variations and propagating offshore at a Rossby-wave speed accompanied by seasonal anomalies in eddy kinetic energy whose vertical extent also deepens moving offshore. The annual- and seasonal-mean circulations exhibit strong intrinsic variability, generated mainly by baroclinic instability of the persistent currents except very near the coastline where lateral-shear instability is also important. The variability is primarily mesoscale, geostrophic currents, although there is a non-negligible ageostrophic component in the surface boundary layer and near the coast. The geographical distribution of this mesoscale variability is similar to those observed in sea level and surface currents, and at the finer model resolutions the variance magnitudes are nearly as large as observed. The mesoscale synoptic structure is a combination of upwelling fronts, offshore squirts and filaments, and eddies, many of which occur as dipoles. The eddies provide an important dissipation mechanism for the mean circulation through instabilities (although not as important as radiation into the subtropical gyre interior) and the prevalent, alongshore, coastal temperature and salinity gradient is maintained by eddy heat fluxes limiting the mean, wind-driven, upwelling advection. Finally, by performing alternative simulations selectively subtracting various model elements, we have demonstrated the significant influences of coastline shape, topographic variations in the near-shore region, and the gradient of the Coriolis frequency through its effects on the alongshore pressure gradient, mean current shears, and the general westward progression in the CCS.

We have analyzed equilibrium CCS solutions with ecosystem dynamics (from a Nutrient-Phytoplankton-Zooplankton-Detritus model) and explored the sensitivity to different parameters, forcings, and resolutions. Results from the coupled model are compared against the statistics of the ocean color (SeaWiFS) climatology, and other observational data (CalCOFI, MBARI moorings, etc). We found that phytoplankton distributions are tightly coupled to the supply of nutrients from below into the upper ocean light bathed region (euphotic zone) as a result of upwelling along the coast and inside cyclonic eddies. The local growth coupled with the horizontal advection of the phytoplankton inside jets and filaments leads then to a rich structure of simulated chlorophyll distribution that looks qualitatively very similar to the observed fields. Primary production essentially occurs in the nearshore eutrophic region while export production occurs in the offshore more oligotrophic region. This is a consequence of very efficient cross-shore transport by the mesoscale eddies. Analyses of the local balance of phytoplankton growth and death in our model revealed that an important condition for the development of the simulated phytoplankton blooms is the spatial displacement of the phytoplankton growth from zooplankton predation. This is consistent with the observation that nutrient-rich upwelled waters are dominated by large phytoplankton (diatoms) which can temporarily escape their large zooplankton predators (Euphausids and Copepods) as the latter regenerate via larval stages with reproductive delay. While our present simulations are successful in capturing the overall levels and dynamics of phytoplankton in the near-shore regions, we clearly underpredict the phytoplankton concentrations in the off-shore regions. This is a well known problem for ecosystem models that are based on a single phytoplankton compartment that has been chosen to represent the typical near-shore rapidly growing phytoplankton community. To better account for these offshore oligotrophic regions in the model, smaller species better adapted to nutrient-limited regions will have to be included in the future.

Discussion

We intentionally posed our calculations here without synoptic and interannual forcing, in order to expose the central role of intrinsic variability in the CCS. The successes of the simulations, in approximately matching much of the available observations, suggest that the large-scale structure of the CCS is substantially a deterministic response to the low-frequency, large-scale atmospheric forcing—whether local or remote and transmitted through the regional boundaries—while the mesoscale variability is intrinsic, hence chaotic with limited predictability. Yet this variability is an essential ingredient in establishing the structure and conducting the dynamics of the CCS. The least satisfactory aspect of these simulations is the sensitivity of even the large-scale circulation (especially the Davidson Current) to uncertainties in the large-scale, low-frequency wind analyses used to force the model. Finally, our simulations may be near a resolution threshold with respect to small-scale, ageostrophic instabilities, whose role is to reinforce and redistribute the mesoscale eddy field. As a result, primary productivity may increase particularly in the offshore, more oligotrophic region (high coupled model experiments have yet to be realized, but a recent coupled model in an idealized oligotrophic regime suggest that these sub-mesoscales may contribute to doubling primary production). On the other hand, basin-scale climate change affects the regional upwelling process, and thus also affects the mesoscale and submesocale activity. Therefore, it is expected that the impact of El Nino and basin-scale climate change on the upwelling ecosystems can be amplified through fluctuations in the generation of local hydrodynamical instabilities.