Eastern boundary current systems are characterized by strong upwelling of nutrient-rich deep waters and high biological productivity, yet the coupling of physical processes and ecosystem response is not well understood, partly because of the importance of small-scale, short-term variability. We address this issue by investigating these processes in the California Current system with the Regional Oceanic Modeling System (ROMS).
Our U.S. west coast setup of ROMS has a horizontal resolution of about 15 km with a nested model covering the central California coast with a resolution of 5 km. The biogeochemical model is an NPZD-type model with nitrogen as the limiting nutrient. The model results (e.g. Chlorophyll concentrations) agree fairly well with observations from remote sensing (SeaWiFS) and shipboard measurements (CalCOFI). However, the simulated productivity is less than estimates derived from remotely sensed or other observed data. We will discuss possible reasons for this discrepancy.
We studied the fluxes of nitrogen through the system under climatological
forcing. The coupling of ocean physics and biogeochemistry has a strong
impact on the dynamics of the system. New production is fueled by the upwelling
of nitrate along the coast. It is highest in jets and filaments originating
from capes and ridges, with the f-ratio reaching values of 0.6. Further
offshore, regenerated production becomes more dominant with f-ratios as
low as 0.2. Both new and regenerated production undergo a distinct seasonal
cycle, with production peaking between spring (Southern California Bight)
and summer (central California). Zooplankton exerts a strong control on
the maximum concentration of phytoplankton. Over large scales in time and
space, new and export production equal each other. However, strong decoupling
between the two is observed on small temporal and spatial scales as discussed
in detail by Plattner et al.
For citations, please use:
Frenzel, H., N. Gruber, J. C. McWilliams, and G.-K. Plattner, Simulation of plankton ecosystem dynamics and upper ocean biogeochemistry in the California Current system, Eos Trans. AGU, 85(47), Fall Meet. Suppl., Abstract OS54A-03, 2004.