Modeling the effects of coastal wind- and wind–stress curl-driven upwellings on plankton dynamics in the Southern California current system

We use a Nitrogen-Phytoplankton-Zooplankton-Detritus (NPZD) biogeochemical model implemented in a time-dependent box model scheme to simulate the temporal dynamics of the pelagic ecosystem in the Southern California Current System (SCCS). The model was forced by winds, sea surface temperature and light. Nutrient inputs to the modeled box were driven by coastal upwelling or upwelling due to wind-stress curl in order to assess the importance of each process in the temporal dynamics of the SCCS ecosystem. Model results were compared to the CalCOFI dataset, both in terms of climatological annual cycles and actual values. This comparison led to modifications of the basic model structure to better represent the coastal ecosystem, particularly phytoplankton growth and zooplankton mortality terms. Wind-stress curl-induced upwelling was found to be significant only in the offshore regions while coastal upwelling better represented the dynamics of the inshore areas. The two upwelling mechanisms work in synchrony, however, to bring nutrients to surface waters during the same time periods. Finally, the effect of low-frequency perturbations, such as those associated with the ENSO and NPGO, were assessed by comparing model results and data. Since the NPGO cycle largely impacts the SCCS through modifications of upwelling-favorable winds, its effects were well represented in the model results. In contrast, ENSO responses were poorly captured in the simulations because such perturbations alter the system by changing surface water mass distributions via mechanisms that were not included in the model forcing.

► NPZD models in a simplified box-framework are useful to study the dynamics of complex coastal systems.
► Key parameters of the model's expressions and parameters values need to be adjusted to the studied ecosystems.
► Coastal upwelling is only relevant to explain the ecosystem behavior in the near coast regions of the southern California Current System.
► Off-shore dynamics is more related with wind-curl forcing although both processes act in synchrony.
► NPGO modulates the productivity of the region through alterations of the wind-driven upwelling while ENSO acts by other processes.