An ecosystem model for the North Pacific embedded in a general circulation model: Part II: Mechanisms forming seasonal variations of chlorophyll

Mechanisms forming seasonal variations of the oceanic ecosystem are examined using an ecosystem model embedded in an ocean general circulation model for the North Pacific. The North Pacific can be divided into seven areas based on seasonal variation patterns of chlorophyll. The areal division compares well with that based on observations. For example, a distinct spring bloom can be found in the Bering Sea while the standing stock of phytoplankton is somewhat constant in most of the subpolar gyre throughout the year. It is shown that physical factors determining the modeled seasonal variations are the annual mean vertical flow, vertical mixing, and, less importantly, solar radiation. The annual mean vertical flow can affect the seasonal variation patterns through modifying the relation between the ecosystem and the mixed layer depth (MLD). Various combinations of these factors result in diverse patterns. But it is possible to classify them in terms of the amplitude of the seasonal variation of MLD and the annual mean vertical flow. The results suggest that the difference in physical environments can yield much of the diversity of the observed seasonal variation patterns.