Decadal Variability in the North Pacific Ocean in a Coupled Physical-Ecosystem Model

A basin-wide interdecadal change in both the physical state and the ecology of the North Pacific occurred near the end of 1976. Here we use a physical-ecosystem model to examine whether changes in the physical environment associated with the 1976-77 transition influenced the lower trophic levels of the food web and if so by what means. The physical component is an ocean general circulation model, while the biological component contains 10 compartments: 4 zooplankton, 3 nitrogen, 2 silicate and CO2. The model is forced with observed atmospheric fields during 1960-1999. During spring, when the mean plankton biomass peaks in the model, there is a strong (20%) reduction in plankton biomass after the 1976 transition. The epoch difference in plankton appears to be controlled by the mixed layer depth (MLD). The enhancement of Ekman pumping in the latter period caused the halocline to shoal, and thus the MLD could not penetrate as deep in the central Gulf of Alaska during winter. As a result, more phytoplankton remained in the euphotic zone and phytoplankton concentrations began to increase earlier in spring during 1977-88 relative to 1970-76. Zooplankton populations also increased but then grazing pressure lead to a strong decrease in phytoplankton by April followed by a drop off in zooplankton by May. Essentially the mean seasonal cycle of plankton biomass is shifted earlier in the year. Finally, there is a rebound in plankton concentrations leading to an enhancement in zooplankton biomass by mid summer after 1976 but the increase is much smaller than observed.