Regime Shifts in the Gulf of Alaska Simulated by Ocean Biogeochemical Models

Regime shifts in the late 1970s and 1980s were observed in the Gulf of Alaska, indicated by abrupt change in the physical conditions and major shifts in the catch of many fish species. The extent and mechanisms leading to such regime shifts is not yet fully understood due to data paucity in time and space and limitations from statistical techniques available for their detection. Here we investigate the ability of a suite of ocean biogeochemistry models of varying complexity to simulate regime shifts in the Gulf of Alaska by testing the presence of abrupt changes in time series of physical variables (sea surface temperature and mixed layer depth), nutrients and biological variables (chlorophyll, primary productivity and plankton biomass). We use a change-point detection methodology allowing detecting abrupt changes and distinguishing forced shifts from an unforced random reorganization of the system. Our study demonstrates that ocean biogeochemical models are able to simulate the late 1970s shift, as an abrupt increase in sea surface temperature, followed by an abrupt decrease in nutrients and biological productivity. Some models also simulate a shift in the late 1980s, suggesting controls from changes in mixed layer depth. We further investigate whether the biological response in the late 1970s and 1980s was a linear response to the physical forcing or whether thresholds were crossed. Our study demonstrates the usefulness of ocean biogeochemical models to investigate the underlying mechanisms of regime shifts by simulating how the shift is propagating through the ecosystem.