Decadal fluctuations in ocean salinity, nutrients, chlorophyll, a variety of zooplankton species, and fish stocks in the Northeast Pacific have been unexplained for many years. They are often poorly correlated with the most widely used indicator of large-scale climate variability in the region: the Pacific Decadal Oscillation (PDO). Researchers Emanuele Di Lorenzo of the Georgia Institute of Technology and Niklas Schneider of the University of Hawaii recently defined a new pattern of climate change—the North Pacific Gyre Oscillation (NPGO)—and showed that its variability is significantly correlated with the previously unexplained fluctuations of salinity, nutrients, and chlorophyll.
Fluctuations in the NPGO are driven by the same fundamental processes that control salinity and nutrient concentrations. In the California Current System, the NPGO particularly reflects changes in the winds that cause coastal upwelling, the process by which subsurface cold water that is rich in nutrients is brought up to the surface. These results strongly support the use of the NPGO as the primary indicator of upwelling strength and nutrient fluxes, and, therefore, the potential for ecosystem change in the California Current System region. Changes in nutrient fluxes drive fluctuations in modeled chlorophyll concentration—an indicator of phytoplankton concentration—that are highly correlated to observed chlorophyll. The model simulations support the hypothesis that variations in phytoplankton biomass in the California Current System region are primarily driven by changes in wind-driven upwelling correlated with the NPGO. The NPGO thus provides a strong indicator of changes in the mechanisms driving oceanic ecosystem dynamics.
This "bottom-up" forcing is consistent with previous fish catch data and satellite-derived chlorophyll concentration, and underscores the need to better understand the influences of physically forced nutrient fluxes on higher food-chain levels in the ocean. The researchers have also shown that the NPGO pattern extends beyond the North Pacific and is part of a global mode of climate variability that is evident in global sea-level trends and sea surface temperature. The amplification of the NPGO variability found in observations and in model simulations of global warming scenarios implies that the NPGO may play an increasingly important role in forcing global-scale decadal changes in marine ecosystems.