Seasonality of the North Pacific Oligotrophic Gyre area in the past two decades and a modelling perspective for the 21st century

Abstract

As the largest oligotrophic ocean globally, the North Pacific oligotrophic ocean gyre (NPOG) exhibits pronounced variability on seasonal, decadal, and centennial time scales. Notably, changes in the seasonality of the NPOG are thought to have larger effects on marine ecosystems than changes in its annual mean state. However, the interannual variability of NPOG seasonality and its response to climate processes remain unclear. Here, we investigate the amplitude of the seasonal cycle in NPOG area and its linkage with climate variability and change. Our results show that the El Niño–Southern Oscillation (ENSO) modulated the seasonal maximum of NPOG area in boreal summer, and thus the amplitude of the seasonal cycle during 1998–2021. This is primarily due to ENSO-induced changes in nutrient transport via equatorial upwelling and thermal stratification, as well as changes in the chlorophyll-to-carbon ratio in phytoplankton cells (photoacclimation). Future projections based on Coupled Model Intercomparison Project Phase 5 (CMIP5) modelling results and an Elman neural network indicate a significant decrease in the seasonal amplitude of NPOG area by 2100, attributed to the growing seasonal minimum of NPOG area in winter along the anthropogenic increase in atmospheric CO2. The findings highlight the importance of considering seasonal differences in future research on the interannual variability of oligotrophic gyres and underscore the need for models to distinguish between the effects of climate variability and change.