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

Abstract. As the largest ocean desert, the North Pacific Ocean Desert (NPOD) exhibits pronounced variations across seasonal, decadal, and centennial time scales. Notably, changes in the seasonality of the NPOD are thought to have larger effects on marine ecosystems than variability in the annual-mean state of the NPOD. However, the interannual variability of NPOD seasonality and its response to climate processes remain unclear. Here, we investigate the amplitude of the seasonal cycle in NPOD area and its linkage with climate variability and change. Our results show that the El Niño - Southern Oscillation (ENSO) modulated the seasonal maximum of NPOD 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. 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 NPOD area by 2100, attributed to the growing seasonal minimum of NPOD 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 ocean desert and underscore the need for models to distinguish between the effects of climate variability and change.