Continued weakening of the equatorial Pacific upwelling annual cycle in CMIP5 future projections

This study explores the dynamics of the equatorial Pacific upwelling annual cycle under global warming using the Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations. Through a linear-weighted theory developed recently, the theoretical upwelling annual cycles under global warming helped reasonably characterize the patterns of the original upwelling annual cycles simulated in CMIP5 models; however, an apparent weakening in magnitude as compared to that during the present stage was observed. To verify the above, we divided 90-year outputs in the CMIP5 future projections into three 30-year windows and set side by side. The long-term evolution of the upwelling annual cycle reconfirmed an overall weakening tendency in the entire equatorial Pacific. Moreover, the weakening of the Ekman upwelling could most likely be attributed to the meridional surface wind stress divergence, while the gradually smoothing inclination in the overall equatorial thermocline depth was responsible for the weakening of the wave upwelling. The weakening of the wave upwelling in the east and the Ekman upwelling in the west jointly contributed to the gradual weakening of the equatorial Pacific upwelling annual cycle. The above projected changes are robust among the 19 chosen CMIP5 models. Equatorial upwelling largely influences the sea surface temperature, associated atmosphere-ocean interactions, and convection and precipitation in tropical areas; hence, a continuous weakening of the upwelling annual cycle over the equatorial Pacific Ocean could likely affect the major climate phenomena variability with strong seasonal-locking characteristics by modifying the background strength at their peak phases in the future. The theoretical results can provide us the equatorial upwelling annual cycle patterns based on the Ekman and wave dynamics, which would be a strong tool for our investigations on the climate variability under global warming.