933 / 2024-09-19 21:01:23

Salinity-effect Strengthens ENSO Forced by Freshwater Flux in Response to Global Warming in the Tropical Pacific within Flux-Anomaly-Forced Model Intercomparison Project

salinity,freshwater flux,stratification,global warming,ENSO

The increase of CO₂ emissions resulting from human activities has significantly intensified the global water cycle, leading to an evident impact on ocean physics. However, there is no clear consensus on how to distinguish climate responses to freshwater flux (FWF) forcing during future warming climate. We investigate salinity change and its effect on ENSO in response of FWF forcing due to doubling CO₂ emissions using the Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP) experiments in phase 6 of the Coupled Model Intercomparison Project (CMIP6). The result reveals an amplification of surface salinity spatial patterns by 7.9% (16±10% K^-1) in the tropical Pacific by FWF forcing. Notably, there is a decrease in salinity observed in the Pacific Ocean, which increases east-west contrast of sea surface salinity, and freshens upper-ocean at above 100 m, particularly in the warm pool, leading to enhanced upper-ocean stability. Furthermore, the change of salinity effect makes the mixing layer shallower, the barrier layer thicker, and enhancing the effect of salinity on the upper layer temperature. We further hypothesize that salinity mean state can strengthen variability of the salinity barrier layer in the tropical Pacific, which may increase the likelihood of extreme ENSO events by trapping more heat in the surface ocean in future. Consequently, freshwater flux forcing can increase the amplitude of ENSO interannual variability by 10%, increase ENSO asymmetry by 40%, and lengthen its period. It is suggested under future anthropogenic forcing, more frequent strong El Niño events are anticipated. The FAFMIP experiment will provide information on the sensitivity of the ocean salinity to distribution of the forcing response to global warming, the difference between climate change when a model is undisturbed and when it is forced by CO₂ to a specific process in the model.