Modulation of El Nino-Southern Oscillation by Freshwater Flux and Salinity Variability in the Tropical Pacific

The El Nino-Southern Oscillation (ENSO) is modulated by manyfactors; most previous studies have emphasized the roles of wind stress andheat flux in the tropical Pacific. Freshwater flux (FWF) is anotherenvironmental forcing to the ocean; its effect and the related ocean salinityvariability in the ENSO region have been of increased interest recently.Currently, accurate quantifications of the FWF roles in the climate remainchallenging; the related observations and coupled ocean-atmosphere modelinginvolve large elements of uncertainty. In this study, we utilizedsatellite-based data to represent FWF-induced feedback in the tropical Pacificclimate system; we then incorporated these data into a hybrid coupledocean-atmosphere model (HCM) to quantify its effects on ENSO. A new mechanismwas revealed by which interannual FWF forcing modulates ENSO in a significantway. As a direct forcing, FWF exerts a significant influence on the oceanthrough sea surface salinity (SSS) and buoyancy flux (Q_B) in thewestern-central tropical Pacific. The SSS perturbations directly induced byENSO-related interannual FWF variability affect the stability and mixing in theupper ocean. At the same time, the ENSO-induced FWF has a compensating effecton heat flux, acting to reduce interannual Q_B variability during ENSOcycles. These FWF-induced processes in the ocean tend to modulate the verticalmixing and entrainment in the upper ocean, enhancing cooling during La Ninaand enhancing warming during El Nino, respectively. The interannual FWFforcing-induced positive feedback acts to enhance ENSO amplitude and lengthenits time scales in the tropical Pacific coupled climate system.