Effects of Freshwater Flux Forcing on Interannual Climate Variability and Predictability in the Tropical Pacific

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Freshwater flux (FWF) is an important atmospheric forcing to the ocean, involving interactions among the Earth's water cycle, ocean circulation, and climate. Recent modeling studies have shown that the FWF-related ocean salinity in the tropical Pacific is important to the tropical dynamics and El Niño-Southern Oscillation (ENSO), and also has important implications for the use and interpretation of sea level, and for ocean data assimilation. However, FWF forcing and its related salinity effects in the tropical Pacific have not been realistically represented in many state-of-the-art coupled climate models; the FWF forcing has not been even included in most simplified coupled models used for ENSO forecasts. This project will investigate the roles of FWF forcing in modulating interannual variability and predictability for the tropical Pacific climate system using observational data and a hybrid coupled model (HCM). The investigators will test a hypothesis that FWF forcing in the tropical Pacific induces a positive climate feedback, which presents a new mechanism for the modulation of ENSO and significant tropical bias sources for ENSO simulation and prediction. They will develop an empirical model for interannual FWF variability from historical precipitation and evaporation data. This Sea Surface Temperature (SST)-dependent, prognostic representation of anomalous FWF forcing allows for an interactive feedback between FWF and SST during ENSO cycles. Then, this FWF model will be incorporated into an HCM of the tropical Pacific to take into account FWF forcing and to represent FWF-induced climate feedback. The realistic inclusion of FWF forcing in a coupled ocean-atmosphere model is expected to lead to better ENSO simulations and predictions. Various numerical experiments will be conducted to quantify the extent to which FWF forcing can contribute to seasonal-to-interannual climate variability and predictability in the region. In particular, the effect of FWF forcing on the ENSO modulation will be a focus. The broader impacts of this project include (1) the potential for direct improvements in ENSO simulations and predictions in coupled ocean-atmosphere models by taking into account the FWF-induced climate feedback; (2) important implication for understanding global water cycle in a changing climate due to global warming; (3) interdisciplinary learning and training experience for undergraduate and graduate students, and postdoctoral researchers; (4) wide dissemination of the results through publications and meeting/workshop presentations.