Separating freshwater flux effects on ENSO in a hybrid coupled model of the tropical Pacific

Freshwater flux (FWF) at the sea surface, defined as precipitation minus evaporation, is a major atmospheric forcing to the ocean that affects sea surface salinity (SSS) and buoyancy flux (Q(B)). Physically, there exist two pathways through which interannual FWF variability can affect the ocean: one through SSS and the other through Q(B). The roles of the interannual FWF variability in modulating the El Nino-Southern Oscillation (ENSO) through its effects on SSS or Q(B) are separately examined using a hybrid coupled model (HCM) of the tropical Pacific; its ocean component is a layer model in which the topmost layer (the first layer) is treated as a mixed layer (ML) whose depth (H-m) is explicitly predicted using an embedded bulk ML model with H-m being directly affected by Q(B), whereas in level ocean models, Q(B) does not have a direct and explicit effect on H-m. Four experiments are conducted using the HCM that is designed to illustrate the effects of these processes on coupled simulations systematically. It is demonstrated that interannual FWF variability serves as a positive feedback on ENSO through its collective effects on both SSS and Q(B). Individually, the interannual FWF effect through SSS accounts for about 80% in terms of ENSO amplitude in the Nino 3.4 area, while that through buoyancy flux accounts for about 26%. This indicates that ocean models without explicitly taking into account the direct FWF effect on Q(B) (typically in level ocean models) could underestimate the positive feedback on ENSO compared with layer ocean models in which the FWF effects are collectively represented on both SSS and Q(B). Further implications for model biases associated with FWF effects are discussed.