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

• Published in: Climate dynamics 2020-06, Vol.54 (11-12), p.4605-4626
• Main Authors: Gao, Chuan, Zhang, Rong-Hua, Karnauskas, Kristopher B., Zhang, Lei, Tian, Feng
• Format: Article
• Language: English
• Subjects: Analysis; Atmospheric forcing; Atmospheric models; Buoyancy; Buoyancy flux; Climatology; Computer simulation; Earth and Environmental Science; Earth Sciences; El Nino; El Nino phenomena; El Nino-Southern Oscillation event; Evaporation; Feedback; Fluctuations; Flux; Freshwater; Geophysics/Geodesy; Inland water environment; Mixed layer; Ocean models; Oceanography; Oceans; Positive feedback; Salinity; Sea surface; Southern Oscillation; Surface salinity; Tropical climate; Variability
• ISSN: 0930-7575; 1432-0894
• DOI: 10.1007/s00382-020-05245-y

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 Niño-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 Niño 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.