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

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Abstract

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 (QB). Physically, there exist two pathways through which interannual FWF variability can affect the ocean: one through SSS and the other through QB. The roles of the interannual FWF variability in modulating the El Niño-Southern Oscillation (ENSO) through its effects on SSS or QB 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 (Hm) is explicitly predicted using an embedded bulk ML model with Hm being directly affected by QB, whereas in level ocean models, QB does not have a direct and explicit effect on Hm. 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 QB. 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 QB (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 QB. Further implications for model biases associated with FWF effects are discussed.

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Acknowledgements

The authors wish to thank the anonymous reviewers for their comments and suggestions that helped to improve the original manuscript. This research was supported by the National Key Research and Development Program of China (No. 2017YFC1404102(2017YFC1404100)), the National Natural Science Foundation of China (Grant Nos. 41705082, 41690122(41690120) and 41421005), the NSFC-Shandong Joint Fund for Marine Science Research Centers (U1406402), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA19060102) and the Taishan Scholarship. All figures in the paper are created by the authors using the Grid Analysis and Display System (GrADS) which is available at https://www.iges.org/grads/grads.html.

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Authors and Affiliations

  1. CAS Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, and Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao, 266071, China

    Chuan Gao, Rong-Hua Zhang & Feng Tian

  2. Pilot National Laboratory for Marine Science and Technology, Qingdao, 266237, China

    Chuan Gao, Rong-Hua Zhang & Feng Tian

  3. University of Chinese Academy of Sciences, Beijing, 10029, China

    Chuan Gao, Rong-Hua Zhang & Feng Tian

  4. Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO, USA

    Kristopher B. Karnauskas & Lei Zhang

  5. Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA

    Kristopher B. Karnauskas

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  1. Chuan Gao
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  2. Rong-Hua Zhang
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  3. Kristopher B. Karnauskas
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  4. Lei Zhang
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  5. Feng Tian
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Correspondence to Rong-Hua Zhang.

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Gao, C., Zhang, RH., Karnauskas, K.B. et al. Separating freshwater flux effects on ENSO in a hybrid coupled model of the tropical Pacific. Clim Dyn 54, 4605–4626 (2020). https://doi.org/10.1007/s00382-020-05245-y

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