ENSO modulation: real and apparent; implications for decadal prediction
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Because of the impact of the El Niño-Southern Oscillation (ENSO) phenomenon on regional and global weather and climate, its potential long-term predictability is an important area of study. One plausible avenue of approach that has been attempted is to study the low-frequency modulation of the ENSO phenomenon. We consider here specifically the modulation of ENSO on decadal timescales by the Pacific Decadal Oscillation (PDO). Since each phase of the PDO has decadal persistence, it was hoped that its influence on the ENSO phenomenon might have decadal predictability. However, the nature of such an influence is not yet understood. There is no doubt that PDO can affect the tropical sea-surface temperature (SST), mostly in the equatorial Central Pacific, but, is this influence through the nonlinear mechanism of amplitude and frequency modulation of ENSO? Or is it through a linear superposition of the two climate modes on tropical SST? By showing that it is largely the latter, we suggest that the problem of decadal predictability should be recast into predicting not the ENSO itself but the tropical Pacific SST, possibly opening up another avenue of research for this difficult problem.
YF was supported by Natural Science Foundation of China under Grant 41776027 and China Scholarship Council during her exchange study at University of Washington, when the work reported here was carried out. This forms part of her Ph.D. thesis under Prof. J. Y. Hu, who provided advice and support. KKT was supported in part by the Frederic and Julia Wan Endowed Professorship and by National Science Foundation, under NSF1536175. We thank Professor Norden Huang for motivating us to examine the nonlinear modulation problem and for sharing with us information on the HHS method. We thank Professor Xianyao Chen for his assistance in statistical analysis.
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