The Optimal Precursors for ENSO Events Depicted Using the Gradient-definition-based Method in an Intermediate Coupled Model

• Published in: Advances in atmospheric sciences 2019-12, Vol.36 (12), p.1381-1392
• Main Authors: Mu, Bin, Ren, Juhui, Yuan, Shijin, Zhang, Rong-Hua, Chen, Lei, Gao, Chuan
• Format: Article
• Language: English
• Subjects: Atmospheric Sciences; Computer simulation; Dependence; Earth and Environmental Science; Earth Sciences; El Nino; El Nino events; El Nino phenomena; El Nino-Southern Oscillation event; Geophysics/Geodesy; Meteorology; Ocean currents; Original Paper; Perturbation; Precursors; Sea surface; Sea surface temperature; Seasons; Southern Oscillation; Surface temperature
• ISSN: 0256-1530; 1861-9533
• DOI: 10.1007/s00376-019-9040-y

The predictability of El Niño-Southern Oscillation (ENSO) has been an important area of study for years. Searching for the optimal precursor (OPR) of ENSO occurrence is an effective way to understand its predictability. The CNOP (conditional nonlinear optimal perturbation), one of the most effective ways to depict the predictability of ENSO, is adopted to study the optimal sea surface temperature (SST) precursors (SST-OPRs) of ENSO in the IOCAS ICM (intermediate coupled model developed at the Institute of Oceanology, Chinese Academy of Sciences). To seek the SST-OPRs of ENSO in the ICM, non-ENSO events simulated by the ICM are chosen as the basic state. Then, the gradient-definition-based method (GD method) is employed to solve the CNOP for different initial months of the basic years to obtain the SST-OPRs. The experimental results show that the obtained SST-OPRs present a positive anomaly signal in the western-central equatorial Pacific, and obvious differences exist in the patterns between the different seasonal SST-OPRs along the equatorial western-central Pacific, showing seasonal dependence to some extent. Furthermore, the non-El Niño events can eventually evolve into El Niño events when the SST-OPRs are superimposed on the corresponding seasons; the peaks of the Niño3.4 index occur at the ends of the years, which is consistent with the evolution of the real El Niño. These results show that the GD method is an effective way to obtain SST-OPRs for ENSO events in the ICM. Moreover, the OPRs for ENSO depicted using the GD method provide useful information for finding the early signal of ENSO in the ICM.