The predictability of El Nino-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).
Researchers from Tongji University, Institute of Oceanology of the Chinese Academy of Sciences and Shanghai Central Meteorological Observatory found that the optimal precursors of sea surface temperature (SST-OPRS) using an adjoint-free method for ENSO showed certain seasonal dependence in the IOCAS ICM model. Their study was published in Advances of Atmospheric Sciences.
"The OPRs (optimal precursors) of ENSO can be represented by the initial perturbation that most likely evolves into an ENSO event," said Prof. YUAN Shijin, corresponding author of the study.
To seek the SST-OPRs of ENSO in the ICM, the researchers chose non-ENSO events simulated by the ICM as the basic state; they employed the gradient-definition-based method (GD method) to solve the CNOP for different initial months of the basic years to obtain the SST-OPRs.
In order to study the predictability of ENSO by using the adjoint-free method, the team has been devoting considerable effort to exploring some new methods for solving the CNOP since 2010, and the GD method is one of them.
The SST-OPRs in the IOCAS ICM model using the GD method showed a positive anomaly signal in the western-central equatorial Pacific, and obvious differences existed in the patterns between the different seasonal SST-OPRs along the equatorial western-central Pacific, showing seasonal-dependence to some extent.
Furthermore, non-El Nino events could eventually evolve into El Nino events when the SST-OPRs were superimposed on the corresponding seasons; the peaks of the Nino3.4 index occurred at the ends of the years, which was consistent with the evolution of the real El Nino.
"The SST OPRs for ENSO depicted using the GD method provide useful information for finding the early signal of ENSO in the ICM," said Prof. YUAN.
Schematic diagram of a normal, El Nino and La Nina year. (Image by IOCAS)
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