[Climate Predictability]Towards optimal observational array for dealing with challenges of El Niño-Southern Oscillation predictions due to diversities of El Niño

There exist large differencesof the impacts on the global climate between the eastern Pacific (EP) El Niño and central Pacific (CP) El Niño events. It is therefore important todistinguish the types of El Nino in predictions. However, a useful prediction skill for predicting two types of El Niño eventswas only possible within a one- to four-month lead time and expected to have a great improvement.A new observational plan (TPOS2020; Tropical Pacific Observational Systems 2020) has been implementing on the purpose of improving the forecast skill for two types of El Nino. Then where is the additional observations deployed to increase the forecast skill to the most extent?

Recently, Prof. DUAN Wansuo, Drs. LI Xuquan and TIAN Ben investigated the optimal observational array for dealing with challenges of El Niño-Southern Oscillation (ENSO) predictions due to diversities of El Niño. The areas for targeting observations are identified by calculating the optimally growing errors (OGEs) of the Zebiak-Cane model, as corrected by the approach of optimal forcing vector (OFV) that is determined by assimilating the observed sea surface temperature anomalies (SSTAs). It is found that although the OGEs have similar structures for different start months of predictions, the regions covered by much large errors for the SSTA component tend to locate at different zonal positions and depends on the start months. Furthermore, these regions are also in difference between two types of El Niño events. The regions covered by large errors of OGEs represent the areas for targeting observations. Considering the dependence of the targeted areas on related El Niño types and the start months of predictions, the study propose a quantitative frequency method to determine the targeting areas for increasing additional observations associated with two types of El Niño predictions, which is expected to be applicable for both types of El Niño predictions with different start months. As a result, the targeted areas that describe the array of target observations are presented with a reversal triangle-like shape locating in the eastern Pacific, specifically the area of 120°W-85°W, 0°-11°S, and an extension to the west along the equator and then gathering at the 180° longitude and the western boundary. Hindcast experiments demonstrated that such observational array is very useful in distinguishing two types of El Niño and superior to the TAO/TRITON array. It is therefore suggested that the observational array provided in the present study is towards the optimal one and the original TAO/TRITON array should be further optimized when applied to predictions of the diversities of El Niño events, which certainly provides useful idea for the observational plan of the TPOS2020.

Fig. 1. The location of the optimal observational array for improving the prediction skills of two types of El Niño events.

This study is recently published in Climate Dynamics (doi: 10.1007/s00382-018-4082-x).

Reference:
Duan, W., Li, X., & Tian, B. (2018). Towards optimal observational array for dealing with challenges of el niño-southern oscillation predictions due to diversities of El Niño. Climate Dynamics, 1-18. https://doi.org/10.1007/s00382-018-4082-x

Contact:DUAN Wansuo, duanws@lasg.iap.ac.cn