Exploring sensitive area in the tropical Indian Ocean for El Niño prediction: implication for targeted observation

Based on initial errors of sea temperature in the tropical Indian Ocean that are most likely to induce spring predictability barrier (SPB) for the El Nino prediction, the sensitive area of sea temperature in the tropical Indian Ocean for El Nino prediction starting from January is identified using the CESM1.0.3 (Community Earth System Model), a fully coupled global climate model. The sensitive area locates mainly in the subsurface of eastern Indian Ocean. The effectiveness of applying targeted observation in the sensitive area is also evaluated in an attempt to improve the El Nino prediction skill. The results of sensitivity experiments indicate that if initial errors exist only in the tropical Indian Ocean, applying targeted observation in the sensitive area in the Indian Ocean can significantly improve the El Nino prediction. In particular, for SPB-related El Nino events, when initial errors of sea temperature exist both in the tropical Indian Ocean and the Pacific Ocean, which is much closer to the realistic predictions, if targeted observations are conducted in the sensitive area of tropical Pacific, the prediction skills of SPB-related El Nino events can be improved by 20.3% in general. Moreover, if targeted observations are conducted in the sensitive area of tropical Indian Ocean in addition, the improvement of prediction skill can be increased by 25.2%. Considering the volume of sensitive area in the tropical Indian Ocean is about 1/3 of that in the tropical Pacific Ocean, the prediction skill improvement per cubic kilometer in the sensitive area of tropical Indian Ocean is competitive to that of the tropical Pacific Ocean. Additional to the sensitive area of the tropical Pacific Ocean, sensitive area of the tropical Indian Ocean is also a very effective and cost-saving area for the application of targeted observations to improve El Nino forecast skills.