The Initial Errors in the Tropical Indian Ocean that Can Induce a Significant “Spring Predictability Barrier” for La Niña Events and Their Implication for Targeted Observations

Initial errors in the tropical Indian Ocean (IO-related initial errors) that are most likely to yield Spring Prediction Barrier (SPB) for the La Nina forecasts are explored by using the CESM model. These initial errors can be classified into two types. Type-1 initial error consists of positive sea temperature in the western Indian Ocean and negative sea temperature errors in the eastern Indian Ocean, while the spatial structure of Type-2 initial error is nearly opposite. Both kinds of IO-related initial errors induce positive prediction errors of sea temperature in the Pacific Ocean, leading to under-prediction of La Nina events. Type-1 initial error in the tropical Indian Ocean mainly influence the SSTA in the tropical Pacific Ocean via atmospheric bridge, leading to the developments of localized sea temperature error in the eastern Pacific Ocean. While, for Type-2 initial error, its positive sea temperature error in the eastern Indian Ocean can induce downwelling error and influence the La Nina predictions through oceanic channel Indonesian ThroughFlow. Based on the large value area of these SPB-related initial errors, the sensitive area in the tropical Indian Ocean for the La Nina predictions is identified. Furthermore, applying targeted observations in the sensitive area are shown to be very useful in decreasing prediction errors of La Nina by sensitivity experiments. It is therefore promising to adopt targeted observation strategy in the tropical Indian Ocean to increase the ENSO prediction skills.