Effect of the mean flow on the anomalous anticyclone over the Indo-Northwest Pacific in post-El Niño summers

Abstract

A large-scale anomalous anticyclone (AAC) is a recurrent pattern in post-El Nino summers, extending from the tropical Northwest Pacific (NWP) to the North Indian Ocean. In boreal summer, there is a strongly confluent lower-level flow between the monsoonal westerlies and easterly trades over the Indo-Northwest Pacific. The effect of this basic state confluent flow on the AAC is investigated with energetics analysis and numerical modeling. The results show that the lower-level mean flow over the Indo-Northwest Pacific aids the AAC development. Specifically, the conversion of kinetic energy from the mean confluent flow to perturbations helps amplify easterly anomalies over the Indo-Northwest Pacific in post-El Nino summers. The enhanced easterly wind anomalies provide a positive feedback onto the AAC by inducing surface Ekman divergence to suppress convection over the NWP. Moreover, the structure of the optimal diabatic heating for the AAC pattern is determined using a method similar to the Green’s function approach. The optimal forcing features heating in the tropical Indian Ocean and cooling in the NWP. This suggests that barotropic energy conversion in the confluence zone and the El Nino-induced positive (negative) sea surface temperature anomalies over the TIO (NWP) together lead to the AAC development over the Indo-Northwest Pacific in post-El Nino summers.