On the perturbation potential energy in the synergistic effect of ENSO and South China Sea summer monsoon on the Indian Ocean dipole development

The Indian Ocean dipole (IOD) has been proven to be synergistically influenced by the South China Sea summer monsoon (SCSSM) and El Niño-Southern Oscillation (ENSO) through the regional Hadley and Walker circulations. However, the atmosphere circulation variations are essentially controlled by the atmosphere energetics changes, this study investigates the energetic processes of the synergistic effect of the SCSSM and ENSO on the IOD development from the perspective of the perturbation potential energy (PPE). An anomalous meridional PPE dipole over the western North Pacific (WNP) and southern Maritime Continent (SMC) associated with the independent SCSSM events induces the regional Hadley circulation through energy conversion, leading to the strong east and weak west poles of IOD. Response to the independent ENSO events, the Walker circulation is reinforced by an anomalous zonal PPE dipole over the central-eastern Pacific and SMC. Meanwhile, the significantly uniform troposphere PPE anomalies (in line with troposphere temperature mechanism) over the central-eastern Pacific can extend eastward to the tropical eastern Indian Ocean as the form of Kelvin wave and further stabilize the local environment. These two mechanisms cooperate over the western Indian Ocean and offset over the eastern Indian Ocean, resulting in the strong west and weak east poles of IOD. As the SCSSM and ENSO events coexist, the east and west poles of IOD are both strengthened, much larger than that induced by the isolated SCSSM or ENSO events, demonstrating the synergistic effect of the SCSSM and ENSO on the IOD development. This situation can persist from boreal summer to autumn with the increase of the zonal gradient over the tropical Indian Ocean, contributing to the culminated IOD. In addition, the PPE anomalies are distinctly different in vertical profile, which is mainly contributed by the heat source in the upper troposphere and heat sink in the lower troposphere. Consequently, the PPE serves as the atmosphere bridge in the synergistic effect of the SCSSM and ENSO on the IOD development.