The MJO-driven Indo-Pacific barotropic see-saw

Abstract

Recent discovery of a see-saw in intraseasonal barotropic sea level driven by boreal winter Madden-Julian Oscillation winds in the tropical Indo-Pacific basin has renewed interest in the barotropic dynamics in the tropics, which was otherwise known to be prominently a region of baroclinic dynamics at intraseasonal timescales. In this study, using a reference model simulation and several sensitivity experiments, we provide comprehensive details of the dynamics associated with this see-saw. It is found that the narrow Indonesian straits are instrumental in relaying barotropic excitations from one basin to the other. The intraseasonal barotropic circulation associated with this see-saw dynamics appears well organized at basin scale, both in the Indian Ocean and in the Pacific Ocean. During the positive phase of the see-saw, the Madden-Julian Oscillation winds over the Maritime Continent drive a basin-wide anti-clockwise circulation of ∼2 Sv around the Australian Continent with the eastern arm of this circulation located along the west African coast. Concurrently, it also drives a clockwise circulation in the southern Pacific Ocean. This circulation pattern reverses during the negative phase of the see-saw. In the Indonesian Throughflow region, the relatively narrow Ombai Strait and Lombok Strait facilitate this circulation in comparison to the wider Timor Passage. We discuss and point out the significant differences in this flow pattern with the mean flow across the Indonesian seas. We also address how some regions in the northern Pacific Ocean are decoupled from this large-scale dynamics. We show that it takes about a week for both the basins to adjust and to establish the see-saw, once the Madden-Julian Oscillation winds reach the Maritime Continent.