Lagrangian decomposition of the Indian Ocean shallow meridional overturning circulation

Based on the high-resolution OFES output, the shallow meridional overturning circulation (SMOC) in the Indian Ocean is decomposed by mass transports from open boundaries with Lagrangian tracing method. The tracers are released on the eastern boundary of the Indian Ocean at 8°S, which mainly concludes the ITF transport, and the southern boundary at 34°S trans-ocean section, which is used to capture transport exchange between the Indian Ocean and the Southern Ocean. The results reveal that ITF and the inflows through the southern boundary have almost equal contribution on SMOC in the tropical Indian Ocean. Nevertheless, the SMOC inferred from trajectories originating from the ITF is mainly constrained in the upper 200m while that from the Southern Ocean expands to upper 700m layers. More important, the particls from two open boundaries experiences different dynamics in the interior. Most of the ITF water does not experience strong downwelling, while the surface ITF water ( 50m) moves southward and merges with transports from southern boundary. In the subtropical south Indian Ocean, the particls experiences strong subduction, then upwells and moves northward, finally pass the equator in the form of the western boundary current.