Dipole Response of Mesoscale Eddy Formation to Monsoon Transition in the Southeast Tropical Indian Ocean

The Southeast Tropical Indian Ocean (SETIO), dominated by the Indian Ocean monsoon, is an important source region for strong mesoscale eddies. To date, the impacts of the Indian Ocean monsoon on mesoscale eddies have not been clarified. Here we report on the dipole response of mesoscale eddy formation to monsoon transition in the SETIO, using satellite and reanalysis data sets. During the summer monsoon season, anticyclonic eddies are mainly concentrated north of 12 degrees S, while cyclonic eddies are south of 12 degrees S. This situation reverses during the winter monsoon season. We attribute this dipole feature to the oceanic perturbations and current shear during the different monsoon periods. A geographical boundary along 12 degrees S aligns with meridional changes in eddy potential energy, which delineates the generation and direction of the newly-formed eddies. The hot spot region, rich in eddy energy properties, tends to promote eddy formation and endurance during the monsoon periods. The Southeast Tropical Indian Ocean (SETIO) is a typical region of strong mesoscale (similar to 10-100 km) eddy generation. Eddies are circular currents that are important in moving heat, nutrients, and marine life around the ocean. The SETIO is also dominated by the Indian Ocean monsoon, which is a seasonal weather pattern that typically occurs in two main phases: the southwest monsoon from June to September, and the northeast monsoon from December to March. To date, the impacts of the Indian Ocean monsoon on the mesoscale eddies remain unclear. Based on satellite and reanalysis data sets, we found that there is a natural latitudinal change in the direction of eddies (anticlockwise/clockwise) formed north/south of 12 degrees S in the summer monsoon, and that this pattern switches in the winter monsoon. The monsoon transition and associated changes to the ocean and its currents drives the dual-pattern. The geographical boundary along 12 degrees S occurs because it aligns with latitudinal changes in the energy stored in the eddies, which delineates a change in the direction of the newly-formed eddies. This hot spot region, rich in eddy energy properties, promotes eddies formation and endurance during the monsoon periods. Strong mesoscale eddies in the Southeast Tropical Indian Ocean are generated in a clear seasonal cycle The eddies present a distinct dipole response to the monsoon transition in the region Changes in oceanic perturbation and current shear modulated by monsoon transition is responsible for this dipole response of eddies