How does the Red Sea Overflow Water reach the Southeastern Indian Ocean?

This study is motivated by a relatively old and unsolved puzzle involving the spreading of the salty Red Sea Overflow Water (RSOW or Red Sea Water) at the Indian Ocean intermediate layer (500-1500 m). The RSOW spreads within the 27-27.6 kg/m3 (potential) density layer mainly along the Indian Ocean western boundary, a pathway that has received the most attention since the early 20th century. However, several studies report this salty water mass (or rather a diluted version) far away in the southeastern Indian Ocean between 10°S and 24°S. How does it arrive there? When we look at the mean intermediate-layer circulation maps, the advective pathway for the RSOW to the southeastern Indian Ocean is obscure.  As it moves southward, the RSOW mixes and interleaves with the fresher and colder Antarctic Intermediate Water (AAIW) and Indonesian Intermediate Water (IIW), becoming progressively less saline before the salinity maximum fades out. This makes it difficult to trace the RSOW pathways in the entire Indian Ocean based on its salinity fingerprint alone. Here, a different approach is used. The present study investigates the paths of the RSOW to the southeastern Indian Ocean using particle tracking Lagrangian simulations, a valuable tool to determine large-scale pathways and connectivity between oceanic regions. The simulations are based on the eddy-rich GLORYS12 (GLobal Ocean ReanalYses and Simulations 1/12° resolution) reanalysis that incorporates most satellite and in-situ observations in the altimeter era. Sets of virtual particles were released every five days between 1993 and 2014 in the westernmost part of the Gulf of Aden at three isopycnic levels within the RSOW layer and tracked for 300 years. We stopped seeding particles in 2014 because the cumulative number of released particles reached about 106, sufficiently large to infer pathways statistically. For performing the simulations, the GLORYS12 Eulerian velocity fields were repeated sequentially over the simulation period. Analysis of probability maps and pseudo-Eulerian velocity fields indicate that the equatorial area is the preferential pathway for the RSOW transport to the eastern Indian Ocean, which reveals the critical role of the Equatorial Intermediate Current in the western-eastern connectivity. A convoluted secondary eastward pathway is seen between 5S and 10S. After reaching the Java-Sumatra area, particles are transported southward along the southeastern boundary. However, most particles return to the western side around 12°S, and fewer particles flow further south offshore Western Australia.