A Lagrangian view of the Red Sea Overflow Water journey from the Gulf of Aden to the Arabian Sea

The Red Sea produces one of the saltiest water masses of the global oceans, known as the Red Sea Overflow Water (RSOW). This warm, salty and dense water escapes from the shallow and narrow Strait of Bab al-Mandab to the Gulf of Aden and is a source of oxygen for the (sub) anoxic Arabian Sea. However, before arriving into the Arabian Sea, the RSOW is strongly transformed in the Gulf of Aden with salinity/temperature decreasing from 39.5/23^oC to 36/12^oC. Despite this brutal transformation, essential aspects of the RSOW in the Gulf are not well understood due to the scarcity of historical in situ observations in the RSOW layer (σ₀ 27.3-27.6 kg/m³; 600- 800m). In the present work, a set of particle tracking experiments using Ocean Parcels is performed to quantitatively characterize the regions of vigorous mixing and the RSOW residence time in the Gulf. Two recently released high-resolution (1/12^o) datasets are used: HYCOM GOFS3.1 (1994-2015) and GLORYS12V1 (1993-2018), both assimilating most satellite and in situ observations to date. Comparison of the reanalysis-based trajectories and in-situ trajectories from 49 isobaric RAFOS floats deployed around 700 m during the REDSOX project (2001-2003) shows that the reanalysis distributions are statistically similar to the in-situ observations, which is confirmed by the two-sample Kolmogorov-Smirnov test. Complementary temperature/salinity observations from the REDSOX project and Argo floats are also analyzed to characterize the RSOW spicity changes in the Gulf.