In the Indian Ocean, salty water masses from the Persian Gulf and Red Sea are important sources of salt, heat, and nutrients. Across the Arabian Sea these outflows impact human and biological activities, their thermohaline characteristics and shapes exhibiting important spatial and seasonal variability. Knowledge of the water masses properties is important to validate realistic simulations of the Indian Ocean. A classical approach to study these water masses is to track them on specific isopycnal levels. Nevertheless, their peaking thermohaline characteristics are not always found at a specific density but rather spread over a range. Here, we develop a detection algorithm able to capture the full vertical structure of the outflows, that we applied to a dataset of about 126,000 vertical profiles. We are thus able to quantify the changes in their thermohaline signatures and in their vertical structures, characterized here by the intensity of the salinity peaks of the water masses and lateral injection of fresh and salty waters, and describe their spatial variability. Across the northwestern Indian Ocean, the salty outflows undergo several changes, diminishing their thermohaline signatures and peaks and layering. In their early stages in the narrow Gulf of Oman and Aden, the outflows present configurations indicative of diapycnal mixing. In the same regions and along the western edge of the Arabian Sea, these water masses are subject to lateral mixing. All over the Arabian Sea salt fingering conditions are met for lower layers of the outflows.

Plain Language Summary

In the northwestern Indian Ocean, water masses with different characteristics are measured. Two of them are some of the most salty found globally, namely Persian Gulf Water and Red Sea Water. In the Arabian Sea, these outflows transport salt and heat, affecting the biological activity. Furthermore, knowledge of such quantities is necessary to simulate the circulation of the basin. A classical approach to analyze the characteristics of these outflows is to measure them at specific densities. It has been observed that peaks in salinity are more often found over a range of densities, rather than at specific values. In this study, we develop a detection algorithm able to capture the full vertical structure of the outflows. Thus, we quantify the temperature and salinity regional distribution of the outflows, and describe their vertical configurations. Along their paths, the salty outflows undergo several changes in their vertical structures, with a diminution of their peaks in salinity and lateral injections of saltier and fresher waters, particularly near their straits of exit. Finally, all over the Arabian Sea, the outflows are in salt fingering conditions, losing salt at smaller vertical scales.