Spreading and vertical structure of the Persian Gulf and Red Sea outflows in the northwestern Indian Ocean
Pierre L'Hegaret1, Charly de Marez1, Mathieu Morvan2, Thomas Meunier3 and Xavier J Carton4, (1)Laboratoire d'Océanographie Physique et Spatiale, Plouzané, France, (2)Laboratoire d'Océanographie Physique et Spatiale, France, (3)WHOI, Physical Oceanography, Woods Hole, MA, United States, (4)Laboratoire de Physique des Océans, Brest, France
Abstract:
In the Indian Ocean, salty outflows from the Persian Gulf and Red Sea are important sources of salt, heat, and nutrients below the mixed layer. Their spreading and evolution across the Arabian Sea impact human and biological activities, as well as the vertical stratification of the water column, a key measure to set realistic simulations of the Indian Ocean. A classical approach to study these water masses is to track them on a specific isopycnal level. Nevertheless, their peaking thermohaline characteristics are not always found at a specific density but rather spread over a range of density. Here, we develop a detection algorithm able to capture the full vertical structure of the outflows, their lower and upper limits set as the local lows on the temperature vs salinity profiles, and their centers as the peaking value. This method is applied to all available observations of the water column in the Arabian Sea, creating a dataset containing over 126,000 vertical profiles.
From their basins of evaporation to the open Arabian Sea, we quantify the mean temperature and salinity dilutions of the water masses as well as their spatial and temporal variability. Their vertical structures are decomposed into a component quantifying the curvature (smooth peaks), and a second underlining the presence of layering (lateral injections).
Along their paths out of the Arabian Sea, the salty outflows undergo several mixing processes, diluting their thermohaline signatures and quickly breaking their plume structures into submesoscale features unlike to remain coherent in a turbulent mesoscale eddy field. In their early stages in the narrow Sea of Oman and Gulf of Aden, and further offshore in the western Arabian Sea, the outflows lose salt and heat via diapycnal mixing. In the same regions and along the western edge of the basin, these water masses are stirred by mesoscale eddies and subject to lateral mixing. There, the upper layers of the outflows are in favorable conditions for diffusive convection, while salt fingering conditions are met for the lower layers all over the Arabian Sea .
