Character and dynamics of the Red Sea and Persian Gulf outflows

Historical hydrographic data and a numerical plume model are used to invest igate the initial transformation, dynamics, and spreading pathways of Red S ea and Persian Gulf outflow waters where they enter the Indian Ocean. The a nnual mean transport of these outflows is relatively small (< 0.4 Sv), but they have a major impact on the hydrographic properties of the Indian Ocean at the thermocline level because of their high salinity. They are differen t from other outflows in that they flow over very shallow sills (depth < 20 0 m) into a highly stratified upper ocean environment and they are located at relatively low latitudes(12 degrees N and 26 degrees N). Furthermore, th e Red Sea outflow exhibits strong seasonal variability in transport. The fo ur main results of this study are as follows. First, on the basis of observ ed temperature-salinity (T-S) characteristics of the outflow source and pro duct waters we estimate that the Red Sea and Persian Gulf outflows are dilu ted by factors of similar to 2.5 and 4, respectively, as they descend from sill depth to their depth of neutral buoyancy. The high-dilution factor for the Persian Gulf outflow results from the combined effects of large initia l density difference between the outflow source water and oceanic water and low outflow transport. Second, the combination of low latitude and low out flow transport (and associated low outflow thickness) results in Ekman numb ers for both outflows that are O(1). This indicates that they should be tho ught of as frictional density currents modified by rotation rather than geo strophic density currents modified by friction. Third, different mixing his tories along the two channels that direct Red Sea outflow water into the op en ocean result in product waters with significantly different densities, w hich probably contributes to the multilayered structure of the Red Sea prod uct waters. In both outflows, seasonal variations in source water and ocean ic properties have some effect on the T-S of the product waters, but they h ave only a minor impact on equilibrium depth. Fourth, product waters from b oth outflows are advected away from the sill region in narrow boundary curr ents, at least during part of the year. At other times, the product water a ppears more in isolated patches.