Observed characteristics of flow, water mass, and turbulent mixing in the Preparis Channel

Preparis Channel is the very important exchange path of energy and materials between the northern Bay of Bengal and Andaman Sea (AS). A set of hydrographic measurements, a microstructure profiler, and a deep mooring were used to determine the characteristics of water masses, turbulent mixing, and flows in the Preparis Channel. The unprecedented short-term mooring data reveal that a deep current in the deep narrow passage (below 400 m) of the Preparis Channel flows toward the Bay of Bengal (BoB) with a mean along-stream velocity of 25.26 cm/s at depth of 540 m; above the deep current, there are a relatively weak current flows toward the AS with a mean along-stream velocity of 15.46 cm/s between 500 m and 520 m, and another weak current flows toward the BoB between 430 m and 500 m. Thus, a sandwiched vertical structure of deep currents (below 400 m) is present in the Preparis Channel. The volume transport below 400 m is 0.06 Sv (1 Sv = 10⁶ m³/s) from the AS to the BoB. In the upper layer (shallower than 300 m), the sea water of the AS is relatively warmer and fresher than that in the BoB, indicating a strong exchange through the channel. Microstructure profiler observations reveal that the turbulent diffusivity in the upper layer of the Preparis Channel reaches O(10⁻⁴ m²/s), one order larger than that in the interior of the BoB and over the continental slope of the northern AS. We speculate that energetic high-mode internal tides in the Preparis Channel contribute to elevated turbulent mixing. In addition, a local “hotspot” of turbidity is identified at the deep mooring site, at depth of about 100 m, which corresponds to the location of elevated turbulent mixing in the Preparis Channel.