Observed characteristics of flow, water mass, and turbulent mixing in the Preparis Channel
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Abstract: 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.
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Key words:
- deep flow /
- turbulent mixing /
- water mass /
- Preparis Channel
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Figure 1. Location of in situ observation measurements and bathymetric map of the Preparis Channel. a. Bottom topography of the Bay of Bengal and Andaman Sea from the global bathymetry data with 1′ resolution (Smith and Sandwell, 1997); the black box indicates the study area. b. Bottom topography of the Preparis Channel; the red star marks the deep mooring, brown filled circles indicate the CTD/LADCP stations, and black dots denoting VMP stations; Sections 1 and 2 (cyan curves) are defined to evaluate the spatial distributions of water mass in the Preparis Channel. c. Expanded view of the bottom topography of the deep mooring station in the Preparis Channel; the dashed black line indicates the section topography displayed in Fig. 4b; the orange contour represents the 400-m isobath, and the magenta contour is the 600-m isobath.
Figure 3. Detided along-stream (a) and cross-stream (b) velocities acquired by the moored upward-looking 300-kHz ADCP. The dashed line indicates zero velocity, and the reference coordinate of redecomposition is indicated in d; c. detided along-stream (red line) and cross-stream (black line) velocities acquired by the deepest instrument recording current meter (RCM); d. mean detided velocity vector obtained by the RCM, where the orange contour represents the 400-m isobaths, and the purple contour is the 600-m isobath.
Figure 4. Vertical profiles of the mean detided along-stream velocity vector in the Prepari Channel; the red arrows indicate the flow toward the BoB, and the blue arrows show the flow toward the AS (a); section of topography extracted along the dashed black line in Fig. 1c, based on the 1′-resolution global bathymetric data (Smith and Sandwell, 1997); the numbers are the estimated volume transports, and the horizontal dashed black lines indicate the boundaries of the transport layers (b).
Figure 5. Vertical sections of potential temperature (a), salinity (b), dissolved oxygen concentration (c), and turbidity (d) along Section 1 shown in Fig. 1b. The solid black contours indicate potential density, and green triangles with vertical dashed lines denote CTD profiles.
Figure 6. Vertical sections of potential temperature (a), salinity (b), dissolved oxygen concentration (c), and turbidity (d) along Section 2 shown in Fig. 1b. The solid black contours indicate potential density, and green triangles with vertical dashed lines denote CTD profiles.
Figure 7. T-S diagrams of water mass across the Preparis Channel. Dashed red and blue lines show the mean T-S profiles within the Andaman Sea and Bay of Bengal directly outside the channel, respectively, estimated from the World Ocean Database 2018 (WOD18). Solid lines with colors from blue to red indicate the profiles of CTD stations along Section 1 (a) and Section 2 (b) from the Bay of Bengal to the Andaman Sea. Red (blue) stars indicate depths of 50 m, 100 m, 150 m, 200 m, 300 m, and 500 m for the T-S profiles in the Andaman Sea (the Bay of Bengal).
Figure 8. Examples of shear spectra at different depths of turbulent mixing station S6 (shown in Fig. 1b). Shear wavenumber spectra (solid blue and red lines), the upper integration bound (dashed thin vertical red and blue lines), and the Namsyth spectra (thick dashed red and blue lines) are shown. PSD: power spectral density; cpm: cycle per meter.
Figure 9. From top to bottom: four sets of profiles of turbulent mixing stations S1, S6, S7, and S9 (shown in Fig. 1b). For each station, quantities plotted are (from left to right) potential density, shear variance (red) and buoyancy frequency squared (black), Richardson number (the vertical line for Ri=0.25), observed original (red line) and depth-averaged (black line) turbulent dissipation rates, and observed turbulent diffusivity (vertically averaged over 10-m bins).
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