Shipboard velocity and hydrographic profiles collected in December 2013 along with drifter observations, satellite altimetry, global ocean nowcast/forecast products, and coupled model simulations were used to examine the circulation in the southern Bay of Bengal as part of ongoing international research efforts in the region. The observations captured the southward flowing East India Coastal Current (EICC) off southeast India and east of Sri Lanka. The EICC was approximately 100 km wide, with speeds exceeding 1 m s−1 in the upper 75 m. East of the EICC, a subsurface‐intensified 300 km‐wide, northward current was observed, with maximum speeds as high as 1 m s−1 between 50 m and 75 m. The EICC moved low‐salinity water out of the bay and the subsurface northward flow carried high‐salinity water into the bay during typical northeast monsoon conditions during a time period when the central equatorial Indian Ocean was experiencing a westerly wind burst related to the Madden‐Julian Oscillation (MJO) event. While the northward subsurface high‐salinity flow has previously been observed during the southwest monsoon, it was observed during the northeast monsoon. The observations are consistent with northward high‐salinity subsurface flow in numerical model solutions. The analysis suggests that direct forcing along the equator may play a significant role for high‐salinity intrusions east of Sri Lanka.

Breaking surface waves generate layers of bubble clouds as air parcels entrain into the upper ocean through the action of turbulent motions. The turbulent diffusivity in the bubble cloud layer is investigated by combining measurements of surface winds, waves, bubble acoustic backscatter, currents, and hydrography. These measurements were made at water depths of 60–90 m on the shelf of the Gulf of Alaska near Kayak Island during late December 2012, a period when the ocean was experiencing winds and significant wave heights up to 22 m s−1 and 9 m, respectively. Vertical profiles of acoustic backscatter decayed exponentially from the wave surface with e-folding lengths of about 0.6 to 6 m, while the bubble penetration depths were about 3 to 30 m. Both e-folding lengths and bubble depths were highly correlated with surface wind and wave conditions. The turbulent diffusion coefficients, inferred from e-folding length and bubble depth, varied from about 0.01 to 0.4 m2 s−1. Analysis suggests that the turbulent diffusivity in the bubble layer can be parameterized as a function of the cube of the wind friction velocity with a proportionality coefficient that depends weakly on wave age. Furthermore, in the bubble layer, on average, the shear production of the turbulent kinetic energy estimated by the diffusion coefficients is a similar order of magnitude as the dissipation rate predicted by the wall boundary layer theory.

High-resolution currents and hydrographic fields were measured at six deep-water moorings in the southern Bay of Bengal (BoB) by the Naval Research Laboratory as part of an international effort focused on the dynamics of the Indian Ocean. Currents, temperature, and salinity were sampled over the upper 500 m for 20 months between December 2013 and August 2015. One of the major goals is to understand the space–time scales of the currents and physical processes that contribute to the exchange of water between the BoB and the Arabian Sea. The observations captured Southwest and Northeast Monsoon Currents, seasonally varying large eddies including a cyclonic eddy, the Sri Lanka dome (SLD), and an anticyclonic eddy southeast of the SLD. The observations further showed intraseasonal oscillations with periods of 30–70 days, near-inertial currents, and tides. Monthly averaged velocities commonly exceeded 50 cm s−1 near the surface, and extreme velocities exceeded 150 cm s−1 during the southwest monsoon. Tides were small and dominated by the M2 component with velocities of about 3 cm s−1. The average transport into the BoB over the measurement period was 2 Sv (1 Sv ≡ 106 m3 s−1) but likely exceeded 15 Sv during summer of 2014. This study suggests the water exchange away from coastal boundaries, in the interior of the BoB, may be largely influenced by the location and strength of the two eddies that modify the path of the Southwest Monsoon Current. In addition, there is a pathway below 200 m for transport of water into the BoB throughout the year.

[1] The bubble cloud depth and its correlation with extreme winds are key elements of bubble-mediated gas injection, which are critical to the determination of the global gas budgets. The characteristics of bubble cloud depth were examined from measurements collected during the passage of a category-4 hurricane with winds up to 50 m s −1 . The bubble cloud depth increases linearly with wind speed for winds less than 35 m s −1 . Our findings are consistent with previous observations at low to moderate wind speeds. However, the rate of increase is reduced significantly at winds higher than 35 m s −1 .

Pressure differences across topography generate a form drag that opposes the flow in the water column, and viscous and pressure forces acting on roughness elements of the topographic surface generate a frictional drag on the bottom. Form drag and bottom roughness lengths were estimated over the East Flower Garden Bank (EFGB) in the Gulf of Mexico by combining an array of bottom pressure measurements and profiles of velocity and turbulent kinetic dissipation rates. The EFGB is a coral bank about 6 km wide and 10 km long located at the shelf edge that rises from 100-m water depth to about 18 m below the sea surface. The average frictional drag coefficient over the entire bank was estimated as 0.006 using roughness lengths that ranged from 0.001 cm for relatively smooth portions of the bank to 1–10 cm for very rough portions over the corals. The measured form drag over the bank showed multiple time-scale variability. Diurnal tides and low-frequency motions with periods ranging from 4 to 17 days generated form drags of about 2000 N m−1 with average drag coefficients ranging between 0.03 and 0.22, which are a factor of 5–35 times larger than the average frictional drag coefficient. Both linear wave and quadratic drag laws have similarities with the observed form drag. The form drag is an important flow retardation mechanism even in the presence of the large frictional drag associated with coral reefs and requires parameterization.