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Bio-optical and physical measurements were collected in the Mississippi Sound (Northern Gulf of Mexico) during the spring of 2018 as part of the Integrated Coastal Bio-Optical Dynamics project. The goal was to examine the impact of atmospheric and tidal fronts on fine-scale physical and bio-optical property distributions in a shallow, dynamic, coastal environment. During a 25-day experiment, eight moorings were deployed in the vicinity of a frontal zone. For a one-week period in the middle of the mooring deployment, focused ship sampling was conducted with aircraft and unmanned aerial vehicle overflights, acquiring hyperspectral optical and thermal data. The personnel in the aircraft located visible color fronts indicating the convergence of two water masses and directed the ship to the front. Dye releases were performed on opposite sides of a front, and coincident aircraft and unmanned aerial vehicle overflights were collected to facilitate visualization of advection/mixing/dispersion processes. Radiometric calibration of the optical hyperspectral sensor was performed. Empirical Line Calibration was also performed to atmospherically correct the aircraft imagery using in situ remote sensing reflectance measurements as calibration sources. Bio-optical properties were subsequently derived from the atmospherically corrected aircraft and unmanned aerial vehicle imagery using the Naval Research Laboratory Automated Processing System. [ABSTRACT FROM AUTHOR]

Observational data are successfully assessed to investigate wave power (wave energy flux per unit of wave-crest) trends within four coastal regions around the US, a parameter that is deemed vital to those responsible for coastal protection and community resilience. This study tests for shifting observational inter-annual wave power trends using a newly developed, unique, United States Army Corps of Engineers Quality Controlled Consistent Measurement Archive, and offers a viable methodology to remove documented observational time series data discontinuations. This study is one of the first to show spatially and temporally comparative observational and model wave power results, providing new information on the accuracy of model wave power estimates, while showcasing in situ wave power trends at 29 sites around the U.S. coastline. Overall, the majority of the eastern Pacific Ocean and Hawaii wave power trends are downward, with mixed slope wave power trends apparent within the Atlantic Ocean and the Gulf of Mexico. Observational and model results are similar with respect to timing, but not magnitude, of wave power peaks in long-term inter-annual trends, with the moored buoy data presenting smaller wave power ranges for two (eastern Pacific Ocean and Hawaii) of the four regions. Additionally, the detection of a noticeable variability in the wave power trend direction within each region suggests that site-specific wave power trends should not be generalised to represent a large region. This work demonstrates that observational data are essential in local and regional wave climate studies to accurately estimate wave power for coastal planners and engineers. [ABSTRACT FROM AUTHOR]

Velocity, hydrographic, and microstructure observations collected under moderate to high winds, large surface waves, and significant ocean-surface heat losses were utilized to examine coherent velocity structures (CVS) and turbulent kinetic energy (TKE) budget in the mixed layer on the outer shelf in the northern Gulf of Mexico in February 2017. The CVS exhibited larger downward velocities in downwelling regions and weaker upward velocities in broader upwelling regions, elevated vertical velocity variance, vertical velocity maxima in the upper part of the mixed layer, and phasing of crosswind velocities relative to vertical velocities near the base of the mixed layer. Temporal scales ranged from 10 to 40 min, and estimated lateral scales ranged from 90 to 430 m, which were 1.5–6 times as large as the mixed layer depth. Nondimensional parameters, Langmuir and Hoenikker numbers, indicated that plausible forcing mechanisms were surface-wave-driven Langmuir vortex and destabilizing surface buoyancy flux. The rate of change of TKE, shear production, Stokes production, buoyancy production, vertical transport of TKE, and dissipation in the TKE budget were evaluated. The shear and Stokes productions, dissipation, and vertical transport of TKE were the dominant terms. The buoyancy production term was important at the sea surface, but it decreased rapidly in the interior. A large imbalance term was found under the unstable, high-wind, and high–sea state conditions. The cause of this imbalance cannot be determined with certainty through analyses of the available observations; however, our speculation is that the pressure transport is significant under these conditions. [ABSTRACT FROM AUTHOR]

Evolution of a thermally stratified diurnal warm layer (DWL), including the formation and decay of a daytime surface-layer jet, high-frequency internal waves, and mixing were examined from observations collected during July 2016, near 93.75°W, 28°N, on the outer Louisiana-Texas continental shelf in the Gulf of Mexico, when the ocean surface was experiencing a weak sea breeze (<5ms-1) and strong solar insolation. While winds and surface waves were weak, the DWL was formed with stratification strengthening and stability frequency reaching 14 cycles per hour at 2-m depth, while inhibiting turbulence below the DWL. A surface-intensified jet developed during afternoon hours. The jet, oriented to the right of the wind stress with a speed of about 10 cm s21 at 2m, veered and decreased with depth. The magnitude of the diurnal jet was correlated with the heat content anomaly in the DWL. Internal waves with periods ranging from 5 min to 4 h were observed in the upper 4m. Temperature fluctuations were ; 60.28C, and the corresponding vertical displacements varied from 0.5 to 1m. These fluctuations appeared during afternoon hours when the Richardson number dropped below the critical value of 0.25 followed by energetic mixing. The daytime jet and the high-frequency fluctuations disappeared a few hours after sunset. Internal waves were likely excited by Kelvin-Helmholtz instabilities and by surface wave and internal wave interactions. Mixing resulting from the dissipation of daytime internal waves is an important factor in regulating sea surface temperature in the DWL. [ABSTRACT FROM AUTHOR]

Optically-active constituents vary over short time and space scales in coastal waters, and they are impacted by a variety of complex, inter-related forcing processes. As part of the Integrated Coastal Bio-Optical Dynamics (ICoBOD) project, we conducted a field campaign in Mississippi Sound in the northern Gulf of Mexico during spring 2018 to examine the impact of the passage of atmospheric and tidal fronts on fine-scale physical and bio-optical property distributions in a shallow, dynamic, coastal environment. During a 25-day experiment, we deployed eight moorings over a roughly 7 × 7 km box encompassing a frontal zone, to collect a time series of physical and bio-optical measurements. We describe changes in diver visibility related to the passage of a short-duration, high-turbidity surface plume and nepheloid layer development/decay during a tidal cycle. Maximum nepheloid layer development was observed during low tide and lasted about 9–12 h. The strongest turbidity signal extended about 4–5 m above the bottom (approximately half of the water column), although anomalously elevated values were observed all the way to the surface. In addition, high-resolution (50 m) hydrodynamic model simulations provide insight into the frontal dynamics and aid interpretation of the observed patterns. Mooring observations confirmed model-predicted heat flux changes associated with the passage of an atmospheric cold front. [ABSTRACT FROM AUTHOR]

The diurnal variability of currents and temperature in the near-surface layer was examined from observations collected for a period of 11 days in July 2016 on the outer continental shelf in the Gulf of Mexico as part the U.S. Naval Research Laboratory study. During the first 7 days of the experiment, sea surface temperature, air temperature, and wind speed and direction varied diurnally, and the next 4 days weather conditions became calm representing daytime heating, low winds varying from almost zero to 5 m/s with directions rotated from northerly to southerly also following a diurnal cycle, and surface wave heights were less than 0.5 m. When winds and waves were weak, a large diurnal cycle of temperature developed in the upper 10 m with stratification as large as 18 cycles per hour at 2 m depth. Internal waves like motions were observed within the highly stratified diurnal layer, where near-surface temperature varied by 0.2C with a periods of 3 to 4 hours, and velocity fluctuations as large as 20 cm/s were found in the upper 10 m. The dominant wave period of velocity fluctuations is similar to the wave period found in temperature records. Generation mechanisms of these trapped high-frequency waves to their evolution during the diurnal cycle will be presented. [ABSTRACT FROM AUTHOR]

The article presents information on the generation of extreme waves owing to a hurricane on the outer continental shelf in the northeastern Gulf of Mexico. On September 15, 2004, the center of Hurricane Ivan passed directly over six wave-tide gauges deployed by the Naval Research Laboratory. Researchers calculated significant wave height and maximum individual wave height, which are the two parameters commonly used to characterize wave fields. Measurements of the extremely large waves directly under Ivan may act as a starting point for improving the understanding of the waves generated by the most powerful hurricanes.

The U.S. Naval Research Laboratory conducted a three-week field experiment to investigate coherent structures in the mixed layer (ML), and their effects on mixing and ML depth during wintertime under variable wind and surface-wave conditions. Atmospheric flux, ocean current, and hydrographic observations were collected in February 2017 on the outer shelf (depths from 80 to 100 m) in the Gulf of Mexico. Wintertime weather was dominated by passages of atmospheric cold fronts followed by dry and low-wind periods. Winds varied from 2 to 17 m/s and significant surface-wave heights (Hs) were between 0.3 and 3.2 m, while ocean currents were eastward with three-week mean speeds of about 24 cm/s near the surface and 4 cm/s near the bottom. The ML depth ranged from about 2 to 77 m. Winds were greater than 10 m/s and Hs was larger than 1.5 m when atmospheric cold fronts passed over the study area. At the onset of high-wind/large surface-wave events, current-shear increased, Richardson number (Ri) dropped below the critical value of 0.25 near the surface, and the ML began to deepen indicating initial development of shear- and wave-driven mixing. As wind/wave forcing continued larger values of shear and low Ri occurred as the ML deepened further. The high-shear region extended below the ML depth. Moreover, velocity observations indicated development of coherent structures that extended from the sea surface to the bottom of the ML with along-wind, cross-wind, and vertical velocities as high as 10, 10, and 5 cm/s, respectively. These structures seemed to have characteristics of Langmuir circulations and persisted until winds and surface waves subsided. [ABSTRACT FROM AUTHOR]