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The Gulf of Mexico Coastal Hypoxia Glider Experiment was designed to assess the feasibility of using ocean glider technology in the coastal hypoxic zone of the northern Gulf of Mexico in Summer/Fall 2014. The objectives were (1) to coordinate and operate multiple autonomous buoyancy ocean gliders in depths less than 50 m and (2) to determine how close to the bottom gliders can reliably reach without making contact. Strong vertical and horizontal stratification gradients, strong coastal currents, and the low-oxygen conditions that occur within the lower water column characterize the coastal area of the northern Gulf of Mexico. These environmental conditions combine with the presence of more than 5,000 surface piercing oil/gas structures to make piloting and navigation in the region challenging. We quantify glider performance to assess the usefulness of buoyancy gliders to address the National Oceanic and Atmospheric Administration Action Plan goal to monitor the spatial extent, duration, and severity of the Gulf hypoxic zone. We find that the gliders, despite the operational challenges, were consistently able to travel from the surface to the oxygen-depleted depths of subpycnocline waters, that is, within 2 m of the ocean bottom. Our assessment is that gliders are able to provide real-time observations suitable to monitor coastal hypoxia. [ABSTRACT FROM AUTHOR]

This study incorporates observations from Array of Real-time Geostrophic Oceanography (ARGO) floats and surface drifters to identify seasonal circulation patterns at the surface, 1000 m, 1500 m, and 2000 m in the northwest Indian Ocean, and quantify velocities associated with them. A skill comparison of the Simple Ocean Data Assimilation (SODA) reanalysis output was also performed to contribute to the understanding of the circulation dynamics in this region. Subsurface currents were quantified and validated using the ARGO float data. Surface currents were identified using surface drifter data and compared to the subsurface observations to enhance our previous understanding of surface circulations. Quantified Southwest Monsoon surface currents include the Somali Current (v max = 179.5 cm/s), the East Arabian Current (v max = 52.3 cm/s), and the Southwest Monsoon Current (v max = 51.2 cm/s). Northeastward flow along the Somali coast is also observed at 1000 m (v max = 26.1 cm/s) and 1500 m (v max = 12.7 cm/s). Currents associated with the Great Whirl are observed at the surface (v max = 161.4 cm/s) and at 1000 m (v max = 16.2 cm/s). In contrast to previous studies, both ARGO and surface drifter data show the Great Whirl can form as early as the boreal Spring intermonsoon, lasting until the boreal Fall intermonsoon. The Arabian Sea exhibits eastward/southeastward flow at the surface, 1000 m, 1500 m, and 2000 m. Quantified Northeast Monsoon surface currents include the Somali Current (v max = 97.3 cm/s), Northeast Monsoon Current (v max = 30.0 cm/s), and the North Equatorial Current (v max = 28.5 cm/s). Southwestward flow along the Somali coast extends as deep as 1500 m. Point-by-point vector and scalar correlations of SODA output to ARGO and surface drifter data showed that surface SODA output and surface drifter data generally produced a strong correlation attributed to surface currents strongly controlled by the monsoons, while subsurface correlations of SODA output and ARGO were mostly insignificant due to variability associated with intermonsoonal transitions. SODA output produced overall smaller velocities than both observational datasets. Assimilating ARGO velocities into the SODA reanalysis could improve subsurface velocity assimilation, especially during the boreal fall and spring when ARGO observations suggest that flow is highly variable. [ABSTRACT FROM AUTHOR]

Most commercially, important food fishes in the tropics reproduce in spawning aggregations, which serve to replenish their populations via larval dispersal. Dispersal pathways are not well understood. This study examines the dispersal hypothesis assumed for most dispersal modeling studies, i.e. that prevailing currents transport freshly released eggs offshore and away from spawning sites, where they are advected into far-field ocean currents and contribute to long-distance larval transport. We used GPS-tracked surface drifters, quantitative ichthyoplankton tows, and microsatellite DNA markers to assess egg dispersal pathways from a well-documentedLutjanus cyanopterusspawning aggregation site. We conducted and plotted 95 drifter tracks over a bathymetric map of the study area and used geoprocessing techniques to determine the direction, speed, and variability of these tracks. Our field-collected data do not support the hypothesis that surface currents transport Cubera snapper gametes offshore during peak spawning. Eggs were transported onshore toward the barrier reef and potentially through the channel, supporting the idea that local retention may be more important than long-distance transport. These results are highly relevant for parameterizing larval connectivity models. Furthermore, we demonstrate the use of a low-cost and easily replicable method to track currents and egg dispersal from spawning aggregation sites. [ABSTRACT FROM PUBLISHER]

The spatial structure and temporal characteristics of sea breeze and the associated coastal ocean response in the northwest Gulf of Mexico are investigated using moored instruments, hydrographic stations, and wind measurements. Near the study area of 30°N, motions in the diurnal–inertial band (DIB) may be significantly enhanced by a near-resonant condition between local inertial and diurnal forcing frequencies. Wavelet analysis is used to quantify the results. Results indicate that diurnal sea-breeze variability peaks in summer and extends at least 300 km offshore with continuous seaward phase propagation. The maximum DIB oceanic response occurs in June when there is a shallow mixed layer, strong stratification, and an approximately 10-day period of continuous sea-breeze forcing. DIB current variance decreases in July and August as the consequence of the deepening of the mixed layer and a more variable phase relationship between the wind and current. River discharge varies interannually and can significantly alter the oceanic response during summer. The “great flood” of the Mississippi River in 1993 deepened the summer mixed layer and reduced the sea-breeze response during that year. Vertically, DIB currents are surface intensified, with a first baroclinic modal structure. The significance of these DIB motions on the shelf is that they can provide considerable vertical mixing in summer, as seen by the suppression of the bulk Richardson number (by a factor of 30) during strong DIB events. This provides a potential mechanism to ventilate seasonally occurring near-bottom hypoxic waters of the coastal ocean. [ABSTRACT FROM AUTHOR]

Abstract: The circulation and distribution of water properties in the water column of the Gulf of Mexico influence the flux of carbon to the benthic environment. The eddy field of the upper 1000m creates environmental conditions that are favorable for biological productivity in an otherwise oligotrophic subtropical ocean. This eddy field results in the transport of nutrients and organic matter into the photic zone through cross-margin flow of shelf waters, upwelling in cyclones, and uplift from the interaction of anticyclones with bathymetry. These conditions then allow the productivity that becomes a possible source of carbon to the benthos. Data from four cruises during summers of 2000–2002 are used to describe the currents and water property distributions in the deepwater Gulf of Mexico, which consists of water depths greater than 400m. Comparisons are made to historical data sets to provide an understanding of the persistence of the characteristics of the Gulf and the processes that occur there. The currents in the Gulf are surface intensified, have minimum in 800–1000m depths, and also exhibit bottom intensification, especially near sloping topography. Historical time series records show current speeds near-bottom reach 50–100cms−1. At basin scales, these currents tend to flow cyclonically (counter-clockwise) along the bathymetry. These near-bottom, episodic, high-speed currents provide a mechanism for the transport of organic material, in both large and small particle sizes, from one benthic area to another. The distributions of temperature, salinity, nutrients, and dissolved oxygen during the study appear to be unchanged from historical findings. The source waters for the deep Gulf are the water masses brought into the Gulf by the Loop Current system. The properties in the upper 100–200m are the most variable of the water column, consistent with their proximity to wind mixing, river discharge mixing, and atmospheric influences. Below 1500m, there are no major horizontal variations in these water properties. [Copyright &y& Elsevier]