Edward H. Ryan

Evidence of Preferential Directions for Gravity Wave Propagation Due to Wind Filtering in the Middle Atmosphere

All-sky TV images of wave structure in the near-infrared hydroxyl (OH) nightglow emission were recorded over a 3-month period during May, June, and July 1988 from a high-altitude site at the Mountain Research Station (40.0° N, 105.6° W, 3050 m), near Nederland, Colorado. Well-defined, coherent wave patterns associated with the passage of short period (<1 hour) gravity waves were observed on a total of 22 occasions. The wave motions exhibited similar spatial and temporal properties during each month but a distinct tendency for northward propagation (68% of the wave azimuths within ± 40.0° N), with some eastward motion in May and June, was observed throughout the campaign. Although it is theoretically well known that upward propagating gravity waves can be blocked at a critical layer produced by the interaction of the waves with the horizontal background wind, observational evidence of this phenomenon is rare. To investigate the possibility that the asymmetry in the wave propagation directions was caused by the critical layer, a model based on mean climatological background winds and numerical tidal wave modes valid for any mid-latitude site and time of the year was constructed to show the regions forbidden to upward gravity wave propagation from critical layer theory. These “blocking diagrams” which vary with height and time were constructed for the OH altitude (∼87 km) for the present paper. Comparison of the predicted (i.e., least restricted) and the observed directions of the wave motion show almost complete agreement. This suggests that middle atmospheric winds can play an important role in determining the flux and the azimuthal distribution of short-period waves reaching the upper atmosphere.

Mean and Near-Inertial Ocean Current Response to Hurricane Gilbert

Abstract The three-dimensional hurricane-induced ocean response is determined from velocity and temperature profiles acquired in the western Gulf of Mexico between 14 and 19 September 1988 during the passage of Hurricane Gilbert. The asymmetric wind structure of Gilbert indicated a wind stress of 4.2 N m−2 at a radius of maximum winds (Rmax) of 60 km. Using observed temperature profiles and climatological temperature–salinity relationships, the background and storm-induced geostrophic currents (re: 750 m) were 0.1 m s−1 and 0.2 m s−1, respectively. A Loop Current warm core ring (LCWCR) was also located to the right of the storm track at 4–5 Rmax, where anticyclonically rotating near-surface and 100-m currents decreased from 0.9 m s−1 to 0.6 m s−1 at depth. The relative vorticity in the LCWCR was shifted below the local Coriolis parameter by about 6%. In a storm-based coordinate system, alongtrack residual velocity profiles from 0 to 4 Rmax were fit to a dynamical model by least squares to isolate the near...

Predictability of Drifter Trajectories in the Tropical Pacific Ocean

Abstract Predictability of particle motion in the ocean over a timescale of one week is studied using three clusters of buoys consisting of 5–10 drifters deployed in the tropical Pacific Ocean. The analysis is conducted by using three techniques with increasing complexity: the center of mass of the cluster, advection by climatological currents, and a new technique that relies on the assimilation of both velocity and position data from the surrounding drifters into a Markov model for particle motion. A detailed mathematical description of the theory leading to this model is given. The predictability of drifter motion in these clusters is characterized using the data density Nd, defined as the number of drifters over an area scaled by the mean diameter of the cluster. The data density Nd decreases along the drifter trajectories due to the tendency of particles to disperse by turbulent fluid motion. In the first regime, which corresponds to the period after the release of drifters in a tight cluster when Nd ...

Principal component analysis of biological and physical variability in a Gulf Stream meander crest

In September and October 1988 a series of physical and biological observations were collected by the R. V. Endeavor and R. V. Cape Hatteras in a Gulf Stream meander crest. The hydrographic data, vertical chlorophyll a profiles derived from CTD/fluorescence profiles (calibrated with discrete pigment samples), and zooplankton biomass data (20–120m, estimated from chlorophyll a exhibited increased variability in the upper 120 m with a zero-crossing at 55m. The first mode of zooplankton biomass is near-surface intensified and decreases between 40 and 100 m. For each variable, the individual profiles are regressed onto the corresponding modes to generate a spatial/temporal series of the first two principal components. These series are analyzed in a local curvilinear coordinate system, defined by the apex of the meander crest on a daily basis. The spatial structures of the principal components for each of the physical variables in the curvilinear coordinate system are very similar. The temporal variability in this data set is overwhelmed by the spatial variability for all variables except zooplankton biomass, which exhibits a clear diurnal signal because of diel migration. For all the variables, cross-stream variability dominates, but there is some indication of along-stream variability, particularly in the Slope Water. The large-scale Gulf Stream front produces 50–80% of the total data variance in all variables, except the zooplankton. Mesoscale phenomena, including meander-induced vertical motion with associated detrainment/entrainment on the western and eastern flanks of the crest, and a warm-core ring stream interaction, account for 15–30% of the total data variance for all variables. Only 18% of the zooplankton variability is due the large-scale Gulf Stream front, while 50% of the variability can be attributed to a combination of mesoscale phenomena and diel migration. On the order of 10% of the variability in the data is due to submesoscale phenomena (L<20 km). The estimated variability associated with biological processes, other than diel migration, is 20%. The error for the terms in the variance decomposition is on the order of 5%.

Statistical properties of the surface velocity field in the northern Gulf of Mexico sampled by GLAD drifters

The Grand LAgrangian Deployment (GLAD) used multiscale sampling and GPS technology to observe time series of drifter positions with initial drifter separation of O(100 m) to O(10 km), and nominal 5 min sampling, during the summer and fall of 2012 in the northern Gulf of Mexico. Histograms of the velocity field and its statistical parameters are non-Gaussian; most are multimodal. The dominant periods for the surface velocity field are 1–2 days due to inertial oscillations, tides, and the sea breeze; 5–6 days due to wind forcing and submesoscale eddies; 9–10 days and two weeks or longer periods due to wind forcing and mesoscale variability, including the period of eddy rotation. The temporal e-folding scales of a fitted drifter velocity autocorrelation function are bimodal with time scales, 0.25–0.50 days and 0.9–1.4 days, and are the same order as the temporal e-folding scales of observed winds from nearby moored National Data Buoy Center stations. The Lagrangian integral time scales increase from coastal values of 8 h to offshore values of approximately 2 days with peak values of 3–4 days. The velocity variance is large, O(1)m2/s2, the surface velocity statistics are more anisotropic, and increased dispersion is observed at flow bifurcations. Horizontal diffusivity estimates are O(103)m2/s in coastal regions with weaker flow to O(105)m2/s in flow bifurcations, a strong jet, and during the passage of Hurricane Isaac. The Gulf of Mexico surface velocity statistics sampled by the GLAD drifters are a strong function of the feature sampled, topography, and wind forcing

A Biodegradable Surface Drifter for Ocean Sampling on a Massive Scale

AbstractTargeted observations of submesoscale currents are necessary to improve science’s understanding of oceanic mixing, but these dynamics occur at spatiotemporal scales that are currently challenging to detect. Prior studies have recently shown that the submesoscale surface velocity field can be measured by tracking hundreds of surface drifters released in tight arrays. This strategy requires drifter positioning to be accurate, frequent, and to last for several weeks. However, because of the large numbers involved, drifters must be low-cost, compact, easy to handle, and also made of materials harmless to the environment. Therefore, the novel Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifter was designed following these criteria to facilitate massive sampling of near-surface currents during the Lagrangian Submesoscale Experiment (LASER). The drifting characteristics were determined under a wide range of currents, waves, and wind conditions in laboratory se...

A Practical, Hybrid Model for Predicting the Trajectories of Near-Surface Ocean Drifters

A hybrid Lagrangian‐Eulerian model for calculating the trajectories of near-surface drifters in the ocean is developed in this study. The model employs climatological, near-surface currents computed from a spline fit of all available drifter velocities observed in the Pacific Ocean between 1988 and 1996. It also incorporates contemporaneous wind fields calculated by either the U.S. Navy [the Navy Operational Global Atmospheric Prediction System (NOGAPS)] or the European Centre for Medium-Range Weather Forecasts (ECMWF). The model was applied to 30 drifters launched in the tropical Pacific Ocean in three clusters during 1990, 1993, and 1994. For 10-day-long trajectories the forecasts computed by the hybrid model are up to 164% closer to the observed trajectories compared to the trajectories obtained by advecting the drifters with the climatological currents only. The best-fitting trajectories are computed with ECMWF fields that have a temporal resolution of 6 h. The average improvement over all 30 drifters of the hybrid model trajectories relative to advection by the climatological currents is 21%, but in the open-ocean clusters (1990 and 1993) the improvement is 42% with ECMWF winds (34% with NOGAPS winds). This difference between the open-ocean and coastal clusters is due to the fact that the model does not presently include the effect of horizontal boundaries (coastlines). For zero initial velocities the trajectories generated by the hybrid model are significantly more accurate than advection by the mean currents on time scales of 5‐15 days. For 3-day-long trajectories significant improvement is achieved if the drifter’s initial velocity is known, in which case the model-generated trajectories are about 2 times closer to observations than persistence. The model’s success in providing more accurate trajectories indicates that drifters’ motion can deviate significantly from the climatological current and that the instantaneous winds are more relevant to their trajectories than the mean surface currents. It also demonstrates the importance of an accurate initial velocity, especially for short trajectories on the order of 1‐3 days. A possible interpretation of these results is that winds affect drifter motion more than the water velocity since drifters do not obey continuity.

Tracking the Hercules 265 marine gas well blowout in the Gulf of Mexico

On 23 July 2013, a marine gas rig (Hercules 265) ignited in the northern Gulf of Mexico. The rig burned out of control for 2 days before being extinguished. We conducted a rapid-response sampling campaign near Hercules 265 after the fire to ascertain if sediments and fishes were polluted above earlier baseline levels. A surface drifter study confirmed that surface ocean water flowed to the southeast of the Hercules site, while the atmospheric plume generated by the blowout was in eastward direction. Sediment cores were collected to the SE of the rig at a distance of ∼0.2, 8, and 18 km using a multicorer, and demersal fishes were collected from ∼0.2 to 8 km SE of the rig using a longline (508 hooks). Recently deposited sediments document that only high molecular weight (HMW) polycyclic aromatic hydrocarbon (PAH) concentrations decreased with increasing distance from the rig suggesting higher pyrogenic inputs associated with the blowout. A similar trend was observed in the foraminifera Haynesina germanica, an indicator species of pollution. In red snapper bile, only HMW PAH metabolites increased in 2013 nearly double those from 2012. Both surface sediments and fish bile analyses suggest that, in the aftermath of the blowout, increased concentration of pyrogenically derived hydrocarbons was transported and deposited in the environment. This study further emphasizes the need for an ocean observing system and coordinated rapid-response efforts from an array of scientific disciplines to effectively assess environmental impacts resulting from accidental releases of oil contaminants.

Occurrence characteristics of mesospheric gravity waves at 51 °N

Abstract Observations of the OH(8-3) band rotational temperature have been carried out from Calgary (51 °N, 114 °W), Canada, from 1988 to 1990. The measurements have been taken in orthogonal scanning mode and the data set has been analyzed to study salient features of gravity waves in the mesospheric region from this mid-latitude station. Rotational temperature data from 36 nights showed distinct gravity wave activity during this period. The gravity wave characteristics studied include the dominant period, horizontal structure speed, implied horizontal wavelength and horizontal component of direction of propagation. The preferred direction in the horizontal wave propagation at Calgary is towards the north-west. A comparison of the observed horizontal propagation directions with the permitted directions, using model wind profiles for Calgary, shows good agreement. This indicates that the upward flow of wave energy could be modified by the background wind.

Ocean convergence and the dispersion of flotsam

Significance Ocean currents move material released on the ocean surface away from the release point and, over time, spread it over an increasingly large area. However, observations also show high concentrations of the material even after significant spreading. This work examines a mechanism for creating such concentrations: downwelling of water at the boundaries of different water masses concentrates floating material at this boundary. Hundreds of satellite-tracked drifters were released near the site of the 2010 Deepwater Horizon oil spill. Surprisingly, most of these gathered into a single cluster less than 100 m in size, dramatically demonstrating the strength of this mechanism. Floating oil, plastics, and marine organisms are continually redistributed by ocean surface currents. Prediction of their resulting distribution on the surface is a fundamental, long-standing, and practically important problem. The dominant paradigm is dispersion within the dynamical context of a nondivergent flow: objects initially close together will on average spread apart but the area of surface patches of material does not change. Although this paradigm is likely valid at mesoscales, larger than 100 km in horizontal scale, recent theoretical studies of submesoscales (less than ∼10 km) predict strong surface convergences and downwelling associated with horizontal density fronts and cyclonic vortices. Here we show that such structures can dramatically concentrate floating material. More than half of an array of ∼200 surface drifters covering ∼20 × 20 km2 converged into a 60 × 60 m region within a week, a factor of more than 105 decrease in area, before slowly dispersing. As predicted, the convergence occurred at density fronts and with cyclonic vorticity. A zipperlike structure may play an important role. Cyclonic vorticity and vertical velocity reached 0.001 s−1 and 0.01 ms−1, respectively, which is much larger than usually inferred. This suggests a paradigm in which nearby objects form submesoscale clusters, and these clusters then spread apart. Together, these effects set both the overall extent and the finescale texture of a patch of floating material. Material concentrated at submesoscale convergences can create unique communities of organisms, amplify impacts of toxic material, and create opportunities to more efficiently recover such material.