Walter R. Johnson

Mississippi River flood waters that reached the Gulf Stream

Distributions of physical, biological, and chemical parameters in florida Keys coastal waters seaward of the reef track were surveyed on September 9 to 13, 1993, as part of a coordinated multidisciplinary study of surface transport processes. A band of low-salinity water was observed along the shoreward side of the florida Current over the downstream extent of the survey from Miami to Key West. Biological and chemical indicators within the band, together with its large volume, satellite imagery, and a surface drifter trajectory suggested the recent Mississippi River flood as the source.

Overview of the Oil Spill Risk Analysis (OSRA) Model for Environmental Impact Assessment

Abstract The oil spill risk analysis (OSRA) model is an environmental impact assessment tool, producing estimates of the probabilities of oil spill occurrence and contact from prospective commercial oil and gas operations, which may occur broadly on the outer continental shelf of the United States. Extensive environmental data sets drive the model. Much of the data were obtained by numerous MMS-funded environmental studies. The model differs from most trajectory models in that it does not attempt to replicate or predict individual oil spill events. Instead, the model generates an ensemble of thousands to hundreds of thousands (typically) of simulated oil spill trajectories over many years of wind and ocean current input fields. The frequencies of contact of the simulated spills are the estimated probabilities under the implicit assumption that the input spill occurrence rates, winds, and ocean currents will be, in a broad statistical sense, like those that will occur during future oil and gas operations proximate to areas of environmental concern.

A Model of the Near-Surface Circulation of the Santa Barbara Channel: Comparison with Observations and Dynamical Interpretations

Abstract Previous studies indicate the importance of wind, wind curl, and density differences in driving the near-surface circulation in the Santa Barbara Channel (SBC). Here model sensitivity experiments and dynamical analyses of the near-surface currents in the SBC are presented. Various approximations of the wind—from coarse-resolution European Centre for Medium-Range Weather Forecasts (ECMWF) archives to a high-resolution dataset that incorporates buoy, oil-platform, and land-based wind stations—are used. In some experiments, observed temperatures at 10 moorings are also assimilated into the model. Model solutions are sensitive to channel-scale [O(10 km)] wind distribution. Modeled currents forced by the ECMWF wind yield poor results when compared with observations. The simulation using the high-resolution wind (without assimilation) captures the observed spatial and seasonal patterns of the circulation, though the intensity is underestimated. With assimilation, the intensity is increased. In particul...

A Cyclesonde View of Coastal Upwelling

Abstract In August 1973, 320 vertical profiles of temperature and horizontal velocity were recorded during a 64 h period by an array of three Cyclesondes in the coastal upwelling region off Oregon. The mean interior along-shore velocity was geostrophic and a linear function of density, with a near-surface, equatorward jet at mid-shelf, and a poleward undercurrent at the shelf break. The mean cross-shelf flow was relatively weak and substantially ageostrophic; it was suggestive of a two-cell (co-rotating) circulation within the mid-shelf frontal zone and a two-cell (counter-rotating) circulation near the shelf break. The direction of the mean, near-bottom, cross-shelf flow was consistent with a bottom Ekman layer driven by the mean near-bottom alongshore flow. At mid-shelf, near-inertial motions with a vertical wavelength of 50 m, upward phase velocity, and downward group velocity persisted throughout the record. The hourly vector shears indicated a layer of persistent shear instability at the base of the ...

Model Wind over the Central and Southern California Coastal Ocean

Recent studies have shown the importance of high-resolution wind in coastal ocean modeling. This paper tests the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) at the 9-, 27-, and 81-km grid resolutions in simulating wind off the central and southern California coasts, including the Santa Barbara Channel (SBC). The test period is March–May (1999) when the wind changes from its characteristics more typical of winter, to spring when strong gradients exist in the SBC. The model results were checked against wind station time series, Special Sensor Microwave Imager wind speeds, and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis. The high-resolution (9-km grid) COAMPS wind shows expansion fans downwind of major capes where speed increases. The large-scale [O(100 km)] wind turns onshore in the Southern California Bight where both wind and wind stress curl weaken southward along the coast. The formation and evolution of the Catalina eddies are also simulated. These general features agree with observations. The turning appears to be the cumulative effect of synoptic cyclones shed downwind of Point Conception during periods of intense northerly wind. The turning and eddies are much weaker in the ECMWF reanalysis or the COAMPS field at the 81-km grid. Near the coast, observed small-scale (tens of kilometers) structures are reasonably reproduced by COAMPS at the 9-km grid. Results from the 9-km grid generally compare better with observations than the 27-km grid, suggesting that a more accurate model wind may be obtained at even higher resolution. However, in the SBC, simulated winds at both the 9- and 27-km grids show along-channel coherency during May, contrary to observations. The observed winds in the channel appear to be of small localized scales (10 km) and would require an improved model grid and perhaps also boundary layer physics to simulate.

Working Toward Improved Small- scale Sea Ice-Ocean Modeling in the Arctic Seas

Until recently the main motivation in sea ice modeling has been toward the development of large-scale models for climate studies. These models describe sea ice as a plastic material, with a smooth yield surface and ice strength dependent on a thickness distribution that is based on statistical representations of sea ice deformation through ridging. With tuning, they are found to reproduce ice extent and concentration in the Arctic and Antarctic, though velocity fields are overly smooth and many details, such as polynyas and leads, are not captured. There is increasing interest in regional ice modeling. In the near-shore Beaufort and Chukchi seas, there is considerable interest from the oil industry in the formation and breakup of landfast ice, the propagation of oil spills, and prediction of sea ice conditions. The importance of resolving eddies in the ocean and modeling small-scale (sub-10-km) sea ice processes is becoming apparent, as we begin to understand the non-linear effect of small-scale processes on the large-scale motion. Recently, there have been advances in the direction of small-scale process research and regional ice-ocean model development. The most pertinent of these are outlined in this article.

A modeling study of coastal circulation and landfast ice in the nearshore Beaufort and Chukchi seas using CIOM

This study investigates sea ice and ocean circulation using a 3-D, 3.8 km CIOM (Coupled Ice-Ocean Model) under daily atmospheric forcing for the period 1990–2008. The CIOM was validated using both in situ observations and satellite measurements. The CIOM successfully reproduces some observed dynamical processes in the region, including the Bering-inflow-originated coastal current that splits into three branches: Alaska Coastal Water (ACW), Central Channel branch, and Herald Valley branch. In addition, the Beaufort Slope Current (BSC), the Beaufort Gyre, the East Siberian Current (ESC), mesoscale eddies, and seasonal landfast ice are well simulated. The CIOM also reproduces reasonable interannual variability in sea ice, such as landfast ice, and anomalous open water (less sea ice) during the positive Dipole Anomaly (DA) years, vice versa during the negative DA years. Sensitivity experiments were conducted with regard to the impacts of the Bering Strait inflow (heat transport), onshore wind stress, and sea ice advection on sea ice change, in particular on the landfast ice. It is found that coastal landfast ice is controlled by the following processes: wind forcing, Bering Strait inflow, and sea ice dynamics.

A comparison of two current meters on a surface mooring

Abstract An EG&G Vector Measuring Current Meter (VMCM) and an Aanderaa Recording Current Meter (RCM4) were moored below a surface-following buoy. The RCM4 instrument at 4 m depth recorded currents similar to those measured by the VMCM at 2 m, in spite of rotor pumping and direction aliasing. The speeds measured by the RCM4 did show the effect of pumping by an absence of low speeds ( −1 ) during most of the time series. However, the mean velocities and standard deviations of the flow field for the two instruments were similar. The major differences were for current fluctuations of frequency >0.125 cph (0.00208 Hz) where the RCM4 overestimated the energy by up to an order of magnitude. A mooring design using a surface follower buoy in strong currents (50 cm s −1 ) can be used with RCM4 instruments to measure accurately low frequency current fluctuations.

STATISTICAL ESTIMATES OF SHORELINE OIL CONTACT IN THE GULF OF MEXICO

ABSTRACT As steward of the Federal offshore lands known as the Outer Continental Shelf (OCS), the U.S. Department of the Interior (DOI), Minerals Management Service (MMS), is responsible for balanc...

Oil-Spill Risk Analysis for Assessing Environmental Impacts

ABSTRACT The Minerals Management Service (MMS), an agency of the U.S. Department of the Interior, maintains a leasing program for commercial oil and gas development on the Outer Continental Shelf in U.S. territorial waters. The MMS performs an oil-spill risk analysis (OSRA) using, in part, a statistical model of hypothetical oil-spill trajectories. The OSRA Model is driven by analyzed sea surface winds and model-generated ocean surface currents. Instead of focusing on individual oil-spill events, the OSRA examines oil-spill risks over long periods of time, ranging from 5 years to decades. The OSRA Model calculates thousands of oil-spill trajectories over extended areas of the U. S. continental shelf and tabulates the frequencies with which the simulated oil-spills contact the geographic boundaries of designated natural resources within a specified number of days after the simulated spill events. A key element of OSRA Model runs is the particle trajectory simulation based on wind velocities and surface oce...