Peter Brickley

Observations of the Eastern Maine Coastal Current and its offshore extensions in 1994

Cold surface temperatures, reflecting Scotian Shelf origins and local tidal mixing, serve as a tracer of the Eastern Maine Coastal Current and its offshore extensions, which appear episodically as cold plumes erupting from the eastern Maine shelf. A cold water plume emanating from the Eastern Maine Coastal Current in May 1994 was investigated using advanced very high resolution radiometer (AVHRR) imagery, shipboard surveys of physical and biochemical properties, and satellite-tracked drifters. Evidence is presented that suggests that some of the plume waters were entrained within the cyclonic circulation over Jordan Basin, while the major portion participated in an anticyclonic eddy at the distal end of the plume. Calculations of the nitrate transported offshore by the plume show that this feature can episodically export significant quantities of nutrients from the Eastern Maine Coastal Current to offshore regions that are generally nutrient depleted during spring-summer. A series of AVHRR images is used to document the seasonal along-shelf progression of the coastal plume separation point. We speculate on potential causes and consequences of plume separation from the coastal current and suggest that this feature may be an important factor influencing the patterns and overall biological productivity of the eastern Gulf of Maine.

Impacts of Loop Current Frontal Cyclonic Eddies and Wind Forcing on the 2010 Gulf of Mexico Oil Spill

Monitoring and M A Record-Breakin Geophysical Mon Copyright 2011 b 10.1029/2011GM The 2010 Deepwater Horizon Gulf of Mexico oil spill, the largest in U.S. history, highlights the environmental risks inherent in deepwater drilling. These risks were mitigated by rapid access to real-time satellite measurements from passive (optical, IR) and active (synthetic aperture radar, altimetry) sensors. This study employed satellite data, in tandem with in situ current and wind measurements, to track surface oil and to better understand the causes for observed large-scalemotions during the 84 day episode. The analysis revealed the merger of three cyclonic eddies along the Loop Current’s (LC’s) northern margin, ultimately forming a larger and more vigorous cyclonic eddy, measuring 280 130 km on 18 May. This larger cyclonic eddy, in tandem with a smaller anticyclonic eddy and a LCmeander, controlled the motion of the oil/dispersant mixture into deepwater (maximum current speed of 2.25 m s ), tripling the area of surface oiling from 9623 to 33,575 km. Two main events limited the flow of oil to the Florida Straits, the accumulation of oil within the merged eddy and the fact that this eddy did not move substantially for several months. The observed offshore entrainment of oil toward the LCwas successfully hindcast using a particle-tracking model based on geostrophic currents computed from satellite altimetry. This assessment of circulation processes may help to advance numerical circulation modeling efforts in this region of rapid current variability in support of safer deepwater drilling in the northern Gulf.

Three Years of Ocean Data From a Bio‐optical Profiling Float

Ocean color, first measured from space 30 years ago, has provided a revolutionary synoptic view of near-surface fields of phytoplankton pigments. Since 1979, a number of ocean color satellite missions have provided coverage of phytoplankton biomass and other biogeochemical variables on scales of days to years and of kilometers to ocean basin. Because of the nature of visible light and its interaction with absorbing and scattering materials in the ocean and atmosphere, these measurements are biased toward near-surface waters and are obscured by clouds. As a consequence, ocean color satellites miss significant fractions of phytoplankton biomass, marine primary productivity, and particle flux that occur at depths beyond their sensing range. They also miss phytoplankton blooms and other events that occur during periods of extended cloud cover.

Use of air-deployed drogued drifting buoys for oil spill tracking

The magnitude of the recent Gulf of Mexico oil spill resulting from the Macondo oil well blowout, catastrophic explosion, and subsequent sinking of the Deepwater Horizon semi-submersible offshore drilling rig is unprecedented. The complex oceanographic and environmental character of the spill location and the application of vast quantities of chemical dispersants combined to create greater challenges for those tasked with mapping the areal extent of the oil and the advective pathways by which the oil would eventually reach both the nearby shore or become entrained into the vigorous offshore currents of the deepwater Gulf. The action of the chemical dispersants combined with surface winds and wave action resulted in high loads of oil particles throughout the mixed layer and extensive surface slicks. In addition, the oil disaggregated into hundreds of small patches instead of remaining pooled on the surface. Under these unexpected conditions, the tracking and forecasting of the spill presents a challenge. Horizon Marine, Inc., an operational oceanographic monitoring and forecasting company, has been actively involved in the oil spill response efforts and supported the current monitoring program near the Deepwater Horizon incident site. One method employed to delineate the initial area of the oil patch and track its expanding perimeter was the use of air-deployable drifting buoys (Far Horizon Drifters). These buoys are equipped with GPS receivers, transmit new positions hourly via satellite, and were nominally drogued at 5m and 50m to provide coverage of the upper water column. Using these observations, we describe several major transport pathways resulting from physical mechanisms operating over different scales. The entrainment of several buoys into the Loop Current and associated frontal eddies provided early indication of potential pathways taken by both the visible surface and invisible subsurface oil.

A Feature Oriented Regional Modeling System for the North Brazil Current Rings Migration after Retroflection

Southeast of the Trinidad-Venezuela region, the North Brazil Current (NBC) retroflects and forms about 5-8 rings annually. The ensemble of trajectories of rings extends offshore of the 500 m isobath, with a mean translation speed of approximately 14 km/day and a mean length scale of about 100 km (Goni and Johns, 2001, 2003). At least two distinct ring types exist: surface-intensified and thermocline-intensified, with differences evident in both azimuthal velocity and water masses. This paper presents a recent implementation of an operational modeling system for this region. The key to this modeling effort is to implement the feature oriented regional modeling methodology for the NBC rings with an advanced initialization scheme to incorporate the varying ring structure, shape, and associated currents made possible by regular surveillance and deployment of instruments into the ring. Multiple observations provide input and guidance to the ring initial conditions for the feature model system. Based on previous studies and data, a water-mass based feature model system for two distinct NBC rings is developed. The parametric models for temperature and salinity are built to capture the main features observed in vertical structure of those rings. For example, in the case of the thermocline-intensified ring, different empirical-analytical functions with tunable parameters are used to represent (i) the thermocline shoaling up in the intermediate depth of the ring, (ii) the dipping down in the inshore and offshore edges, and (iii) the presence of the maximum salinity water at 50-100 m. These feature models are first calibrated with available sea surface temperature (SST) data and then melded with background climatology in a featureoriented multiscale objective analysis to develop a three-dimensional description of the regional ocean. The feature oriented scheme is used to initialize an operational forecasting system using the Harvard Ocean Prediction System (HOPS) framework. Implementation, calibration, and validation of this system were carried out for multiple case studies during 2006 and 2007 when a number of drifter data sets were available. A hindcast study for 27 January 2010 is used for verifying the forecast system in a semi-operational mode. A fully operational system was launched in July 2010. Introduction The NBC is an intense western boundary current and a dominant circulation feature in the western tropical Atlantic. The NBC separates from the South American coastline near 6° – 8° N and retroflects to feed the eastward North Equatorial Counter Current (NECC). The tip of the retroflection grows and stalls typically in the region around 4-5° N, 50-52° W and pinches off to form a closed circulating eddy every 6-8 weeks between July and March. Between late spring (April) and midsummer (June), the retroflection weakens and the northwestward flow may extend to about 7° N and beyond. The resulting eddies, called NBC rings, migrate northwestward parallel to the shelf/slope contours and eventually reach the Lesser Antilles and impact active drilling sites in the region. Previous studies indicate that ring trajectories extend offshore of the 500 m isobath, with a mean translation speed of approximately 14 km/day and a mean length scale of about 100 km and azimuthal velocities from 70–200 cm/s (Goni and Johns, 2001, 2003; Wilson et al., 2002). Fratantoni and Richardson (2006) observed at least two distinct ring-to-ring differences in the three kinds of observed ring structures; one is in the azimuthal velocity structure and the other is in their water mass composition. In other words, two distinct ring types exist: surfaceintensified and thermocline-intensified.

Loop Current eddy merger exposed by satellites during Gulf of Mexico oil spill

The clockwise flow of water that extends northward into the Gulf of Mexico known as the Loop Current, and its associated eddies, regularly produces strong currents of 2–4 knots in the northern Gulf. Cyclonic (i.e., counterclockwise rotating) eddies, migrating along its outer margin, are difficult to study due to their rapid and unpredictable growth and propagation, as well as persistent cloud cover. We have found that night-time midinfrared satellite images obtained every 30 minutes from geostationary satellites used to quantify sea surface temperature, superimposed with daily updated gridded sea surface height data based on several satellite altimeters, allows us to track the Loop Current and cyclonic eddiesmore effectively than previous methods.1, 2 During the Deepwater Horizon oil spill, we assessed daily changes in the motion of the Loop Current, its cyclonic eddies (called Loop Current frontal eddies), and detached anticyclonic eddies by analyzing daily-updatedmaps of sea surface temperatures and heights. We used ship-board acoustic Doppler current profiler current measurements and time-series positions from satellite-tracked Global Positioning System (GPS) buoys to validate our satellite data interpretations.3 Surface oil is often mapped using readily available passive radiometric optical imaging, such as ‘true-color’ images from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) sensors (see Figure 1). During the Deepwater Horizon oil spill these data were useful, but not totally reliable due to solar interference with the optical sensors. Synthetic aperture radar (SAR) sensors proved more valuable, as the backscatter signal is very sensitive to surface oil and is usable in all weather, day Figure 1. Top: Moderate Resolution Imaging Spectroradiometer (MODIS) ‘true-color’ image depicting the offshore entrainment of the Deepwater Horizon oil toward the Loop Current front on 17May 2010. Middle: Radarsat-2 synthetic aperture radar (SAR) image also depicting the surface oil on 17May 2010. Bottom: Geostationary Operational Environment Satellite East (GOES-East) sea surface temperature image depicting the Loop Current and the large merged cyclone north of the Loop Current on 17–19 May 2010. Superimposed on the color-coded sea surface temperatures are the oiled area, traced from the

An operational modeling implementation for the Trinidad-Venezuela region using feature models

Southeast of the Trinidad-Venezuela region, the North Brazil Current (NBC) retroflects and forms about 5-8 rings annually. In addition to the NBC rings being the most prominent dynamic feature after the retroflection, the shelf flow plays an important role in the regional circulation. We present a recent implementation of an operational modeling system for this region using the feature oriented regional modeling system (FORMS) methodology for the NBC rings and shelf flow. A new formulation of the ring is presented in which the synoptic structures of temperature and salinity profiles are placed at the core and the climatological profiles are placed around the edge. The shelf-break front is feature modeled by interpolating between a representative temperature profile on the shelf and the monthly climatology 200 km offshore of the coastline. The feature oriented scheme is used to initialize an operational forecasting system using the Harvard Ocean Prediction System (HOPS) framework. Three case studies between 2011 and 2012 are presented, wherein real drifters trajectories within several NBC rings and the shelf are compared with those of the model drifters. These examples highlight the advantages of using feature models for short-term operational forecasting, in that, changing ring size and center location, changing level of no motion (LNM) and using drifter data for shelf flow verification allow for better predictability for the 4-5 days forecasts. These capabilities could be extended operationally to provide 2-3 week outlooks to offshore industry in the future.

Ocean Observing in the 4th Dimension--Using Autonomous Gliders for Operational Surveillance of the Gulf of Mexico

The ocean circulation in the Gulf of Mexico is highly dynamic in four dimensions (4D). The circulation varies throughout the interconnected shelf, slope, and deep ocean domains (3 dimensions) and varies greatly in time (the 4 dimension). Obtaining timely, comprehensive, and accurate forecasting of this 4D system remains a challenge for operators on the outer continental shelf (OCS). Long-range autonomous underwater gliding vehicles (AUGVs) enable collection of high spatial resolution sections through the ocean repeatedly, autonomously, and at a very low cost compared to conventional methods. Using these new tools, in-situ data can be continuously assessed to provide a more integrated description of the ocean state. We describe a series of glider observations obtained during extended (3.4 months) observational campaigns in the eastern Gulf of Mexico. One survey period took place from April-August 2011, covered a linear distance of 1400 nm (2700 km), and collected over 1000 vertical profiles to 1000 m. The deployment spanned the Loop Current (LC) growth phase and detachment of Eddy Hadal. Several cross-sections were obtained through the LC front, cyclonic frontal eddies, and in close proximity to working platforms in lease areas of the NGOM slope. The in-situ fields of ocean density help to delineate the subsurface boundaries of these dynamic features, and their geostrophic velocity structure can be inferred from lateral density variations. Sustained in-situ glider observations throughout the eastern Gulf of Mexico will improve the knowledge and forecasting vital for efficient day-to-day operational management decisions of the offshore petroleum industry.