David M. Fratantoni

Collective Motion, Sensor Networks, and Ocean Sampling

This paper addresses the design of mobile sensor networks for optimal data collection. The development is strongly motivated by the application to adaptive ocean sampling for an autonomous ocean observing and prediction system. A performance metric, used to derive optimal paths for the network of mobile sensors, defines the optimal data set as one which minimizes error in a model estimate of the sampled field. Feedback control laws are presented that stably coordinate sensors on structured tracks that have been optimized over a minimal set of parameters. Optimal, closed-loop solutions are computed in a number of low-dimensional cases to illustrate the methodology. Robustness of the performance to the influence of a steady flow field on relatively slow-moving mobile sensors is also explored

UNDERWATER GLIDERS FOR OCEAN RESEARCH

Underwater gliders are autonomous vehicles that profile vertically by buoyancy control and move horizontally on wings. Gliders are reviewed, from their conception by Stommel as an extension of autonomous profiling floats, through their development in 3 models, and including their first deployments singly and in numbers. This paper discusses the basics of glider function as implemented by University of Washington, Seaglider, Scripps Institution of Oceanography, and Webb Research in Slocum. Preliminary results are presented from a recent demonstration project that used a network of gliders off Monterey. A wide range of sensors has already been deployed on gliders, with many under development, and a wider range of future possibilities. Glider networks appear to be among the best approaches to achieving subsurface spatial resolution necessary for ocean research.

North Atlantic surface circulation during the 1990's observed with satellite‐tracked drifters

A new compilation of Lagrangian velocity observations describes the state of the North Atlantic surface circulation during the 1990s. Gridded fields of velocity and eddy kinetic energy (EKE) are constructed from trajectories of more than 1500 15-m drogued satellite-tracked surface drifters in service between January 1990 and December 1999. This time period overlaps a coordinated field study of circulation and variability in the North Atlantic completed between 1996–2000 as part of the World Ocean Circulation Experiment. We describe the construction of a self-consistent drifter climatology, present decadal-mean quasi-Eulerian velocity and EKE fields computed on a 1° grid, and compare these results with contemporary satellite measurements. Detailed discussion of the inferred surface circulation is focused on three regions: (1) The Gulf Stream and North Atlantic Current, (2) the Labrador Sea and subpolar gyre, and (3) the Caribbean Sea. The swiftest drifter motions were found in the equatorial region and along the tropical, subtropical, and subpolar western boundaries. The maximum instantaneous speed determined from a single (quality-controlled and filtered) drifter observation was 273 cm s−1 in the Gulf Stream southeast of Cape Cod. The highest EKE value in the North Atlantic (2790 cm2 s−2) was found in the Gulf Stream just downstream of the New England Seamounts. Over most of the Atlantic basin, drifter-derived EKE values were found to be O(100 cm2 s−2) higher than corresponding values derived from satellite altimetry. In the Labrador Sea a region of sharply elevated EKE appears to be geographically related to the localized ejection of drifters (and by extension, mass and kinetic energy) from the energetic West Greenland Current between 60° and 62°N. When compared to drifter measurements made in the late 1970s our results suggest (but do not statistically confirm) an enhancement and slight northward shift of the zonal Gulf Stream extension. Such a shift is consistent in sign with expectations based on observed interdecadal variations in wind stress and subtropical gyre potential energy associated with the North Atlantic Oscillation.

Multi-AUV Control and Adaptive Sampling in Monterey Bay

Operations with multiple autonomous underwater vehicles (AUVs) have a variety of underwater applications. For example, a coordinated group of vehicles with environmental sensors can perform adaptive ocean sampling at the appropriate spatial and temporal scales. We describe a methodology for cooperative control of multiple vehicles based on virtual bodies and artificial potentials (VBAP). This methodology allows for adaptable formation control and can be used for missions such as gradient climbing and feature tracking in an uncertain environment. We discuss our implementation on a fleet of autonomous underwater gliders and present results from sea trials in Monterey Bay in August, 2003. These at-sea demonstrations were performed as part of the Autonomous Ocean Sampling Network (AOSN) II project

On the Atlantic inflow to the Caribbean Sea

New observations are summarized that lead to the first comprehensive description of the mean inflow distribution in the passages connecting the Atlantic Ocean with the Caribbean Sea. The total Caribbean inflow of 28 Sv is shown to be partitioned approximately equally between the Windward Islands Passages (B10 Sv), Leeward Islands Passages ðB 8S vÞ; and the Greater Antilles Passages ðB10 SvÞ: These results are compared to a numerical model study using a 6-layer, 1=41 resolution Atlantic Basin version of the NRL Layered Ocean Model. Results from two simulations are described, including a purely wind-forced model driven by Hellerman and Rosenstein (J. Phys. Oceanogr. 13 (1983) 1093) monthly winds, and a model with an additional 14 Sv meridional overturning cell driven by inflow/outflow ports at the northern ð651N) and southern ð201S) model boundaries. The purely wind-driven version of the model exhibits a total Caribbean inflow of 17 Sv; consistent with expectations from steady, non-topographic Sverdrup theory. Nearly all of the wind-driven inflow occurs north of Martinique at latitude B151N. The net transport through the Lesser Antilles passages south of 151N (Grenada, St. Vincent, and St. Lucia passages) is nearly zero when the model is forced by winds alone. The addition of a 14 Sv meridional cell in the model increases the net Caribbean inflow to 28 Sv; with nearly all of the additional 11 Sv of inflow entering through the southern Lesser Antilles passages. The modeled inflow distribution resulting from the combined wind and overturning forced experiment is found to compare favorably with the observations. The seasonal cycle of the total inflow in the combined forcing experiment has a mixed annual/semiannual character with maximum in spring and summer and minimum in fall, with a total range of about 4 Sv: The seasonal cycle of the Florida Current resulting from this inflow variation is in good qualitative agreement with observations. Most of the seasonal inflow variation occurs through the Windward Islands passages in the far southern Caribbean, whose annual cycle slightly leads that of the Florida and Yucatan Currents. Variability of the modeled inflow on shorter time scales shows a dramatic change in character moving northward along the Antilles arc. The southern passages exhibit large fluctuations on 30–80 day time scales, which decay to very small amplitudes north of Dominica. Much of this variability is caused by North Brazil Current Rings that propagate northwestward from the equatorial Atlantic and interact with the abrupt island arc topography. The total range of transport variability in individual passages predicted by the model is consistent with observations. However, observations are presently too limited to confirm the seasonal cycles or variability spectra in the Caribbean passages. r 2002 Elsevier Science Ltd. All rights reserved.

Coordinated control of an underwater glider fleet in an adaptive ocean sampling field experiment in Monterey Bay

A full-scale adaptive ocean sampling network was deployed throughout the month-long 2006 Adaptive Sam- pling and Prediction (ASAP) field experiment in Monterey Bay, California. One of the central goals of the field experiment was to test and demonstrate newly developed techniques for coordinated motion control of au- tonomous vehicles carrying environmental sensors to efficiently sample the ocean. We describe the field results for the heterogeneous fleet of autonomous underwater gliders that collected data continuously throughout the month-long experiment. Six of these gliders were coordinated autonomously for 24 days straight using feed- back laws that scale with the number of vehicles. These feedback laws were systematically computed using recently developed methodology to produce desired collective motion patterns, tuned to the spatial and tem- poral scales in the sampled fields for the purpose of reducing statistical uncertainty in field estimates. The implementation was designed to allow for adaptation of coordinated sampling patterns using human-in-the- loop decision making, guided by optimization and prediction tools. The results demonstrate an innovative tool for ocean sampling and provide a proof of concept for an important field robotics endeavor that integrates coordinated motion control with adaptive sampling. C

Control of coordinated patterns for ocean sampling

A class of underwater vehicles are modelled as Newtonian particles for navigation and control. We show a general method that controls cooperative Newtonian particles to generate patterns on closed smooth curves. These patterns are chosen for good sampling performance using mobile sensor networks. We measure the spacing between neighbouring particles by the relative curve phase along the curve. The distance between a particle and the desired curve is measured using an orbit function. The orbit value and the relative curve phase are then used as feedback to control motion of each particle. From an arbitrary initial configuration, the particles converge asymptotically to form an invariant pattern on the desired curves. We describe application of this method to control underwater gliders in a field experiment in Buzzards Bay, MA in March 2006.

Multi-AUV control and adaptive sampling in Monterey Bay

Multi-AUV operations have much to offer a variety of underwater applications. With sensors to measure the environment and coordination that is appropriate to critical spatial and temporal scales, the group can perform important tasks such as adaptive ocean sampling. We describe a methodology for cooperative control of multiple vehicles based on virtual bodies and artificial potentials (VBAP). This methodology allows for adaptable formation control and can be used for missions such as gradient climbing and feature tracking in an uncertain environment. We discuss our implementation on a fleet of autonomous underwater gliders and present results from sea trials in Monterey Bay in August 2003. These at-sea demonstrations were performed as part of the Autonomous Ocean Sampling Network (AOSN) II project.

North Brazil Current Ring Generation and Evolution Observed with SeaWiFS

The earth’s largest oceanic rings are formed by the retroflecting North Brazil Current (NBC) near 8 8N in the western tropical Atlantic. The NBC flows northward across the equator and past the mouth of the Amazon River entraining river-influenced shelf water along its nearshore edge. Enhanced phytoplankton production associated with the nutrient-rich Amazon discharge results in near-surface chlorophyll gradients that delineate the trajectory of the retroflecting NBC. These large-scale gradients, visible from space using Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color imagery, enable visualization of NBC rings during the initial phases of their evolution and northwestward translation. Observations of 18 NBC rings identified between September 1997 and September 2000 are summarized. Six rings formed each year. Although nearly circular at formation the rings quickly deformed as they translated at speeds near 15 cm s 21 toward the Caribbean Sea. Typical core radii of rings near 558W were 100 km and 150 km in the across- and alongshore dimensions, respectively. The contribution of each ring to intergyre mass transport (1.0 6 0.4 Sv) was estimated using SeaWiFS derived surface areas and an estimate of vertical penetration (600 m) based on in situ tracer observations. Several rings were observed (using satellite-tracked surface drifters in combination with SeaWiFS imagery) to violently collide with the Lesser Antilles. At least one ring maintained an organized circulation while passing directly over the island of Barbados.

Rings of the North Brazil Current: their structure and behavior inferred from observations and a numerical simulation

Large anticyclonic rings are shed from the retroflecting North Brazil Current (NBC) near 8°N in the tropical western Atlantic. New subsurface velocity and temperature measurements within three such rings are presented here and are found to be consistent with previous in situ and remotely sensed NBC ring measurements. A high-resolution numerical model of the Atlantic Ocean forced by monthly wind stress and an imposed meridional overturning cell is found to shed NBC rings that approximate those observed. The model rings are more surface-intensified than those observed and somewhat smaller in diameter. Both observed and modeled NBC rings move northwestward along the coast of South America with a speed of 8–16 cm/s, considerably slower than predicted by analytical theories describing westward ring propagation. At least 2–3 rings per year separate from the NBC retroflection. Annually, 1–3 rings translate intact from their formation region near 50°W to the islands of the southeastern Caribbean, where they disintegrate after a lifetime of about 100 days. The volume of fluid trapped within the core of an NBC ring and isolated from external mixing is estimated using potential vorticity as a tracer. The horizontal limits of the trapped core volume closely coincide with the radius of maximum swirl velocity, while the vertical limit of the core is typically less than the subsurface extent of significant swirl velocity. The core volume of a typical observed ring is 3.2±1.0×1013 m3. This corresponds to an annualized per-ring mass transport near 1 Sv (106 m3/s), similar to previous estimates. This study is the first to make use of subsurface temperature and velocity data to compute the volume of the anomalous ring core. NBC rings may be responsible for 3–4 Sv of direct mass transport across the equatorial-tropical gyre boundary or 20–25% of the total upper ocean cross-gyre transport required by the Atlantic meridional overturning cell. Translating NBC rings may contribute 20% of the total meridional heat transport by the ocean at this latitude.