Figure 1. The COOL room

Enabling discovery based science with Webb Gliders

Oscar Schofield, Josh Kohut, Hugh Roarty, Scott Glenn Coastal Ocean Observation Lab, Institute of Marine and Coastal Sciences,

School of Environment and Biological Sciences, Rutgers University New Brunswick, NJ, 08901, USA

Clayton Jones, Doug Webb

Webb Research Inc., 82 Technology Park E. Falmouth, MA, 02536 USA

Abstract- Buoyancy driven Slocum Gliders were a vision of

Douglas Webb, which Henry Stommel championed in a futuristic vision published in 1989. Slocum Gliders have transitioned from a concept to a technology serving basic research and environmental stewardship. The long duration and low operating costs of Gliders allow them to anchor spatial time series. Large distances, over 600 kilometers, can be covered using a single set of alkaline batteries. Since the initial tests, a wide range of physical and optical sensors have been integrated into the Glider allowing measurements of temperature, salinity, depth averaged currents, surface currents, fluorescence, apparent and inherent optical properties. The ability to operate Gliders for extended periods of time are making them the central in situ technology for the evolving ocean observatories. Off shore New Jersey Gliders have occupied a cross shelf transect and has documented the annual variability in shelf wide stratification on the Mid-Atlantic Bight and the role of storms in sediment resuspension. The sustained data permits scientists to gather regional data critical to addressing if, and how, the oceans are changing. One of next major regions we will use this technology is to study the climate induced impacts on the food webs along the West Antarctic Peninsula.

The motivation. It is critical to understand the interactions between humans and the oceans. While only 10% of the oceans are coastal, they contain an abundance of natural resources and by 2050, 75% of Earth’s human population will live within its coastal watersheds. For example, in our state of New Jersey, 60% of the population lives in coastal counties, and the annual economic impact of the coastal ocean is ~$80 billion. Coastal ecosystems span from watersheds to the deep sea and are extremely complex, hindering planning for sustainable development, energy policy, homeland security and emergency response. This lack of understanding and the ability to asses the present state of the coastal ocean is fueling governments around the world to build regional integrated coastal ocean observing networks. These systems are dual-use, built to support both basic research and the practical needs of society. These environmental networks are enabled by rapid advances in technology. We have been building an ocean network for over decade based on short two-three year competitively vetted science proposals. For this article we will highlight the evolution of the network, and then specifically highlight the development and success of the Webb Slocum gliders and future projects that will be enabled by their transformational capabilities. The Coastal Ocean Observation Lab (COOL) is a shore-based system that remotely controls the ocean observing assets deployed world-wide (Figure 1). It provides scientists direct control over remote assets collecting data in the field. The

observatory operates three key technologies that allow for fine scale characterization of the coastal ocean over wide areas, including the acquisition of data streams from U.S. and foreign satellites in space, a network of high-frequency radars deployed along the shore, and a fleet of robotic gliders flying beneath the ocean surface. Data is delivered in near real-time, analyzed, and posted to the world-wide-web (academic web

site: http://rucool.marine.rutgers.edu; education web site http://www.coolclassroom.org; public outreach web site http://www.thecoolroom.org/). The potential of the comprehensive system has been reviewed elsewhere [1-3] so this manuscript will focus exclusively on the capabilities provided by the Webb Gliders which allow scientists to maintain a sustained subsurface presence in the oceans. Rutgers Webb Glider Fleet. A number of different gliders have been developed and are being used by many organizations; however for this paper we will only discuss the field efforts conducted by Rutgers University (RU) and Webb Research Corporation (WRC). We emphasize that the successes of this group are matched by other groups at other institutions. Our “take home” message is that Gliders are a robust technology capable of anchoring large field campaigns. The Glider used by this group is the Webb Slocum Glider (Figure 2). The Slocum Glider is a 1.8 m long torpedo-shaped, winged AUV. It maneuvers through the ocean at a forward speed of 20-30 cm/s in a sawtooth-shaped gliding trajectory, deriving its forward propulsion by means of a buoyancy change and steering by means of a tail fin rudder. The

978-1-4244-2268-5/08/$25.00 ©2008 IEEE

Figure 2. A Webb glider at the surface.

Figure 3. Some of the selected deployments of the Webb glider around the world since Fall 2003.

altimeter and depth sensor enable preprogrammed sampling of the full water column. The primary vehicle navigation system uses an on-board GPS receiver coupled with an attitude sensor, depth sensor, and altimeter to provide dead-reckoned navigation, with backup positioning and communications provided by an Argos transmitter. Two-way communication with the vehicle is maintained by RF modem or the global satellite phone service Iridium. All antennas are carried within the tail fin that is raised out of the water when the vehicle is

commanded to surface at some predetermined interval. Operational endurance, utilizing alkaline batteries, is 25 to 50 days, depending on sensor payload and sampling regimes. Horizontal distance traveled averages 24 km per day. The vehicle is operational in 5 to 200 meters of water depth and can be optimized for 30, 100, and 200 meters operation with select gearboxes. Newly developed is the Slocum 1km (1000 meter depth rated) vehicle. Over the years we through competitive research grants have acquired 17 Webb gliders to dedicate to our science efforts. We have deployed Gliders over the last four years spanning waters offshore Australia, Europe, North America, and Antarctica (Figure 3). In all these deployments the command and control was always maintained from the COOL operations center in New Jersey. The Iridium communication also allows shore-side scientists to maneuver Gliders through treacherous costal conditions which include areas of fishing debris, ship traffic and moorings (Figure 4). During the last three years, our group has flown 129 missions which represent over 40,415 kilometers and 1932 days at sea. This effort represents a single University group, and given the Glider’s capabilities it represents a technology that can be cost effectively scaled up by larger institutions and/or federal agencies.

Use of the Glider. Rutgers initiated a long term time series to monitor the hydrography of the Mid-Atlantic Bight using Webb Gliders that

Figure 4. Glider deployments in May of 2006 offshore the coast of

New Jersey. The red lines indicate ship traffic in and out of New York Harbor. Red boxes indicate zones of fishing debris and old wrecks

which must be avoided. The black circles indicate moorings. The blue and black lines indicate the paths taken by two gliders. Green triangle

indicates the starting position of the Glider, and the red triangle indicates the final point of the glider mission when it was recovered. Glider progress was continually monitored by shore-side scientists in

the COOL room.

LongitudeLongitude

Figure 5. the complexity in the optical signatures in the coastal waters

offshore New Jersey.

Figure 6. Two Webb gliders sitting side-by-side on the deck of ship

before the leaving the docks. The glider in the back is a standard size while the one in the forefront is a new stretch glider. The longer body allows for extended missions as more batteries can be carried at sea.

transect from the nearshore electro-optic sea floor cabled observatory (LEO) to the edge of the continental shelf (~150 kilometers offshore, Figure 3) in October 2003. The interannual data shows the erosion of late autumn shelf stratification by storms and surface cooling. This autumn period is followed by a rapid cooling during the month of December. The winter months (November through April) are associated with a well-mixed water column with the MAB cold pool representing one of the warmer water masses on the shelf. Numerous storms keep the water column well-mixed in the winter and associated with these storms are dramatic benthic resuspension events (Figure 5). These resuspension events are some of the most prominent particle transport events on the shelf and would not be visible by satellites as they occur during cloudy stormy atmospheric conditions. The physics and sediment resuspension revealed by gliders is reviewed in a paper by Glenn et al (this volume). The spatial complexity observed in the optics illustrates the importance of these sensor packages; therefore a great deal of effort has recently focusing on increasing the range of optical sensors that are available for these efforts.

The value of the Glider surveys will increase as the sensors available for Webb Gliders expands. The main bottleneck for integrating sensors is minimizing their size and power consumption. Several physical (temperature, conductivity) and optical measurements (backscatter, chlorophyll a and colored dissolved organic fluorescence) are now standard measurements. These sensors are now complemented with a range of optical sensors which include inherent optical sensors (http://www.wetlabs.com), multispectral radiometers (http://www.satlantic.com/) and hyperspectral absorption meters [4 Kirkpatrick et al. 2001]. Taken together, backscatter and attenuation measurements provide information on the concentration and composition of the particles present. Measurements of absorption

provide information about phytoplankton taxa, detritus, and colored dissolved organic matter. These concentration measurements are

complemented with rate measurements provided by oxygen sensors. Additionally a kinetic chlorophyll a fluorometer is being miniaturized to allow the glider to make measurements of the photosynthetic efficiencies which provides information on the physiological status of phytoplankton. This is not an exhaustive list of the available sensors that have been flown on Webb Gliders by other research groups. Regardless, the large number of sensors and their associated power requirements is beyond the capacity of a single glider, and therefore fleets of gliders will need to be coordinated to provide a complete biogeochemical picture of the ocean. The key, given the continued advances in sensor technology, is expanding the Glider duration by increasing the power carried on the platforms.

Extended duration. We are pursuing to general approaches for extending the power capabilities. The first approach is expand the space in the gliders. We have accomplished by developing a long body Webb glider (Figure 6) which offers more space to both carry more batteries and carry larger sensors. These new extended bay

bodies have been flown and they have successfully demonstrated that they can extend glider duration significantly.

The second approach to improve glider duration is change to type of battery that these systems carry. These systems have traditionally carried alkaline batteries. Despite the lower power of these batteries ,

depth

depth

Figure 7. The path of the lithium powered glider. The top panel shows

the trajectory during its two month journey. The bottom panel shows the 3-D picture of temperature.

the big advantage is that they are not hazardous. This great improves safety and permits easy shipment of “powered” gliders anywhere in the world. Lithium batteries allow for longer duration and could greatly extend the duration of a glider at sea. Our group is currently conducting field tests of lithium batteries onboard Webb gliders.

The initial test is ongoing during the writing of this extended abstract. The initial test we are conducting is focused on flying a glider from New Jersey in the United States to academic partners in Halifax Canada. The glider was deployed off the New Jersey coast on March 8, 2008 and is currently offshore Canada (4/14/2008) and now it will begin to transect towards Halifax in the coming week (Figure 7). The glider on its journey 1) transected across the Mid-Atlantic Bight, 2) caught and then subsequently “surfed” the Gulf Stream, 3) exited the Gulf Stream and utilized several warm core rings to head to the North, and 4) finally it will conduct the cross-shore transect to just offshore Halifax..

Given the potential for gliders now to carry a wide range of sensors and have sufficient power to remain at sea, what are some of the new challenging scientific projects the Rutgers glider fleet will be applied?

Climate change and the polar oceans. The COOL team will begin to use the gliders in extreme environments which are experiencing the most dramatic changes to climate change in the coming decades. The

potential was demonstrated in January 2007 when a Webb glider transected along the Antarctic Peninsula. In the coming years the COOL team will join the Palmer Long term Ecological Research project which building off an 18 year time series to document climate induced changes in the Southern ocean. Specific motivations and glider plans are outlined below.

The Western Antarctic Peninsula (WAP) is undergoing the most dramatic climate change on Earth (Figure 8). The WAP region has experienced a significant winter warming during the past half century (5.4 times the global average). This warming has shortened the sea ice season and perennial sea ice is gone (Figure 8). The maritime system of the northern WAP is expanding southward, displacing the continental, polar system of the southern WAP while 87% of the glaciers are in retreat. Associated with this warming has been a climate induced migration in the WAP ecosystem spanning the primary, secondary, and higher trophic levels. We do not understand the drivers of the temperature changes or the resulting ecosystem changes, but we know the decline of ice is driving one of the largest and most rapid climate induced shifts in a marine ecosystem on Earth. For example, the Chinstrap (Pygoscelis antarctica) and Gentoo penguins (P. papua) have invaded the region displacing the long-resident Adélie penguins (P. adeliae).

Despite dramatic changes, our understanding remains woefully inadequate as the Southern ocean is a dangerous and hostile environment for humans. The National Science Foundation (NSF) supports traditional ship sampling; however traditional sampling provides data from only a limited time at sea. The resulting data gaps limit our ability to resolve climate forcing of ocean food webs. A holistic understanding requires a new capability to maintain a continued presence in the Southern ocean. We believe a fleet of coastal gliders operating a coordinated swarm could overcome much of the chronic under-sampling of the WAP and provide an integrated set of measurements expanding understanding of how the climate change alters polar ecosystems.

As one major limitation, despite the improvements, is the available to power onboard instruments comes at the expense of the glider’s mission duration, a single glider cannot carry all the sensors that would be required to characterize ecosystem structure and dynamics. Therefore a fleet of gliders carrying a distributed suite of sensors is required. For this project we will construct the glider fleet capable of carrying distributed sensor suite that will characterize the physical structure (temperature, salinity, currents), phytoplankton (concentrations and health), and higher trophic levels (zooplankton, whales). These three gliders will need to fly using swarming algorithms that will allow the distributed data to be combined. All the systems will carry CTDs allowing standardizations between the systems. The proposed sensors and gliders that will be diedicated to this effort in the near future include:

Glider 1: 1000 meter profile & physical oceanography focus: This glider will be dedicated to mapping the hydrography and particle loads in the WAP from the surface to 1000 meters. The system will be outfitted to measure temperature, salinity, spectral backscatter, and chlorophyll fluorescence. The temperature and salinity data will allow the major water masses to be mapped given the 1000 m depth coverage. These

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Figure 8. A) The regional warming of the West Antarctic Peninsula measured at (black line,1944-2005) the former British Faraday Station (65°12’S,

64°16’W; became the Ukraine Vernadsky Base in 1997) and at Palmer Station (red) indicating a mean winter warming rate of ~1°C per decade. B) Changes in perennial ice in the WAP. C) Decadal changes in ice-dependent (Adelie penguins, Weddell Seals) and ice avoiding species (Chinstrap &

Gentoo penguins, Fur & Elephant seals) at Palmer Station. D) The 20 and 5 year changes in phytoplankton productivity along the WAP measured with the CZCS and SeaWIFs satellites

deep profiles will be complemented by 0-200 meter profiles collected by the other two gliders focused on mapping the biology in the upper euphotic zone where most of the biological activity occurs.

Glider 2: 200 meter profile & phytoplankton physiology focus: A second glider will provide phytoplankton physiology and biomass measurements. Over the last decade, the pump-and-

probe and Fast Repetition Rate (FRRF) Fluorometers have provided unprecedented insight into the factors controlling phytoplankton physiology and primary production in the ocean. The technique relies on active stimulation and highly resolved detection of the induction and subsequent relaxation of chlorophyll fluorescence yields on micro- and millisecond time scales. Analysis of the induced variable fluorescence provides

a comprehensive suite of fluorescent and photosynthetic parameters of the phytoplankton. From these, a detailed picture of photosynthetic status can be deduced. The measured parameters provide the basis to model gross primary productivity and assess the impact of environmental stresses. The principles of variable fluorescence measurements and instrumentation are well established in peer-reviewed literature. The first glider variable fluorometer is being developed through a National Ocean Partnership Program. The first sensor has recently been completed by Satlantic Inc. and it will deployed this summer. A new Fluorescence Induction and Relaxation (FIRe) System will be constructed for this project hardened for heavy ice conditions and dedicated to mapping phytoplankton biomass and physiology.

Glider 3: 200 meter profile & higher trophic level acoustics. This glider will be outfitted with an Acoustic Doppler Current Profiler which will provide estimates of zooplankton (here krill and salps). The system will also provide vertically resolved measurements of the upper ocean currents to complement the depth-averaged and surface currents provided by a standard Webb Gliders. The development and integration of the ADCP is being leveraged with funds provided by the Office of Naval Research. This active acoustic system will be complemented with a passive acoustic system. The active acoustics will be calibrated against net tows and population counts conducted during the annual LTER cruise. Efforts will focus on defining the acoustic cross-section and animal number. The passive acoustic system will be used to record fish and mammal vocalizations. These vocalizations will be correlated in both space and time with the concentrations of zooplankton and the high phytoplankton biomass.

In conclusion, the ocean ecosystems are changing exponentially, and developing the capacity to understand these changes and their implications for humanity is a clarion call for oceanographers. Traditional sampling modes are insufficient and we must develop systems capable of operating for sustained periods at sea and in harsh environments. The proposed modular, robotic network would be applicable to any coastal shelf, thus providing a significant technological advance needed by the scientific community. The proposed use of robots will provide a revolutionary tool to study the dynamic interactions between atmospheric forcing, physical mixing, phytoplankton concentration/metabolism, and higher trophic levels. Web-enabled adaptive sampling will allow the robotic network to adjust on the fly as the ecosystem evolves in response to the physical forcing of the ocean.

ACKNOWLEDGMENT

We thank our sponsors from the Office of Naval Research, National Science Foundation, and the National Partnership Program. We also thank the stupendous efforts of the technicians, students and post-doctoral researchers who make the COOL room one of the most stimulating and exciting places to work every day.

REFERENCES

[1] O. Schofield, T. Bergmann, W. P. Bissett, F. Grassle, D. Haidvogel, J. Kohut, M. Moline, and S. Glenn “ Linking regional coastal observatories to provide the foundation for a national ocean observation network.” J. Ocean. Eng. Vol 27, pp. 146-154, 2002.

[2] S. Glenn and O. Schofield “Observing the oceans from the COOLroom: Our history, experience, and opinions.” Oceanogr. vol 16, pp 37-52, 2003.

[3] O. Schofield, J. Kohut, amd S. Glenn “Coastal observing networks and dawn in a new millennium of well sampled oceans” Sea Tech. vol 49, pp. 31-36, 2008.

[4] G. J. Kirkpatrick, C. Orrico, M. Moline, M. Oliver, and O. Schofield. “Continuous hyperspectral absorption measurements of colored dissolved organic material in aquatic systems” App. Opt. vol 42, pp. 6564-6568, 2003.