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2017
Rudnick, DL, Zaba KD, Todd RE, Davis RE.  2017.  A climatology of the California Current System from a network of underwater gliders. Progress in Oceanography. 154:64-106.   10.1016/j.pocean.2017.03.002   AbstractWebsite

Autonomous underwater gliders offer the possibility of sustained observation of the coastal ocean. Since 2006 Spray underwater gliders in the California Underwater Glider Network (CUGN) have surveyed along California Cooperative Oceanic Fisheries Investigations (CalCOFI) lines 66.7, 80.0, and 90.0, constituting the world's longest sustained glider network, to our knowledge. In this network, gliders dive between the surface and 500 m, completing a cycle in 3 h and covering 3 km in that time. Sections extend 350500 km offshore and take 2-3 weeks to occupy. Measured variables include pressure, temperature, salinity, and depth-average velocity. The CUGN has amassed over 10,000 glider-days, covering over 210,000 km with over 95,000 dives. These data are used to produce a climatology whose products are for each variable a mean field, an annual cycle, and the anomaly from the annual cycle. The analysis includes a weighted least-squares fit to derive the mean and annual cycle, and an objective map to produce the anomaly. The final results are variables on rectangular grids in depth, distance offshore, and time. The mean fields are finely resolved sections across the main flows in the California Current System, including the poleward California Undercurrent and the equatorward California Current. The annual cycle shows a phase change from the surface to the thermocline, reflecting the effects of air/sea fluxes at the surface and upwelling in the thermocline. The interannual anomalies are examined with an emphasis on climate events of the last ten years including the 2009-2010 El Nino, the 2010-2011 La Nina, the warm anomaly of 2014-2015, and the 2015-2016 El Nino. (C) 2017 Elsevier Ltd. All rights reserved.

Davis, RE, Sherman JT.  2017.  Evaluating a lithium-seawater battery on gliders. Journal of Atmospheric and Oceanic Technology. 34:1175-1182.   10.1175/jtech-d-16-0151.1   AbstractWebsite

Neutral-buoyancy vehicles demand high-density energy sources and lithium is light with high oxidation energy. PolyPlus Battery Company has developed a prototype lithium-seawater battery that is attractive for powering long-duration autonomous oceanographic vehicles (floats and underwater gliders). These batteries were tested in the laboratory and at sea. PolyPlus batteries use "Protected Lithium Electrodes'' with proprietary "windows'' protecting the volatile lithium anode from water while passing lithium ions. The cathode reduces oxygen dissolved in seawater, or hydrolyzes seawater to produce hydrogen. Not requiring additional electrolyte, fuel, or pressure cases, these cells have impressive weight advantages. Good electrode-seawater mass transfer is required but can increase drag and be impeded by biofouling. Tests assessing robustness of the PolyPlus batteries in oceanographic use, evaluating mass transfer issues, and observing biofouling impacts are reported. In sea trials, two cells were tested for 69 days mounted on a Spray glider. Findings are as follows: 1) the cells were robust over 900 dives, most to 400 m; 2) without antifouling measures, the cells became substantially biofouled, but their performance was undiminished; and 3) performance was complex, depending on current density, oxygen concentration, and flow conditions. For dissolved oxygen concentration above 1mL L-1, the cells delivered 9Wm(-2) of electrode surface at 3V. For low oxygen, the cell shifted to hydrolysis near 2.3 V, but mass transfer was less critical so current density could be increased and observed power reached 5Wm(-2). This could be increased using a lower resistance load.

2016
Rudnick, DL, Davis RE, Sherman JT.  2016.  Spray underwater glider operations. Journal of Atmospheric and Oceanic Technology. 33:1113-1122.   10.1175/jtech-d-15-0252.1   AbstractWebsite

Operational statistics for the Spray underwater glider are presented to demonstrate capabilities for sustained observations. An underwater glider is an autonomous device that profiles vertically by changing buoyancy and flies horizontally on wings. The focus has been on sustained observations of boundary currents to take advantage of the glider's small size, which allows it to be deployed and recovered from small vessels close to land, and the fine horizontal resolution delivered by the glider, which is scientifically desirable in boundary regions. Since 2004, Spray underwater gliders have been deployed for over 28 000 days, traveling over 560 000 km, and delivering over 190 000 profiles. More than 10 gliders, on average, have been in the water since 2012. Statistics are given in the form of histograms for 297 completed glider missions of longer than 5 days. The statistics include mission duration, number of dives, distance over ground, and horizontal and vertical distance through water. A discussion of problems, losses, and short missions includes a survival analysis. The most extensive work was conducted in the California Current system, where observations on three across-shorelines have been sustained, with 97% coverage since 2009. While the authors have certain advantages as developers and builders of the Spray underwater glider and Spray may have design and construction advantages, they believe these statistics are a sound basis for optimism about the widespread future of gliders in oceanographic observing.

2014
Ganachaud, A, Cravatte S, Melet A, Schiller A, Holbrook NJ, Sloyan BM, Widlansky MJ, Bowen M, Verron J, Wiles P, Ridgway K, Sutton P, Sprintall J, Steinberg C, Brassington G, Cai W, Davis R, Gasparin F, Gourdeau L, Hasegawa T, Kessler W, Maes C, Takahashi K, Richards KJ, Send U.  2014.  The Southwest Pacific Ocean circulation and climate experiment (SPICE). Journal of Geophysical Research-Oceans. 119:7660-7686.   10.1002/2013jc009678   AbstractWebsite

The Southwest Pacific Ocean Circulation and Climate Experiment (SPICE) is an international research program under the auspices of CLIVAR. The key objectives are to understand the Southwest Pacific Ocean circulation and the South Pacific Convergence Zone (SPCZ) dynamics, as well as their influence on regional and basin-scale climate patterns. South Pacific thermocline waters are transported in the westward flowing South Equatorial Current (SEC) toward Australia and Papua-New Guinea. On its way, the SEC encounters the numerous islands and straits of the Southwest Pacific and forms boundary currents and jets that eventually redistribute water to the equator and high latitudes. The transit in the Coral, Solomon, and Tasman Seas is of great importance to the climate system because changes in either the temperature or the amount of water arriving at the equator have the capability to modulate the El Nino-Southern Oscillation, while the southward transports influence the climate and biodiversity in the Tasman Sea. After 7 years of substantial in situ oceanic observational and modeling efforts, our understanding of the region has much improved. We have a refined description of the SPCZ behavior, boundary currents, pathways, and water mass transformation, including the previously undocumented Solomon Sea. The transports are large and vary substantially in a counter-intuitive way, with asymmetries and gating effects that depend on time scales. This paper provides a review of recent advancements and discusses our current knowledge gaps and important emerging research directions. Key Points Southwest Pacific WBCs transport large volumes toward the equator and the pole Pathways are complex; water properties tend to erode during the transit Variations due to seasons, ENSO and the SPCZ modulate the relative WBC strengths

2013
Ohman, MD, Rudnick DL, Chekalyuk A, Davis RE, Feely RA, Kahru M, Kim HJ, Landry MR, Martz TR, Sabine CL, Send U.  2013.  Autonomous ocean measurements in the California Current ecosystem. Oceanography. 26:18-25. AbstractWebsite

Event-scale phenomena, of limited temporal duration or restricted spatial extent, often play a disproportionately large role in ecological processes occurring in the ocean water column. Nutrient and gas fluxes, upwelling and downwelling, transport of biogeochemically important elements, predator-prey interactions, and other processes may be markedly influenced by such events, which are inadequately resolved from infrequent ship surveys. The advent of autonomous instrumentation, including underwater gliders, profiling floats, surface drifters, enhanced moorings, coastal high-frequency radars, and satellite remote sensing, now provides the capability to resolve such phenomena and assess their role in structuring pelagic ecosystems. These methods are especially valuable when integrated together, and with shipboard calibration measurements and experimental programs.

2012
Davis, RE, Kessler WS, Sherman JT.  2012.  Gliders Measure Western Boundary Current Transport from the South Pacific to the Equator. Journal of Physical Oceanography. 42:2001-2013.   10.1175/jpo-d-12-022.1   AbstractWebsite

"Spray" gliders, most launched from small boats near shore, have established a sustainable time series of equatorward transport through the Solomon Sea. The first 3.5 years (mid-2007 through 2010) are analyzed. Coast-to-coast equatorward transport through the Solomon Sea fluctuates around a value of 15 Sv (1 Sv equivalent to 10(6) m(3) s(-1)) with variations approaching +/- 15 Sv. Transport variability is well correlated with El Nino indices like Nino-3.4, with strong equatorward flow during one El Nino and weak flow during two La Ninas. Mean transport is centered in an undercurrent focused in the western boundary current; variability has a two-layer structure with layers separated near 250 m (near the core of the undercurrent) that fluctuate independently. The largest variations are in midbasin, confined to the upper layer, and are well correlated with ENSO. Analysis of velocity and salinity on isopycnals shows that the western boundary current within the Solomon Sea consists of a deep core coming from the Coral Sea and a shallow core that enters the Solomon Sea in mid basin. Analysis of the structure of transport and its fluctuations is presented.

Todd, RE, Rudnick DL, Mazloff MR, Cornuelle BD, Davis RE.  2012.  Thermohaline structure in the California Current System: Observations and modeling of spice variance. Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007589   AbstractWebsite

Upper ocean thermohaline structure in the California Current System is investigated using sustained observations from autonomous underwater gliders and a numerical state estimate. Both observations and the state estimate show layers distinguished by the temperature and salinity variability along isopycnals (i.e., spice variance). Mesoscale and submesoscale spice variance is largest in the remnant mixed layer, decreases to a minimum below the pycnocline near 26.3 kg m(-3), and then increases again near 26.6 kg m(-3). Layers of high (low) meso-and submesoscale spice variance are found on isopycnals where large-scale spice gradients are large (small), consistent with stirring of large-scale gradients to produce smaller scale thermohaline structure. Passive tracer adjoint calculations in the state estimate are used to investigate possible mechanisms for the formation of the layers of spice variance. Layers of high spice variance are found to have distinct origins and to be associated with named water masses; high spice variance water in the remnant mixed layer has northerly origin and is identified as Pacific Subarctic water, while the water in the deeper high spice variance layer has southerly origin and is identified as Equatorial Pacific water. The layer of low spice variance near 26.3 kg m(-3) lies between the named water masses and does not have a clear origin. Both effective horizontal diffusivity, kappa(h), and effective diapycnal diffusivity, kappa(v), are elevated relative to the diffusion coefficients set in the numerical simulation, but changes in kappa(h) and kappa(v) with depth are not sufficient to explain the observed layering of thermohaline structure.

2011
Sherman, AD, Hobson BW, McGill PR, Davis RE, McClune MC, Smith KL.  2011.  Lagrangian sediment traps for sampling at discrete depths beneath free-drifting icebergs. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:1327-1335.   10.1016/j.dsr2.2010.11.008   AbstractWebsite

The recent proliferation of icebergs in the Southern Ocean and the chemical and biological enrichment recently identified in the surrounding water raise the question of whether these icebergs are major contributors to drawdown of CO2 and the export of organic carbon to the deep ocean. The Lagrangian sediment trap (LST) was developed to measure the carbon export associated with these free-drifting icebergs. The core of the LST is a neutrally buoyant Sounding Oceanographic Lagrangian Observer (SOLO) float (Davis et al., 2001) that contains a variable-buoyancy engine, enabling it to sink to a set depth (600 m for the iceberg study), drift at that depth, then resurface on preset time intervals. Four sediment-trap funnels and opening/closing sample cups mounted around each SOLO float collect sinking particulate matter. Additionally, an upward-looking acoustic system mounted on the float detects the presence of ice cover above. In March/April 2009, three LSTs were deployed in the NW Weddell Sea for a total of five successful missions. Four of these LST deployments were made near or under a large iceberg. The fifth deployment was made at a control site 74 km from the nearest large iceberg. Sinking particulate matter was collected on each deployment. Despite the high-risk nature of the deployments, the LSTs successfully sampled particulate matter beneath drifting icebergs. (C) 2010 Elsevier Ltd. All rights reserved.

Dohan, K, Davis RE.  2011.  Mixing in the Transition Layer during Two Storm Events. Journal of Physical Oceanography. 41:42-66.   10.1175/2010jpo4253.1   AbstractWebsite

Upper-ocean dynamics analyzed from mooring-array observations are contrasted between two storms of comparable magnitude. Particular emphasis is put on the role of the transition layer, the strongly stratified layer between the well-mixed upper layer, and the deeper more weakly stratified region. The midlatitude autumn storms occurred within 20 days of each other and were measured at five moorings. In the first storm, the mixed layer follows a classical slab-layer response, with a steady deepening during the course of the storm and little mixing of the thermocline beneath. In the second storm, rather than deepening, the mixed layer shoals while intense near-inertial waves are resonantly excited within the mixed layer. These create a large shear throughout the transition layer, generating turbulence that broadens the transition layer. Details of the space time structure of the frequencies in both short waves and near-inertial waves are presented. Small-scale waves are excited within the transition layer. Their frequencies change with time and there are no clear peaks at harmonics of inertial or tidal frequencies. Wavelet transforms of the inertial oscillations show the evolution as a spreading in frequency, a deepening of the core into the transition layer, and a shift off the inertial frequency. A second near-inertial energy core appears below the transition layer at all moorings coincident with a rapid decay of mixed layer currents. An overall result is that direct wind-generated motions extend to the depth of the transition layer. The transition layer is a location of enhanced wave activity and enhanced shear-driven mixing.

Todd, RE, Rudnick DL, Mazloff MR, Davis RE, Cornuelle BD.  2011.  Poleward flows in the southern California Current System: Glider observations and numerical simulation. Journal of Geophysical Research-Oceans. 116   10.1029/2010jc006536   AbstractWebsite

Three years of continuous Spray glider observations in the southern California Current System (CCS) are combined with a numerical simulation to describe the mean and variability of poleward flows in the southern CCS. Gliders provide upper ocean observations with good across-shore and temporal resolution along two across-shore survey lines while the numerical simulation provides a dynamically consistent estimate of the ocean state. Persistent poleward flows are observed in three areas: within 100 km of the coast at Point Conception, within the Southern California Bight (SCB), and offshore of the SCB and the Santa Rosa Ridge (SRR). Poleward transport by the flows within the SCB and offshore of the SRR exceeds the poleward transport off Point Conception, suggesting that the poleward flows are not continuous over the 225 km between observation lines. The numerical simulation shows offshore transport between the survey lines that is consistent with some of the poleward flow turning offshore before reaching Point Conception. The poleward current offshore of the SRR is unique in that it is strongest at depths greater than 350 m and it is observed to migrate westward away from the coast. This westward propagation is tied to westward propagating density anomalies originating in the SCB during the spring-summer upwelling season when wind stress curl is most strongly positive. The across-shore wave number, frequency, and phase speed of the westward propagation and the lack of across-shore transport of salinity along isopycnals are consistent with first-mode baroclinic Rossby dynamics.

Todd, RE, Rudnick DL, Davis RE, Ohman MD.  2011.  Underwater gliders reveal rapid arrival of El Nino effects off California's coast. Geophysical Research Letters. 38   10.1029/2010gl046376   AbstractWebsite

The 2009-2010 El Nino marked the first occurrence of this climate phenomenon since the initiation of sustained autonomous glider surveillance in the California Current System (CCS). Spray glider observations reveal the subsurface effects of El Nino in the CCS with spatial and temporal resolutions that could not have been obtained practically with any other observational method. Glider observations show that upper ocean waters in the CCS were unusually warm and isopycnals were abnormally deep during the El Nino event, but indicate no anomalous water masses in the region. Observed oceanic anomalies in the CCS are nearly in phase with an equatorial El Nino index and local anomalies of atmospheric forcing. These observations point toward an atmospheric teleconnection as an important mechanism for the 2009-2010 El Nino's remote effect on the mid-latitude CCS. Citation: Todd, R. E., D. L. Rudnick, R. E. Davis, and M. D. Ohman (2011), Underwater gliders reveal rapid arrival of El Nino effects off California's coast, Geophys. Res. Lett., 38, L03609, doi:10.1029/2010GL046376.

2010
Leonard, NE, Paley DA, Davis RE, Fratantoni DM, Lekien F, Zhang FM.  2010.  Coordinated Control of an Underwater Glider Fleet in an Adaptive Ocean Sampling Field Experiment in Monterey Bay. Journal of Field Robotics. 27:718-740.   10.1002/rob.20366   AbstractWebsite

A full-scale adaptive ocean sampling network was deployed throughout the month-long 2006 Adaptive Sampling 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 autonomous 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 feedback 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 temporal 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) 2010 Wiley Periodicals, Inc.

Davis, RE.  2010.  On the coastal-upwelling overturning cell. Journal of Marine Research. 68:369-385.   10.1357/002224010794657173   AbstractWebsite

The overturning cell driven by alongshore wind and coastal upwelling off central California is examined using direct observations from four underwater gliders equipped with Acoustic Doppler Profilers. These show the onshore flow that compensates for offshore Ekman transport during the month of August 2006 to occur primarily just below the Ekman Layer so that the net offshore heat transport, and by inference the net vertical transport of nutrients, is much less than if this compensating flow occurred throughout the water column. Correlation of cross-shelf flow and wind stress is used to infer the profile of Ekman transport while glider-measured alongshore gradients of density determine the geostrophic cross-shelf flow, which is strongly surface-intensified. The sum of Ekman and geostrophic flows approximates the independently and directly measured cross-shelf flow profile. This underscores the importance of the alongshore pressure gradient in the overturning circulation and makes understanding it a high priority.

Testor, P, Meyers G, Pattiaratchi C, Bachmayer R, Hayes D, Pouliquen S, Petit de la Villeon L, Carval T, Ganachaud A, Gourdeau L, Mortier L, Claustre H, Taillandier V, Lherminier P, Terre T, Visbeck M, Karstensen J, Krahmann G, Alvarez A, Rixen M, Poulain P-M, Osterhus S, Tintore J, Ruiz S, Garau B, Smeed D, Griffiths G, Merckelbach L, Sherwin T, Schmid C, Barth JA, Schofield O, Glenn S, Kohut J, Perry MJ, Eriksen C, Send U, Davis R, Rudnick D, Sherman J, Jones C, Webb D, Lee C, Owens B.  2010.  Gliders as a component of future observing systems. Proceedings of OceanObs'09: Sustained Ocean Observations and Information for Society (Vol. 2), . ( Hall J, Harrison DE, Stammer D, Eds.)., Venice, Italy: ESA Publication WPP-306   10.5270/OceanObs09.cwp.89   Abstract

The aim of this community white paper is to make recommendations for a glider component of a global ocean observing system. We first recommend the adoption of an ARGO-like (Global array of free-drifting profiling floats) data system for gliders. Then, we argue that combining glider deployments with the other components (ships, moorings, floats and satellites) will considerably enhance our capacity for observing the ocean by filling gaps left by the other observing systems. Gliders could be deployed to sample most of the western and eastern boundary circulations and the regional seas (around 20 basins in the world) which are not well covered by the present global ocean observing system and near fixed-point time series stations. These plans already involve people scattered around the world in Australia, Canada, Cyprus, France, Germany, Italy, Norway, Spain, UK, and the USA, and will certainly expand to many other countries. A rough estimate of resources required is about 13M$/Euro for ~20+ gliders permanently at sea during five years in the world ocean, based on present scientific infrastructures.

Send, U, Davis R, Fischer J, Imawaki S, Kessler W, Meinen C, Owens B, Roemmich D, Rossby T, Rudnick D, Toole J, Wijffels S, Beal L.  2010.  A global boundary current circulation observing network. Proceedings of OceanObs'09: Sustained Ocean Observations and Information for Society (Vol. 2), . ( Hall J, Harrison DE, Stammer D, Eds.)., Venice, Italy: ESA Publication WPP-306   10.5270/OceanObs09.cwp.78   Abstract

Western and eastern boundary currents are key regions for understanding and monitoring the ocean's influence on and response to climate change processes. Yet the present global ocean observing is poorly suited for capturing the small scales, intense currents, often large vertical extent, the eddy-rich conditions, and (for eastern boundary currents) the biogeochemical and ecosystem variables needed. This paper reviews available technologies and methods, none of which can satisfy all requirements for the needed observing system. Therefore, merged hybrid approaches are proposed, which need to be evaluated in each case since conditions vary strongly. A global network is presented as a vision, of which 50% has at least partial implementations already.

2009
Simons, FJ, Nolet G, Georgief P, Babcock JM, Regier LA, Davis RE.  2009.  On the potential of recording earthquakes for global seismic tomography by low-cost autonomous instruments in the oceans. Journal of Geophysical Research-Solid Earth. 114   10.1029/2008jb006088   AbstractWebsite

We describe the development and testing of an autonomous device designed to revolutionize Earth structure determination via global seismic tomography by detecting earthquakes at teleseismic distances in the oceans. One prototype MERMAID, short for Mobile Earthquake Recording in Marine Areas by Independent Divers, was constructed and tested at sea. The instrument combines two readily available, relatively low-cost but state-of-the-art components: a Sounding Oceanographic Lagrangian Observer, or SOLO float, and an off-the-shelf hydrophone, with custom-built data logging hardware. We report on the development of efficient wavelet-based algorithms for the detection and discrimination of seismic events and analyze three time series of acoustic pressure collected at a depth of 700 m in pilot experiments conducted offshore San Diego, CA. In these tests, over 120 hours of data were gathered, and five earthquakes, of which one was teleseismic, were recorded and identified. Quantitative estimates based on these results suggest that instruments of the MERMAID type may collect up to a hundred tomographically useful teleseismic events per year. The final design will also incorporate a Global Positioning System receiver, onboard signal processing software optimized for low-power chips, and high-throughput satellite communication equipment for telemetered data transfer. With these improvements, we hope to realize our vision of a global array of autonomous floating sensors for whole-earth seismic tomography.

Roemmich, D, Johnson GC, Riser S, Davis R, Gilson J, Owens WB, Garzoli SL, Schmid C, Ignaszewski M.  2009.  The Argo Program Observing the Global Ocean with Profiling Floats. Oceanography. 22:34-43.   10.5670/oceanog.2009.36   AbstractWebsite

The Argo Program has created the first global array for observing the subsurface ocean. Argo arose from a compelling scientific need for climate-relevant ocean data; it was made possible by technology development and implemented through international collaboration. The float program and its data management system began with regional arrays in 1999, scaled up to global deployments by 2004, and achieved its target of 3000 active instruments in 2007. US Argo, supported by the National Oceanic and Atmospheric Administration and the Navy through the National Oceanographic Partnership Program, provides half of the floats in the international array, plus leadership in float technology, data management, data quality control, international coordination, and outreach. All Argo data are freely available without restriction, in real time and in research-quality forms. Uses of Argo data range from oceanographic research, climate research, and education, to operational applications in ocean data assimilation and seasonal-to-decadal prediction. Argo's value grows as its data accumulate and their applications are better understood. Continuing advances in profiling float and sensor technologies open many exciting possibilities for Argo's future, including expanding sampling into high latitudes and the deep ocean, improving near-surface sampling, and adding biogeochemical parameters.

Todd, RE, Rudnick DL, Davis RE.  2009.  Monitoring the greater San Pedro Bay region using autonomous underwater gliders during fall of 2006. Journal of Geophysical Research-Oceans. 114   10.1029/2008jc005086   AbstractWebsite

Glider surveys of the greater San Pedro Bay region in the Southern California Bight during the fall of 2006 demonstrated the utility of autonomous underwater gliders in a coastal region with complex flow and significant anthropogenic inputs. Three Spray gliders repeatedly surveyed between Santa Catalina Island and the coast of Southern California collecting profiles of temperature, salinity, and chlorophyll fluorescence and estimates of vertically averaged currents. These observations provided context for shelf transport studies during the Huntington Beach 2006 experiment and showed the transition from summer to winter conditions. Vertically averaged currents were predominantly poleward following topography with horizontal scales of approximately 20 km. The gliders surveyed a small cyclonic eddy near Santa Catalina Island and provided a unique view of the structure of the eddy. Nitrate concentration within the euphotic zone was estimated to be 19% greater within the eddy and led to significantly elevated chlorophyll concentrations at the subsurface maximum. Glider observations of salinity reliably detected the distinctly fresh signature of the effluent plume from an ocean outfall near Huntington Beach, California. The salinity anomaly caused by the plume was used to track the spread of the plume as it was advected poleward and away from the coast while remaining subsurface.

Chao, Y, Li ZJ, Farrara J, McWilliams JC, Bellingham J, Capet X, Chavez F, Choi JK, Davis R, Doyle J, Fratantoni DM, Li P, Marchesiello P, Moline MA, Paduan J, Ramp S.  2009.  Development, implementation and evaluation of a data-assimilative ocean forecasting system off the central California coast. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 56:100-126.   10.1016/j.dsr2.2008.08.011   AbstractWebsite

The development and implementation of a real-time ocean forecast system based on the Regional Ocean Modeling System (ROMS) off the coast of central California are described. The ROMS configuration consists of three nested modeling domains with increasing spatial resolutions: the US West coastal ocean at 15-km resolution, the central California coastal ocean at 5 km, and the Monterey Bay region at 1.5 km. All three nested models have 32 vertical sigma (or terrain-following) layers and were integrated in conj. unction with a three-dimensional variational data assimilation algorithm (3DVAR) to produce snapshots of the ocean state every 6 h (the reanalysis) and 48-h forecasts once a day. This ROMS forecast system was operated in real time during the field experiment known as the Autonomous Ocean Sampling Network (AOSN-II) in August 2003. After the field experiment, a number of improvements were made to the ROMS forecast system: more data were added in the reanalysis with more careful quality control procedures, improvements were made in the data assimilation scheme, as well as model surface and side boundary conditions. The results from the ROMS reanalysis are presented here. The ROMS reanalysis is first compared with the assimilated data as a consistency check. An evaluation of the ROMS reanalysis against the independent measurements that are not assimilated into the model is then presented. This evaluation shows the mean differences in temperature and salinity between reanalysis and observations to be less than 1 degrees C and 0.2 psu (practical salinity unit), respectively, with root-mean-square (RMS) differences of less than 1.5 degrees C and 0.25 psu. Qualitative agreement is found between independent current measurements and the ROMS reanalysis. The agreement is particularly good for the vertically integrated current along the offshore glider tracks: the ROMS reanalysis can realistically reproduce the poleward California Undercurrent. Reasonably good agreement is found in the spatial patterns of the surface current as measured by high-frequency (HF) radars. Preliminary results concerning the ROMS forecast skill and predictability are also presented. Future plans to improve the ROMS forecast system with a particular focus on assimilation of HF radar current measurements are discussed. (C) 2008 Elsevier Ltd. All rights reserved.

Shulman, I, Rowley C, Anderson S, DeRada S, Kindle J, Martin P, Doyle J, Cummings J, Ramp S, Chavez F, Fratantoni D, Davis R.  2009.  Impact of glider data assimilation on the Monterey Bay model. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 56:188-198.   10.1016/j.dsr2.2008.08.003   AbstractWebsite

Glider observations were essential components of the observational program in the Autonomous Ocean Sampling Network (AOSN-II) experiment in the Monterey Bay area during summer of 2003. This paper is focused on the impact of the assimilation of glider temperature and salinity observations on the Navy Coastal Ocean Model (NCOM) predictions of surface and subsurface properties. The modeling system consists of an implementation of the NCOM model using a curvilinear, orthogonal grid with 1-4 km resolution, with finest resolution around the bay. The model receives open boundary conditions from a regional (9 km resolution) NCOM implementation for the California Current System, and surface fluxes from the Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) atmospheric model at 3 km resolution. The data assimilation component of the system is a version of the Navy Coupled Ocean Data Assimilation (NCODA) system, which is used for assimilation of the glider data into the NCOM model of the Monterey Bay area. The NCODA is a fully 3D multivariate optimum interpolation system that produces simultaneous analyses of temperature, salinity, geopotential, and vector velocity. Assimilation of glider data improves the surface temperature at the mooring locations for the NCOM model hindcast and nowcasts, and for the short-range (1-1.5 days) forecasts. It is shown that it is critical to have accurate atmospheric forcing for more extended forecasts. Assimilation of glider data provided better agreement with independent observations (for example, with aircraft measured SSTs) of the model-predicted and observed spatial distributions of surface temperature and salinity. Mooring observations of subsurface temperature and salinity show sharp changes in the thermocline and halocline depths during transitions from upwelling to relaxation and vice versa. The non-assimilative run also shows these transitions in subsurface temperature; but they are not as well defined. For salinity, the non-assimilative run significantly differs from the observations. However, the glider data assimilating run is able to show comparable results with observations of thermocline as well as halocline depths during upwelling and relaxation events in the Monterey Bay area. It is also shown that during the relaxation of wind, the data assimilative run has higher value of subsurface velocity complex correlation with observations than the non-assimilative run. (C) 2008 Elsevier Ltd. All rights reserved.

Ramp, SR, Davis RE, Leonard NE, Shulman I, Chao Y, Robinson AR, Marsden J, Lermusiaux PFJ, Fratantoni DM, Paduan JD, Chavez FP, Bahr FL, Liang S, Leslie W, Li Z.  2009.  Preparing to predict: The Second Autonomous Ocean Sampling Network (AOSN-II) experiment in the Monterey Bay. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 56:68-86.   10.1016/j.dsr2.2008.08.013   AbstractWebsite

The Autonomous Ocean Sampling Network Phase Two (AOSN-II) experiment was conducted in and offshore from the Monterey Bay on the central California coast during July 23-September 6, 2003. The objective of the experiment was to learn how to apply new tools, technologies, and analysis techniques to adaptively sample the coastal ocean in a manner demonstrably superior to traditional methodologies, and to use the information gathered to improve predictive skill for quantities of interest to end-users. The scientific goal was to study the upwelling/relaxation cycle near an open coastal bay in an eastern boundary current region, particularly as it developed and spread from a coastal headland. The suite of observational tools used included a low-flying aircraft, a fleet of underwater gliders, including several under adaptive autonomous control, and propeller-driven AUVs in addition to moorings, ships, and other more traditional hardware. The data were delivered in real time and assimilated into the Harvard Ocean Prediction System (HOPS), the Navy Coastal Ocean Model (NCOM), and the jet Propulsion Laboratory implementation of the Regional Ocean Modeling System (JPL/ROMS). Two upwelling events and one relaxation event were sampled during the experiment. The upwelling in both cases began when a pool of cold water less than 13 degrees C appeared near Cape Ano Nuevo and subsequently spread offshore and southward across the bay as the equatorward wind stress continued. The primary difference between the events was that the first event spread offshore and southward, while the second event spread only southward and not offshore. The difference is attributed to the position and strength of meanders and eddies of the California Current System offshore, which blocked or steered the cold upwelled water. The space and time scales of the mesoscale variability were much shorter than have been previously observed in deep-water eddies offshore. Additional process studies are needed to elucidate the dynamics of the flow. (C) 2008 Elsevier Ltd. All rights reserved.

Davis, RE, Leonard NE, Fratantoni DM.  2009.  Routing strategies for underwater gliders. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 56:173-187.   10.1016/j.dsr2.2008.08.005   AbstractWebsite

Gliders are autonomous underwater vehicles that achieve long operating range by moving at speeds comparable to those of, or slower than, typical ocean currents. This paper addresses routing gliders to rapidly reach a specified waypoint or to maximize the ability to map a measured field, both in the presence of significant currents. For rapid transit in a frozen velocity field, direct minimization of travel time provides a trajectory "ray" equation. A simpler routing algorithm that requires less information is also discussed. Two approaches are developed to maximize the mapping ability, as measured by objective mapping error, of arrays of vehicles. In order to produce data sets that are readily interpretable, both approaches focus sampling near predetermined "ideal tracks" by measuring mapping skill only on those tracks, which are laid out with overall mapping skill in mind. One approach directly selects each vehicle's headings to maximize instantaneous mapping skill integrated over the entire array. Because mapping skill decreases when measurements are clustered, this method automatically coordinates glider arrays to maintain spacing. A simpler method that relies on manual control for array coordination employs a first-order control loop to balance staying close to the ideal track and maintaining vehicle speed to maximize mapping skill. While the various techniques discussed help in dealing with the slow speed of gliders, nothing can keep performance from being degraded when current speeds are comparable to vehicle speed. This suggests that glider utility could be greatly enhanced by the ability to operate high speeds for short periods when currents are strong. (C) 2008 Elsevier Ltd. All rights reserved.

2008
Checkley, DM, Davis RE, Herman AW, Jackson GA, Beanlands B, Regier LA.  2008.  Assessing plankton and other particles in situ with the SOLOPC. Limnology and Oceanography. 53:2123-2136.   10.4319/lo.2008.53.5_part_2.2123   AbstractWebsite

We combined a Sounding Oceanographic Lagrangian Observer float with a Laser Optical Plankton Counter (LOPC) and a fluorometer to make an autonomous biological profiler, the SOLOPC. The instrument senses plankton and other particles over a size range of 100 mm to 1 cm in profiles to 300 m in depth and sends data ashore via satellite. Objects sensed by the LOPC include aggregates and zooplankton, the larger of which can be distinguished from one another by their transparency. We hypothesized that the diel production of particles and their loss by sinking and grazing are reflected in the change of the particle distribution. We present data from four deployments of the SOLOPC off California. Particle volume was maximal at the base of the surface mixed layer and correlated with chlorophyll a fluorescence. In a 3-d deployment in 2005, particle volume was greatest in the early evening and smallest in the morning, and average particle size increased with depth. Eigenvector analysis of the particle volume distribution as a function of diameter for each of the deployments yielded size peaks characteristic of planktonic crustaceans. Ship-based measurements showed that the abundance of opaque particles of 1.1-1.7 mm equivalent spherical diameter was positively correlated with copepods of this size and simultaneously collected in nets. This relationship was used with SOLOPC data to estimate the distribution of large copepods, which were most abundant beneath the depth of maximal particle flux, estimated from particle size and published sinking rates. Our data are consistent with a model with diel production of particles and their loss by sinking and grazing.

Davis, RE, Ohman MD, Rudnick DL, Sherman JT, Hodges B.  2008.  Glider surveillance of physics and biology in the southern California Current System. Limnology and Oceanography. 53:2151-2168.   10.4319/lo.2008.53.5_part_2.2151   AbstractWebsite

Since 2005, Spray underwater gliders have been used to sample sections across the southern California Current System (CCS). Properties measured are depth-averaged velocity and profiles of temperature, salinity, Acoustic Doppler Profiler shear, chlorophyll a fluorescence, and 750-kHz acoustic backscatter. Although slow and carrying a relatively small sensor suite, gliders observed multiple 350-700-km sections that reveal details of known elements of the CCS circulation like the California Current and Undercurrent as well as a previously undescribed undercurrent 100-200 km offshore. The long time series with high spatial resolution disclose the close link between fronts in physical and biological variables, both the main pycnocline front and secondary weaker but sharp fronts farther offshore. These fronts appear to organize chlorophyll and acoustic backscatter fields, particularly during spring. Much of the patchiness of chlorophyll is associated with vertical displacements of fields that are much smoother within potential density surfaces. The horizontal structure of both chlorophyll and spice (the dynamically passive variable formed from temperature and salinity) appear to be formed by mesoscale stirring.

Gourdeau, L, Kessler WS, Davis RE, Sherman J, Maes C, Kestenare E.  2008.  Zonal jets entering the coral sea. Journal of Physical Oceanography. 38:715-725.   10.1175/2007jpo3780.1   AbstractWebsite

The South Equatorial Current ( SEC) entering the Coral Sea through the gap between New Caledonia and the Solomon Islands was observed by an autonomous underwater vehicle ( Spray glider) and an overlapping oceanographic cruise during July-October 2005. The measurements of temperature, salinity, and absolute velocity included high-horizontal-resolution profiles to 600-m depth by the glider, and sparser, 2000-m-deep profiles from the cruise. These observations confirm the splitting of the SEC into a North Vanuatu Jet ( NVJ) and North Caledonian Jet ( NCJ), with transport above 600 m of about 20 and 12 Sv, respectively. While the 300-km-wide NVJ is associated with the slope of the main thermocline and is thus found primarily above 300 m, the NCJ is a narrow jet about 100 km wide just at the edge of the New Caledonian reef. It extends to at least a 1500-m depth with very little shear above 600 m and has speeds of more than 20 cm s(-1) to at least 1000 m. An Argo float launched east of New Caledonia with a parking depth fixed at 1000 m became embedded in the NCJ and crossed the glider/cruise section at high speed about 3 months before the glider, suggesting that the jet is the continuation of a western boundary current along the east side of the island and extends across the Coral Sea to the coast of Australia. In the lee of New Caledonia, the glider passed through a region of eddies whose characteristics are poorly understood.