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Evolution of Autonomous Platform for Sustained Ocean Observations Russ E. Davis* In the 16 years until 2025 I believe changes to oceanography will be substantially incremental. This is particularly true for ocean observ- ing where even relatively modest technical developments take a decade. Rather than give broad (and unreliable) prognostications about ocean- ography over these years, I prefer to focus on one specific evolutionary trend. The worldwide Argo program is showing how significant scientific results can be derived from proliferated long-term sampling with autono- mous vehicles. Argo floats were designed to minimize the cost of routine observations with a very limited set of sensors (mainly a CTD) and as a consequence, these relatively inexpensive floats have long design lives, no redundancy, and can support only a few of the lowest-powered sensors. While appropriate for general-circulation studies, Argo floats are not well matched to the upper ocean where comprehensive, expensive, and energy consuming sensor suites are needed. The complex interactions of air-sea fluxes, ocean mixing, primary production, biogeochemistry, marine optics, marine acoustics, and fauna in the upper ocean demand sustained observation with comprehensive sensor suites. The questions to be answered are myriad and of practical and academic interest. For example, we do not understand the main mechanisms supporting air- * Scripps Institution of Oceanography, University of California, San Diego 138

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Russ E. Davis 139 sea fluxes under high winds, or those responsible for stirring within the mixed layer or with the well stratified ocean below, nor how these pro- cesses affect the distribution of passive or living material in the ocean. Models that seek to predict variability of currents, ambient noise, rates of atmospheric CO2 exchange, ecosystem evolution, optical properties, acoustic propagation, and even the oceanic consequences of global change all parameterize these processes, often using simple hypotheses about mechanisms that are calibrated with small data sets from a limited range of conditions. For simple questions like how deep a mixed layer will be, this leads to manageable quantitative errors. For complex questions, like ocean optical structure or nutrient cycling through the food web, it can lead to larger qualitative errors. There are many ways to improve the factual basis for ocean mod- els. New, more comprehensive, sophisticated and accurate sensors are needed. Improved data assimilation procedures for using data to test and improve models are essential. But the complexity of upper ocean biophysical coupling and the spectrum of associated time scales also demand many multi-year, multi-variable time series from available sen- sors deployed in a range of locations to isolate and quantify the key processes and parameterize them. Ships and moorings will often be the right platforms, but they are too expensive for proliferated long-term use. Profiling floats are economical, but today’s platforms neither carry the energy nor provide the reliability to properly support comprehen- sive sensor suites. I believe a new class of platform will become a focus of future ocean observations. Characteristics of this class of vehicles follow: • They will be free-drifting to avoid expensive deployment and recovery operations, will cycle vertically like Argo floats so a single sensor generates a profile, and will communicate only at the surface. Unlike Argo floats, they will carry comprehensive high-value sensor systems, will include system redundancies for reliability, will be re-used, and will have flexible mounting sys- tems for varied sensors and a modular approach to power man- agement, data recording and relay, and real time control of the vehicle and sensors. • Relatively extensive sensor suites will generate profiles when the vehicle cycles to depth and back. Typical sensors to be carried include CTD, multiple wavelength fluorescence, optical back- scatter and/or transmission, Laser Doppler Velocimeter or thrust probe for turbulence, Laser Optical Plankton Counter, chemical sensors for oxygen, pH, CO2 and/or nutrients, and an optically sensed and occasionally flushed sediment trap. ADCPs for abso-

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140 OCEANOGRAPHY IN 2025 lute currents, multi-frequency sonars for acoustic backscatter and biological remote sensing, and/or accelerometer-based surface wave sensors could be used during extended surface intervals. Additional characteristics that cannot be implemented today include: • Novel energy sources or energy renewal from solar, wind or wave harvesting at the surface; • Lightweight or remotely sensing instruments for wind measure- ments, air temperature humidity, and atmospheric optical proper- ties that might profile upward; •  combination of acoustic detection and optical identification that A could provide reproducible measures of large-plankton and fish abundances. What is needed now is not so much a specific instrument as a line of scientific and technical developments leading to a class of multi-use autonomous instruments.