Passive and active acoustic measurements are effective and unique approaches to ocean observation. Both approaches have techniques that are well-established and suitable for immediate deployment as part of an operational observing system.Many of these techniques exploit the remarkable ability of lowfrequency sounds to traverse great distances in the ocean. Passive acoustic measurements of natural or man-made sounds are made for diverse purposes ranging from assessing the environmental impact of human activities, to geophysics, to monitoring changes in biodiversity in response to climate change. Active acoustic systems are employed for tracking instruments and acoustic tomography. Tomography has a role in the ocean observing system that is complementary to altimetry and profiling floats, particularly in the context of data assimilation and ocean modeling. Modest systems of acoustic receivers and sources deployed across ocean basins can make cost effective, sustained observations of a variety of ocean processes.
Acoustic tomography systems have been deployed in Fram Strait over the past decade to complement existing observing systems there. The observed acoustic arrival patterns are unusual, however, consisting of a single, broad arrival pulse, with no discernible repeating patterns or individual ray arrivals. The nature of these arrivals is caused by vigorous acoustic scattering from the small-scale processes that dominate ocean variability in Fram Strait. Simple models for internal wave and mesoscale variability were constructed and tailored to match the variability observed by moored thermisters in Fram Strait. The internal wave contribution to variability is weak. Acoustic propagation through a simulated ocean consisting of a climatological sound speed plus mesoscale and internal wave scintillations obtains arrival patterns that match the characteristics of those observed, i.e., pulse width and travel time variation. The scintillations cause a proliferation of acoustic ray paths, however, reminiscent of "ray chaos." This understanding of the acoustic forward problem is prerequisite to designing an inverse scheme for estimating temperature from the observed travel times.
Individual ray arrivals were not observed. Rather, the arrival patterns consisted of a single, stable, broad arrival pulse of about 100 ms duration. Travel time variations of 60.15 s recorded the vigorous mesoscale environment of the region and the seasonal cycle. To estimate ocean temperature from the tomography data an inverse scheme employed a high-resolution ocean model for Fram Strait as the reference ocean. The information from the tomographic measurements is primarily average temperature. Estimated temperatures, averaged over 0-1000 m depth and over range, had a mean of 1.118C and variations of 60.338C; the uncertainty of the tomography estimates was about 60m8C. Agreement with an alternate inverse approach based on EOFs and a Markov Chain Monte Carlo inversion scheme relying on a matched-peak approach was excellent, indicating a robust estimate for ocean temperature. The inverse estimates for average temperature agreed with the equivalent estimates from hydrographic sections obtained along the acoustic path at the start and end of the program. Among other deficiencies, the ocean model greatly underestimated the intensity of the mesoscale fluctuations and exhibited a warm bias of about 0.388C in section-averaged temperature. Tomographic measurements in Fram Strait offer unique large-scale temperature constraints for ocean models through data assimilation. It is anticipated that these constraints will lead to more accurate estimates of the circulation and transports in Fram Strait.
Rapid Arctic warming drives profound change in the marine environment that have significant socio-economic impacts within the Arctic and beyond, including climate and weather hazards, food security, transportation, infrastructure planning and resource extraction. These concerns drive efforts to understand and predict Arctic environmental change and motivate development of an Arctic Region Component of the Global Ocean Observing System (ARCGOOS) capable of collecting the broad, sustained observations needed to support these endeavors. This paper provides a roadmap for establishing the ARCGOOS. ARCGOOS development must be underpinned by a broadly endorsed framework grounded in high-level policy drivers and the scientific and operational objectives that stem from them. This should be guided by a transparent, internationally accepted governance structure with recognized authority and organizational relationships with the national agencies that ultimately execute network plans. A governance model for ARCGOOS must guide selection of objectives, assess performance and fitness-to-purpose, and advocate for resources. A requirements-based framework for an ARCGOOS begins with the Societal Benefit
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