Characteristics of acoustic propagation to the eastern vertical line array receiver during the summer 1996 New England shelfbreak PRIMER experiment

During July and August of 1996, the summer component of the New England shelfbreak front PRIMER experiment was fielded in the Mid-Atlantic Bight, at a site due south of Martha's Vineyard, MA. This study produced acoustic transmission data from a network of moored sources and receivers in conjunction with very-high-resolution oceanography measurements. This paper analyzes receptions at the northeast array receiver from two 400 Hz acoustic tomography sources, with the transmission paths going from the continental slope onto the continental shelf. These data, along with forward acoustic-propagation modeling based on moored oceanographic data, SeaSoar hydrography measurements, and bottom measurements, reveal many new and interesting aspects of acoustic propagation in a complicated slope-shelf environment. For example, one sees that both the shelfbreak front and tidally generated soliton internal wave packets produce stronger mode coupling than previously expected, leading to an interesting time-and-range-variable population of the acoustic normal modes. Additionally, the arrival time wander and the signal spread of acoustic pulses show variability that can be attributed to the presence of a frontal meander and variability in the soliton field. These and other effects are discussed in this paper, with an emphasis on creating a strong connection between the environmental measurements and the acoustic field characteristics.     

Spatial and temporal variations in acoustic propagation characteristics at the New England shelfbreak front

The spatial and temporal variability of the acoustic field in the region of a strong coastal shelfbreak front are examined, using the high-resolution environmental data from the 1996-1997 New England shelfbreak PRIMER experiment to provide input to acoustic propagation models. Specifically, the "SeaSoar" undulating conductivity-temperature-depth (CTD) probe across-shelf transects provide 1-km along-track resolution, including the front, during the spring, summer, and winter seasons. These data allow one to study the diurnal and seasonal temporal variation of the acoustic field, as well as the varying spatial structure of the field. Using the RAM parabolic equation code, across-shelf acoustic field structure at 200, 400, and 1000 Hz is studied for various source depths. A number of interesting propagation effects are noted, the most interesting of which are the inhibition of strong acoustic-bottom interaction by the warm shelf water beneath the shelfbreak front and the existence of small-scale ducts near the front, due to offshore transport. Data from the vertical line arrays deployed as part of PRIMER offer partial validation of the predictions made. Specifically, it is seen that the mean received levels are in reasonable accord with propagation calculations made using locally measured bottom properties and the SeaSoar water-column measurements.     

Shelfbreak circulation and thermohaline structure in the northern south China Sea-contrasting spring conditions in 2000 and 2001

Two intensive, high-resolution hydrographic surveys during April 2000 and May 2001 are used to characterize the thermohaline and current structure at the shelfbreak in the South China Sea. In 2000, a strong anticyclonic circulation was present in the northern portion of the South China Sea with strong onshore currents east of Dongsha Island. The flow became polarized along isobaths as it encountered shallow water, with northeastward flows of over 0.9 m/s along steep topography. The flow was driven by strong density contrasts between waters of the outer shelf and upper slope. Shelf water was both cooler and more fresh than the water offshore, which had salinities close to that of Kuroshio water. In contrast, the mean flow in the northern South China Sea was predominantly cyclonic in 2001. Flow over the slope was to the southwest at up to 0.2 m/s. The water mass properties of the outer shelf and upper slope were similar, so that there were not the strong cross-shelf density gradients present as in 2000. A potential difference between the water mass structure of the two years was the difference in cooling during the preceding winters. In December, 1999, unusually strong cooling may have resulted in cooler shelf waters relative to the following year. The ASIAEX study area may be a particularly sensitive region to both seasonal and interannual variability, as it is near a bifurcation point associated with the Kuroshio Intrusion into the South China Sea.     

Oceanographic and sound speed fields for the ESME workbench

The authors describe the effort to provide three-dimensional global thermohaline and sound speed fields for use in the effects of sound in the marine environment (ESME) workbench suite of programs. The primary fields used are from the modular ocean data assimilation system (MODAS), developed by Fox et al. The system provides global thermohaline and sound speed fields on a daily basis using environmental inputs from the U.S. Navy as well as remote sensing of sea surface temperature and sea surface height. To examine the MODAS fields, the authors also used data from the Southern California Bight collected by the California Cooperative Fisheries Investigations as well as high-resolution hydrographic data collected over the continental shelf south of New England as part of the shelfbreak PRIMER experiment. MODAS performs well for features such as large-scale boundary currents and eddies but is more limited in resolving features such as shelfbreak and coastal fronts, which have small spatial and temporal correlation scales. Because of the considerable computational needs of other ESME modules and its use as a planning tool, the authors present a pragmatic approach for future applications.     

Climatological Estimation of Environmental Uncertainty Over the Middle Atlantic Bight Shelf and Slope

Historical hydrographic data are used to determine the spatial and seasonal patterns of uncertainty in thermohaline and sound-speed fields in a well-sampled region, the continental shelf and slope in the Middle Atlantic Bight (MAB). Several different historical databases are combined to produce two-dimensional (2-D) plan view and cross-shelf fields of temperature, salinity, and sound speed in two separate regions, the New England shelf and the shelf off Delaware and Maryland. In addition, spatial maps of the sound-speed fields reveal that the maximum variance of the sound speed occurs at the edge of the continental shelf, in the vicinity of the shelfbreak front. The standard deviation of the sound speed was largest during the spring and summer, with magnitudes as large as 14 m/s in a narrow band coinciding with the mean position of the shelfbreak front. During spring the peak in variance was located near the surface outcrop of the front, but during summer the maximum variance was centered at a depth of 30 m, immediately beneath the seasonal thermocline. Comparisons with both synoptic measurements from the Shelfbreak PRIMER experiment as well as moored time series from the Nantucket Shoals Flux Experiment confirm that the shelfbreak front is a "hotspot" of uncertainty (maximum variance), and that the vertical structure of the peak variance is dependent on the presence or absence of the seasonal thermocline     

Interannual variability of the Kuroshio intrusion in the South China Sea

The interannual variability of intrusions of the Kuroshio into the South China Sea (SCS) is investigated using satellite remote sensing data supported by in-situ measurements. The mesoscale circulation of the SCS is predominantly wind-forced by the northeast winter and southwest summer monsoons. Although the region has been studied extensively, considerable uncertainty remains about the annual and interannual mesoscale nature of the circulation. The frequency and characteristics of Kuroshio intrusions and their effect on circulation patterns in the northeast SCS are also not well understood. Satellite observations of Sea Surface Temperature (SST) from the Tropical Rainfall Measuring Mission (TRMM) and the Advanced Very High Resolution Radiometer (AVHRR) and Sea Surface Height Anomalies (SSHA) from TOPEX/ Poseidon for the period 1997–2005 are used here to analyze the annual and interannual variability in Kuroshio intrusions and their effects on the region. Analysis of SST and SSHA shows the formation and characteristics of intrusions vary considerably each year. Typically, the intrusion occurs in the central region of Luzon Strait and results in an anticyclonic circulation in the northeastern SCS. However, in some years, the intrusion is located in the northern portion of Luzon Strait and a cyclonic intrusion results. Wind stress and wind stress curl derived from the National Aeronautics and Space Administration (NASA) QuikSCAT satellite scatterometer are used to evaluate the relationship between wind stress or wind stress curl and the presence of winter Kuroshio intrusions into the SCS.     

Effects of Korean littoral environment on acoustic propagation

Environmental acoustics experiments were recently conducted in shallow to intermediate water depths in the Sea of Japan, east of Korea, along the shelf and slope, covering frequencies from 25 to 800 Hz. These were operational experiments carried out in three different seasons. The primary objectives of the data reported here are: (1) to characterize the Korean coastal environment during May 1998, September 1998, and February 1999 and (2) to assess how complexities of the environment might impact acoustic propagation in May and February, as measured by its transmission loss. Propagation data were obtained from broadband explosive SUS sources and sonobuoy receivers. The tests were conducted over varying bottom depths and slopes, both approximately normal and parallel to the bathymetric contours. Two different source depths were included. Environmental and acoustic data are reviewed and discussed. While many aspects of the observed propagation remain ill understood, on the whole a consistent and useful picture has emerged of acoustic propagation in this region. Environmental impacts on propagation are associated mainly with bottom properties, somewhat less so with source depth in relationship to sound speed profiles, and almost not at all with range-dependent profiles of a water mass front     

Instability of the Kuroshio in Luzon Strait: Effects of ridge topography and stratification

The spectral analyses of moored current velocities in the central Luzon Strait reveal northward (i.e., downstream of the Kuroshio) propagation of a frontal wave with a five-day period, with wave amplitude increasing northward. Estimated from both curve fitting and frequency domain Empirical Orthogonal Function methods, the characteristics of five-day variations have wave speeds ranging from 32 to 40 cm s⁻¹, wavelengths ranging from 130 to 150 km, and e-folding time scales for growth ranging from 0.8 to 3 days. An analytical two-layer model used to explore linear stability characteristics indicates that bottom topography (two meridional ridges) is important for the Kuroshio stability characteristics in the Luzon Strait. In the two-layer model with the two ridges, the flow is stabilized for the long-wave mode but destabilized for the short-wave mode (due to increasing vertical shear in the horizontal velocity). The analytical model produces wavelengths and phase speeds for the most unstable mode which is similar to the observation, but the growth rate is underestimated. However, a spectral numerical model applied with a more realistic stratification and velocity structure does obtain faster growth rates comparable to the observations. Parameter sensitivity tests were conducted using the analytical model. The characteristics of the most unstable mode are most sensitive to the surface front location relative to the bottom topography but not sensitive to varying the density difference and thickness of the upper layer.     

Consideration of fine-scale coastal oceanography and 3-D acoustics effects for the ESME sound exposure model

Results and recommendations for evaluating the effects of fine-scale oceanographic scattering and three-dimensional (3-D) acoustic propagation variability on the Effects of Sound on the Marine Environment (ESME) acoustic exposure model are presented. Pertinent acoustic scattering theory is briefly reviewed and ocean sound-speed fluctuation models are discussed. Particular attention is given to the nonlinear and linear components of the ocean internal wave field as a source of sound-speed inhomogeneities. Sound scattering through the mainly isotropic linear internal wave field is presented and new results relating to acoustic scattering by the nonlinear internal wave field in both along and across internal wave wavefront orientations are examined. In many cases, there are noteworthy fine-scale induced intensity biases and fluctuations of order 5-20 dB.