CTD observations were made off the northern California coast during R/V Wecoma cruise W8905 May 5-14, 1989 as part of the Shelf Mixed Layer Experiment (SMILE). The surveys consisted of two sampling plans - a large-scale grid of four cross-shelf transects extending to both sides of Point Arena and Point Reyes, and a small-scale grid of six cross-shelf transects located near the central SMILE mooring site. All of the cross-shelf transects extended beyond the shelf break and the maximum sampling depth at each station was near-bottom or 1500 m. The average along-shelf separation between cross-shelf transects was about 15 km for the small-scale surveys and 50 km for the large-scale grid. The primary objectives of the hydrographic measurement program were to observe and characterize the temperature, salinity, density, and light transmission fields and their temporal and spatial variability in the surface boundary layer along the continental shelf and slope near the SMILE moored array, and to acquire estimates of the cross- and along-shelf scales over which the mixed-layer depth varies. This report presents a summary in graphic and tabular form of the hydrographic observations made during cruise W8905 on the R/V Wecoma.
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This thesis examines the general task of active sensing by defining a measure of efficiency for sensing in a particular environment. We focus on fine-scale acoustic mapping from an autonomous underwater vehicle (AUV). The constraints on imaging underwater - vehicle power, vehicle hydrodynamics, computational and telemetry requirements, and typical navigational and attitudinal uncertainties along with the underlying physics of the acoustic sensing modality- are considered in defining an entropic measure of sensor efficiency. 675-kHz pencil-beam sonar data acquired using the JASON remotely operated vehicle in a challenging shallow water environment and 200-kHz echo-sounder data acquired using the ABE A UV are used to demonstrate the utility of the en tropic framework. We show the utility of an entropic framework for the following: (i) Optimizing the speed of the AUV for maximizing the information gathered with a particular sensor. (ii) the rate of convergence and the stability of our mapping efforts in the face of typical uncertainties in navigation and attitude; (iii) as a methodology for actual sensor deployment and use on a real vehicle; and (iv) in tasks such as post-mission analysis for applications such as change detection and path planning for subsequent missions.
A set of four hydrographic sections through the Brazil Current are analyzed to identify downstream changes in the Brazil Current. The data, from the Thomas Washington Marathon Cruise, Leg 9, are at 27, 31, 34 and 36°S. The region they span details the change of the current from a relatively small near surface feature to a large, deep current. While the Brazil Current does not appear to develop transports as large as those found in the Gulf Stream, the calculated transports greatly exceed previous estimates. At 27°S the current extends down to approximately 700 m and transports 12 Sv southward; this value is consistent with previous estimates farther north. Downstream, surface layer transport increases, the current deepens, and the transport reaches a maximum of 80 Sv at 36°S. Part of the growth comes from the tight recirculation found just offshore of the Brazil Current. The recirculation strengthens and deepens to the south, with a minimum transport of 4 Sv north at 27°S and a maximum of 33 Sv at 36°S. The change in the current is also reflected in its shear profiles. At 27°S Brazil Current shear is found only in the upper portion of the water column, over the continental slope. Downstream, the current moves off the slope into deeper water and develops top-to-bottom, monotonic shear. To obtain velocity from the shear profiles, zero velocity surfaces are chosen based on conservative use of tracer information. A simple basin-wide model is used at 31°S to tie limits on the size of the Brazil Current and recirculation to various limits on layer-to-layer exchanges south of the section. The multi-layer model - based on changes in depth of several isotherms is used to extend the interpretation of the current beyond that of an isolated ocean feature. The model is required to conserve mass in each layer, either by applying barotropic transports or by allowing layer-to-layer exchanges south of the section. Solutions are deemed acceptable if the sense, or direction, of the various layer-to-layer conversions are consistent with accepted ideas of water mass formation. Initially, a two layer model is employed. Governed by the conservation of mass in each layer, the two layer model has only one constraint on the resulting solutions: a conversion of cold-to-warm water in the south (or the surface layer flowing north and the deep layer flowing south). Such a meridional flow pattern is consistent with the equatorward heat flux in the South Atlantic. The single constraint, however, is not strong enough to limit the solution region in any significant way. The final model presented has four layers, and acceptable solutions have the net transports of the surface layer and the bottom water northward and form intermediate water from North Atlantic Deep Water in the south. The resulting solution set has a fairly small range of transports for the Brazil Current, with surface layer transports between 20 and 35 Sv; this range is consistent with the value calculated from hydrographic data at 31°S. Given the complex interleavings of the South Atlantic water masses, the four layer model performs remarkably well. Finally, total potential vorticity is calculated from the hydrographic sections. Contrary to what one might expect, the reference level choice is not a significant problem: where currents are large, most of the signal in relative potential vorticity comes from the measured shear, and where currents are small, the relative potential vorticity is not significant compared to the planetary vorticity. Unfortunately, the process of taking two horizontal derivatives of the density field results in a jittery relative potential vorticity signal. As a result, a clear potential vorticity profile could not be constructed for the current. This variablitiy may be real -the ocean is frequently much noisier than one imagines. It may also be possible, though, to smooth the data sufficiently so that a cleaner picture emerges. Despite the problems involved in obtaining a quantitative profile of the potential vorticity, qualitative changes are useful in detecting different flow regimes. By comparing the downstream changes in total and planetary potential vorticity, one can deduce frictional and inertial regimes in the different layers. The presence of a frictional regime at the inshore edge suggests that care should be taken in assuming that potential vorticity is conserved in western boundary currents. In addition the potential vorticity sections trace a pattern of the recirculation feeding into the Brazil Current in the upper layers; other tracers did not provide a clear image. The final picture which emerges is not of a small, surface-trapped Brazil Current; rather, it is that of a classic western boundary current, increasing in strength and depth before turning east into the interior ocean.
Seismic studies of the oceanic crust, both experimental and theoretical, often assume a flat seafloor and laterally homogeneous crust. This is done regardless of the appearance in seismic data of obvious effects due to scattering from lateral heterogeneities both on and in the seafloor. Detailed fine scale surveys of mid-ocean ridges, where the upper oceanic crust is exposed, have revealed the presence of lateral heterogeneities in the form of complicated topography, extrusive volcanic structure, and abundant fracturing and faulting. These heterogeneities have a significant affect on the propagation of seismo/acoustic energy through the crust, especially in the immediate vicinity of the seafloor. This thesis deals with the problem of scattering of seismo/acoustic energy from a number of forms of lateral heterogeneity in the upper oceanic crust. A common theme throughout this work is that the size of the heterogeneity on or in the seafloor is of the same order of magnitude as the seismo/acoustic wavelength. This is the realm of scattering theory where the wave-like characteristics of seismic energy have a particularly large influence on the outcome of interaction with structure in the media. The work presented here involves the application of the finite difference modeling technique to problems concerning laterally heterogeneous elastic media. This method is a full wave solution to the elastic wave equation and as such includes all wave interactions with the media. The finite difference formulation is used to study four distinct phenomena; scattering from discrete deterministic seafloor features; wave propagation through continuous randomly heterogeneous upper oceanic crust; scattering from more complicated topographic profiles and the limitations of the method for the rough seafloor problem; and the problem of plane acoustic wave scattering from an infinite elastic cylinder. The principal finding of this work is that lateral heterogeneities in the upper oceanic crust can have a dramatic affect on seismo/acoustic wave propagation. Scattering from rough seafloors and/or volume heterogeneities is often quite similar and causes the occurrence of signal generated 'noise' (coda), decorrelation of primary arrivals, and anomalies in arrival travel time and amplitude. Topographic and volume scatterers acting as secondary sources of seismic energy can cause a resonant coupling of body wave energy into interface (Stoneley) waves at the seafloor. This is possibly one mechanism by which natural seismic and storm generated acoustic energy can be coupled into seafloor noise. The applicability of the use of the finite difference method for non-planar water-solid interfaces is also discussed. Models were calculated which approximate sinusoidal seafloors and plane acoustic wave scattering from an infinite elastic cylinder. The discretization of a rectangular difference grid must be extremely fine to accurately accommodate a smoothly varying water-solid interface which does not align with the grid. Regardless of the discretization concerns, the rough seafloor models presented here demonstrate the arrivals expected from larger scale sinusoidal topography as well as the importance of considering quite small (
