http://hdl.handle.net/1957/1314 Search the Channel search http://ir.library.oregonstate.edu/dspace/simple-search http://hdl.handle.net/1957/8371 Title: Mapping semi-regular autonomous underwater vehicle glider observations onto a cross-shelf section <br/> <br/>Abstract: Two Autonomous Underwater Vehicle Gliders have alternated continuous sampling of a 45-nautical mile transect line (the Newport Hydrographic Line) across the Oregon continental shelf since April, 2006. Strong currents (>25cm/s) push the gliders off their trajectories as they survey this transect line, preventing them from sampling the historically occupied stations exactly. Three methods were used to map the semi-regular glider data onto a cross-shelf line: (1) an algorithm that groups data by isobaths then block-averages the data, (2) an objective analysis that employs fixed along-isobath and cross-isobath correlation scales, and (3) a hybrid combination of the isobath-binning algorithm and objective analysis. To determine validity and accuracy, the mapping procedures are tested by comparison to moored observations at NH-10 on the Newport Line in 80m water depth while varying the spatial and temporal averaging scales. Isobath binning showed the best agreement with moored observations at the monthly timescale, while objective analysis showed the best agreement with moored observations at the weekly timescale. The hybrid method improved the agreement of the objective analysis at larger timescales. <br/> <br/>Description: Graduation date: 2008 Thu, 20 Mar 2008 22:58:59 GMT http://hdl.handle.net/1957/6391 Title: Surface wind modification near mid-latitude ocean fronts : observational and dynamical analysis <br/> <br/>Abstract: Interactions between surface winds and meanders in mid-latitude sea surface temperature (SST) fronts with horizontal length scales of 100-1000 km are investigated from satellite observations and numerical simulations. Observations from the Sea- Winds scatterometer on the QuikSCAT satellite show that the magnitude, direction, curl, and divergence of the surface wind stress and 10-m winds are well correlated with small-scale SST structures associated with large-scale ocean currents. Detailed analysis of the response of the surface winds to SST fronts from these satellite observations exposed shortcomings in previous conceptual hypotheses governing the relationships between surface winds and SST. To gain understanding of the physical mechanisms needed to explain the satellite wind observations, we performed a numerical experiment simulating the atmospheric flow over meandering SST fronts. Based on these results, a new conceptual model is constructed to explain the dynamical response of the surface winds consistent with the satellite observations and numerical simulation analysis. Of particular importance was the finding that the wind stress curl and divergence fields observed from QuikSCAT are linearly related to the crosswind and downwind components of the SST gradient, respectively. This relationship was generally thought to result from modification of the vertical turbulent mixing of momentum within the atmospheric boundary layer (ABL). We show that this mechanism is overly simplistic; nearly all of the terms in the momentum budget are needed to explain these observed statistical relationships, consistent with recent work. SST-induced surface wind changes are a manifestation of more complicated changes to the vertical structure of the dynamic forces within the ABL. Among the most significant of several new findings presented here concerns the influence of SST on the meridional wind field. Since winds are generally westerly at mid-latitudes, SST-induced changes in meridional wind cause changes in the surface wind direction that significantly influence the wind stress curl and divergence fields through modification of streamline curvature and diffluence. From numerical and analytical results, these meridional wind perturbations are shown to result from a baroclinic Ekman adjustment mechanism modified by horizontal advection. <br/> <br/>Description: Graduation date: 2008 Mon, 10 Sep 2007 23:03:25 GMT http://hdl.handle.net/1957/5113 Title: Inner-shelf circulation off the central Oregon Coast <br/> <br/>Abstract: Inner-shelf circulation and mechanisms of across-shelf transport of water masses were examined using seven years of observations collected by the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO) program, a long-term monitoring effort along the central Oregon coast. Since 1998, moored velocity and hydrographic measurements have been obtained during the summer upwelling season in water depths of 30, 15, or 8 m at 3-5 stations along a 75 km stretch of the Oregon shelf. These observations enabled a description of along-shelf variations and upwelling dynamics in an area of intermittent wind forcing but little buoyancy influences. While wind forcing and bathymetry were nearly spatially uniform in the inner-shelf, circulation was spatially variable due to an offshore submarine bank. Classic two-dimensional upwelling existed north of the bank, with bottom stress and acceleration balancing the wind stress in the depth-averaged along-shelf momentum equation. This balance failed onshore of the bank where the pressure gradient and nonlinear advection were needed to close the momentum balance. Driven by along-shelf wind forcing, across-shelf surface transport was 25% of the theoretical Ekman transport at 15 m water depth, 1-2 km offshore, and reached full Ekman transport 5-6 km offshore in 50 m of water. This result, based on season-long comparisons of measured across-shelf transport and theoretical Ekman transport, defines the across-shelf scale of coastal upwelling on the Oregon shelf. However, observations of across-shelf circulation also highlight the rapid movement of water masses and variable residence times in the inner shelf. To quantify the time variability of across-shelf exchange, a numerical model was adapted to estimate vertical eddy viscosity using the velocity measurements. Resulting depth-averaged eddy viscosities ranged from 0.8x10[superscript −3] m [superscript 2] s[superscript −1] during upwelling winds to 2.1x8x10[superscript −3] m [superscript 2] s[superscript −1] during downwelling winds, consistent with previous numerical model results. The difference in eddy viscosities between upwelling and downwelling led to varying across-shelf exchange efficiencies and increased net upwelling over time. These results quantify the structure and variability of circulation in the inner-shelf and have significant implications for ecological processes (e.g., larval recruitment, nutrient availability) in the region. <br/> <br/>Description: Graduation date: 2007 Wed, 23 May 2007 22:58:59 GMT http://hdl.handle.net/1957/3127 Title: Quantifying linear disturbance growth in periodic and aperiodic systems <br/> <br/>Abstract: The mathematical and physical connections between three different ways of quantifying linear predictability in geophysical fluid systems are studied in a series of analytical and numerical models. Normal modes, as they are traditionally formulated in the instabilities theories of geophysical fluid dynamics, characterize the asymptotic development of disturbances to stationary flows. Singular vectors, currently used to generate initial conditions for ensemble forecasting systems at some operational centers, characterize the transient evolution of disturbances to flows with arbitrary time dependence. Lyapunov vectors are an attempt to associate a physical structure with the Lyapunov exponents, which give the rate at which the trajectories of dynamical systems diverge. It is shown that these seemingly divergent ways of quantifying linear disturbance growth are closely related. It is argued that Lyapunov vectors are a natural generalization of normal modes to flows with arbitrary time dependence. Singular vectors are shown to asymptotically converge to orthogonalizations of the Lyapunov vectors. A direct, efficient, and norm-independent method for constructing the n most rapidly growing Lyapunov vectors from the n most rapidly growing forward and the n most rapidly decaying backward asymptotic singular vectors is proposed and demonstrated using several models of geophysical flows. These connections are further studied using a (time-periodic) wave-mean oscillation in an intermediate complexity baroclinic channel model. For time-periodic systems, normal modes may be defined in terms of Floquet vectors. It is argued that Floquet vectors are equivalent to Lyapunov vectors for time-periodic flows. The Floquet vectors of the wave-mean oscillation are found to split into two dynamically distinct classes that have analogs in the classical theories of the baroclinic instability and parallel shear flow. The singular vectors of the oscillation are found to preserve this dynamical splitting. The representations of the singular vectors in terms of the forward and adjoint Floquet vectors display much simpler temporal behavior than the singular vectors or the Floquet vectors individually. It is further demonstrated that while the Floquet vectors point 'onto' the local system attractor, the singular vectors point 'off' the attractor. <br/> <br/>Description: Graduation date: 2007 Fri, 06 Oct 2006 20:36:56 GMT