Coastal Region Dynamics I Posters
Presiding: A C Warn-Varnas, Naval Research Laboratory; K G Lamb, University of Waterloo
OS41A-01 0830h
Physical Structure of the Coastal Transition Zone off Central California
Ten hydrographic cruises were carried out in the coastal transition zone off Central California between February 1997 and October 2002. Stations were spaced 10 nautical miles (nm) apart along sections that were perpendicular to the coast. These sections were spaced 20 nm apart and extended from Pt. Reyes to Pt. Sur although not all sections were occupied on a given cruise. Objective analysis is used to map the depth, spice and acceleration potential on density surfaces (a) of 25.0 and 26.8 kg/m3. Results are compared with the pattern of ocean currents observed by vessel mounted acoustic Doppler current profilers. Mesoscale variability dominates the charts of physical properties. The general pattern circulation pattern reflects offshore flow along the northern and southern portions of the region with onshore flow in the center.
OS41A-02 0830h
Measurement of Longshore Sediment Transport on Galveston Island Texas
Longshore sediment transport in the surf zone on Galveston Island, Texas, was studied using a instrumented sled that was pulled across the surf zone. Determination of the sand transport rate was challenging due to presence of fines. The instrument sled contained a LISST 100 for particle size distribution determination, four OBS sensors, and three velocity sensors. The sled occupied 10 to 15 stations, spaced about 10 m apart, and collected data for about 3 minutes at each station. Data from the LISST-100 and from the 4 OBS's was analyzed to determine the average vertical distribution of sand concentration, assuming that fines were vertically well mixed at each station. A logarithmic profile of average longshore current was calculated based on the velocity data. The longshore sediment transport was calculated as the spatial integral of the product of the sand concentration and velocity, and was related to the wave conditions at the point of breaking. The results indicated that the popular CERC formula, with K = 0.7, did a reasonable job of calculating sediment transport. Longshore sediment transport was greatest in the vicinity of the sand bars.
OS41A-03 0830h
Ambient Hydrographic Effects on the Generation and Propagation of Tidally-Driven Internal Bores and Solitons
The effects of different initial hydrographic representations on the numerical simulations of internal bore and soliton generation and propagation were studied. The initial set of experiments were carried out in the Luzon Strait and the eastern part of the South China Sea using nonhydrostatic models. Internal wave generation was forced by the barotropic tide passing over the sills between islands and south of Taiwan in the Strait. Horizontally homogeneous initial density profiles were derived, via analytical approximations, from three sources: (1) the 2001 Levitus monthly, 1/4 deg climatology; (2) a set of 694 CTD observations collected between April and July of 2001, and (3) numerical simulations for the period of the observations. To obtain high resolution 3-D simulations, only subregions of the Luzon Strait/South China Sea area were considered, with focus on regions in the 118E-122E and 18.5N-22.5N geographic domain. Model resolutions varied from 50m to 1 km in the horizontal and 25m to 100m in the vertical. The choice of temperature and salinity profiles, and the resulting density profile, affected the numerical convergence of the iterations in the model. In addition, the amplitude of the generated internal bores and their propagation were also affected.
OS41A-04 0830h
Simulation the Effect of Internal Wave on the Acoustic Propagation
An acoustic radiation transport model with the Monte Carlo solution has been developed and applied to study the effect of internal wave induced random oceanic fluctuations on the deep ocean acoustic propagation. Refraction in the ocean sound channel is performed by means of bi-cubic spline interpolation of discrete deterministic ray paths in the angle(energy)-range-depth coordinates. Scattering by random internal wave fluctuations is accomplished by sampling a power law scattering kernel applying the rejection method. Results from numerical experiments show that the mean positions of acoustic rays are significantly displaced tending toward the sound channel axis due to the asymmetry of the scattering kernel. The spreading of ray depths and angles about the means depends strongly on frequency. The envelope of the ray displacement spreading is found to be proportional to the square root of range which is different from "3/2 law" found in the non-channel case. Suppression of the spreading is due to the anisotropy of fluctuations and especially due to the presence of sound channel itself.
OS41A-05 0830h
Hydrography of the Entrance to the Gulf of California in November 2004
In November 2004, two high resolution hydrographic sections were occupied at the entrance to the Gulf of California. One section combined velocity data collected with a 300 kHz broadband ADCP with water properties obtained by a CTD which continuously profiled the upper 125 m. The second section combined velocity data collected with a 75 kHz ocean surveyor ADCP with traditional 1000 dbar CTD casts. As with previous sections, inflow was observed along Sinola but outflow occurred in the center third of the section. This meant that the isopycnal dome was located in the eastern half of section at 108.5W. Flow patterns and rates derived from ADCP measurements were in good agreement with those derived from geostrophy. Inflow and outflow extended from the surface to about 700 dbar. The maximum inflow (outflow) was about 35-40 cm/s (20 cm/s) between 100-200 (100-200) dbar and 108.2-108.4W (108.5-108.9W). Unlike previous sections, the portion of the section next to Baja California was marked by weak inflow. Waters in the upper layer were roughly divided into two classes by the isohaline S=34.7. To the east, Tropical Surface Waters (S greater than 34.3) lay between 50 and 90 dbar while to the west, Gulf waters (S less than 34.9) were observed. The Triaxus section showed the high resolution details (0.5 km horizontal and 0.5 dbar vertical) of the upper ocean structure. Highest productivity was observed at the base of the mixed layer in the western portion of the section where the warmest and highest salinity waters were found. Below the mixed layer, the fine structure of Subtropical Subsurface Waters formed in the Gulf is seen at depth along the 25.5 and 26.0 kg/m3 isopycnals. The separation between Tropical Surface and Gulf Waters was nearly vertical along 108.6W and marked by the zero isotach of cross stream flow as well as the zero isotach of inflow between 70 and 100 m.
OS41A-06 0830h
Nonhydrostatic Dynamics of High Amplitude Internal Waves in Canyons and Straits
High Amplitude Internal Waves (HAIWs) and Internal Bores (IBs) are frequently generated by the interaction of internal tides with topography. These interactions are intensified in canyons and straits where the flow is constricted by the geometry. Several mechanisms have been proposed for the generation of HAIWs and IBs from internal tides. To better understand the dynamics of these processes we simulate the generation and propagation of HAIWs and IBs in the Straits of Gibraltar using nonhydrostatic models. The nonhydrostatic models will be forced at the open boundaries using data from the two layer, three dimensional model developed at the University of Cadiz. The results are compared with measurements taken as part of the Gibraltar Mixing (GIMIX) project, funded by the Science and Technology Ministry of the Spanish Government. The results of this work also have implications for the dynamics of the Luzon Straits which is the site of observations and simulations in the near future. We will also present nonhydrostatic simulations of the Hudson canyon and the New Jersey shelf where barotropic tides generate HAIWs that propagate up the canyon onto the shelf.
OS41A-07 0830h
Wavelet analysis of internal gravity waves
A series of model studies of internal gravity waves (igw) have been conducted for several regions of interest. Dispersion relations from the results have been computed using wavelet analysis as described by Meyers (1993). The wavelet transform is repeatedly applied over time and the components are evaluated with respect to their amplitude and peak position (Torrence and Compo, 1998). In this sense we have been able to compute dispersion relations from model results and from measured data. Qualitative agreement has been obtained in some cases. The results from wavelet analysis must be carefully interpreted because the igw models are fully nonlinear and wavelet analysis is fundamentally a linear technique. Nevertheless, a great deal of information describing igw propagation can be obtained from the wavelet transform. We address the domains over which wavelet analysis techniques can be applied and discuss the limits of their applicability.
OS41A-08 0830h
Simulated shelf-break processes in the northern Gulf of Mexico
Simulations that use the HYbrid Coordinate Ocean Model (HYCOM) configured for the Gulf of Mexico are used to elucidate the dynamics of the Gulf and explore the sensitivity to parameter space. HYCOM uses isopycnal coordinates in the deep stratified ocean, pressure coordinates in unstratified regions including the mixed layer, and terrain-following coordinates in shallow coastal regions. The simulations have 20 layers in the vertical and 1/12 or 1/25 degree horizontal resolution and are forced with 6-hourly NOGAPS winds and fluxes and boundary conditions from a 1/12 HYCOM Atlantic simulation during 2000 and 2001. Several different vertical mixing schemes have been examined and will be discussed. All simulations depict a realistic Loop Current and realistic Loop Current Eddy shedding. Upwelling is observed near the Yucatan consistent with observations. Emphasis is placed on the role of cyclones in cross-shelf exchange in the northeastern Gulf. The cyclones tend to orbit around the anticyclonic Loop Current eddies and seldom cross the shelf-break front, although filaments associated with baroclinically unstable eddies do genetrate cross-shelf exchange and interact with largely barotropic flows that tend to follow the shelf-break front.
OS41A-09 0830h
Preliminary mooring results at the entrance to the Gulf of California from May to November 2004.
The preliminary results of two moorings at the entrance to the Gulf of California, from May to November 2004 are discussed. The mooring M1 (23o 28.58' N, 109o 23.34' W), off Cabo Pulmo, Baja California consisted of a ADCP workhorse broadband at 118 m and three MicroCats at 40, 79 and 118 m respectively. The mooring M2 (24o 02.15'N, 107o 51.40' W), off El Dorado, Sinaloa, consisted of a ADP Sontek 250 (KHz) at 115 m and also three MicroCats at 38, 77 and 115 m respectively. The low passed filtered pressures at the three depths in each site are highly coherent while the coherence between the pressures on opposite sides of the entrance at three depths is low, being high only at the diurnal and semidiurnal tidal frequencies. The pressure record at the three depths in the El Dorado side was dominated by two events that occurred on June 10 and October 23, 2004. The first event was preceded by an increase in salinity of about .30 at 40 m, as well as small increases at 80 and 120 m, all of which were associated to an increase in pressure, suggesting an intrusion of a saline vortex. The coherence of temperature and salinity measured at the two moorings was low indicating the difference in changes of thermohaline characteristics. The along shelf current direction was chosen as the principal axes for each current time series (Cabo Pulmo and El Dorado). As expected, the principal axes were aligned with the local isobaths on each shelf. On the Sinaloa shelf (El Dorado), the currents were strongly oriented along the shelf. Along shelf currents were stronger and less variable on the Baja California shelf (Cabo Pulmo). At El Dorado, with the exception of the strong event periods mentioned above when the current reached 0.5 ms-1, flow was weaker and more variable; periods of strong outlfow lasted for about 8 days. The vertically averaged flow at both sides of the entrance to the Gulf was directed out of the Gulf most of the time. This suggests that the persistent cyclonic circulation observed at the entrance to the Gulf by Castro et al. (2000) occurred in deep water.
OS41A-10 0830h
Seasonal Variation of Sea Surface Chlorophyll a in the Gulf of Mexico
The concentration of chl a is related to phytoplankton biomass and is therefore an important parameter characterizing the trophic state of marine ecosystems. The distribution of chl a in surface waters has been monitored on a daily basis by the space-borne SeaWiFS instrument for more than six years. Early algorithms for calculating chl a were susceptible to interference in Case II waters, but recent studies indicate that the new OC4 algorithm performs well in coastal seawaters. We processed six years of daily SeaWiFS images for surface water chl a of the Gulf of Mexico using the OC4 algorithm. Additionally, we conducted an analysis of temporal and spatial variations of surface water chl a based on its daily distributions. Our analysis indicates that the concentrations of chl a were high in coastal and low in offshore waters consistent with the nutrient limitation of its growth and nutrient sources from rivers and continental shelf. Three belts of high chl a regions were observed including: Texas to Alabama shelf, west Florida shelf, and Campeche/Yucatan shelf. These belts coincide with the wide continental shelf and its shallow water, indicating the contribution to surface biomass by benthic and recycled nutrients. Fluvial nutrients are likely a significant source for Texas to Alabama shelf where highest chl a concentrations were observed. Strong seasonal cycles of chl a concentration were observed throughout the Gulf of Mexico. Two phytoplankton blooms were observed near the northern and southern coasts. The blooms near the northern coast were in March and June likely related to the peak of fluvial nutrient input and solar radiation maximum. The blooms near the southern coast were in July to August and in October and coincide with the seasonal maximum of benthic fluxes. Only one phytoplankton bloom was observed in the offshore waters of the Gulf of Mexico likely fueled by winter mixing and its nutrient input.
OS41A-11 0830h
Turbulent Coastal Dynamics
The coastal dynamics model, restoring the three-dimensional hydro-physical structure of the coastal shallow-water area is essentially improved. The improvement involves two-equation of non-localarametrization scheme which is based on turbulentkinetic energyand dissipation rate nonlinear equations, the forcing influence of the surface wind on the drift geostrophic currents and their contribution to the formation of the turbulent exchangein the oceanic shallow-water zone, the interaction betweenatmospheric boundary layer and oceanic surface. We divide atmospheric turbulentmomentum flux into two parts, responsible fordevelopment of thedrift currents and formation of the wind waves. The kinetic turbulent energy flux from the atmosphere to oceanic eddies definedby the collapse of the surface waves. The depth of intensive dissipation oceanic surface layer and atmospheric friction velosity are used for the calculation of the dissipation in this layer. This model describes more fully mechanisms of turbulent coastal dynamics in the oceanic boundary layer than any one developed so far and can be used for solving many applied problems of coastal oceanic dynamics.
OS41A-12 0830h
On a Mechanism for Sediment Resuspension by Solitary Internal Waves
Recent coastal measurements (for example JPO, vol. 27, pg. 1181) suggest that the passage of internal solitary waves can result in an increase in sediment resuspension. In this talk we will discuss numerical simulations of fully nonlinear internal solitary waves that illustrate one mechanism for sediment resuspension. In particular we will demonstrate that efficient sediment resuspension requires the presence of both a solitary wave and a background current directed against the direction of propagation. Furthermore we will show that, while the efficiency of the mechanism is profoundly affected by the wave amplitude, a wave with a recirculating core (or breaking wave) is not required. We will compare the present results to published theories on global instability in the bottom boundary layer beneath the solitary wave, and contrast the cases in which the solitary waves are waves of elevation and depression.