Ocean Sciences General Contributions I Posters
Presiding: B Mullenbach, Texas A&M University; R Hetland, Texas A&M University
OS11A-01 0830h
Topographic Rossby Waves in a Z-Level Ocean Model
Topographic Rossby waves (TRW) play an important role in the ocean dynamics in regions where the slope of the bottom topography is sufficiently large so as to dominate the β -effect. TRW propagate signals in the ocean along the slopes over large distances [Oey and Lee, 2002]. Pacanowski and Gnanadesikan [1998] suggested that when the bottom slope is less than the grid cell aspect ratio (Δz/Δx) in a numerical ocean model with a Z-level vertical coordinate, the model does not accurately resolve topography leading to an inaccurate simulation of topographic waves with a modified dispersion relation. It is important to better understand the consequences of the modelers' choices of vertical grids since coarse vertical resolution ocean models can alter the rate and direction of propagation of waves. Using Navy Ocean Coastal Model, we examine the ability of Z-level models to accurately simulate topographic waves by conducting model experiments with σ-level (terrain following) and Z-level vertical coordinate systems with different vertical resolutions. We will show that Z-level models can accurately reproduce TRWs if the bathymetry is well resolved in the model. Poor vertical resolution leads to an unrealistic representation of the bathymetry, which initiates different waves with different dispersion relations resulting in errors of signal propagation in the model. Also the necessary criteria for the resolution of bathymetry in Z-level models for properly simulating TRW is suggested.
OS11A-02 0830h
Hypoxia in The East China Sea: One of The Largest Hypoxic Areas in The World
Anoxia and/or hypoxia have been widely observed in estuarine, coastal regions in the past few decades, however, there are limited reports focused on the East China Sea (ECS). The ECS is one of the largest continental shelves in the world. It receives significant amounts of allochthonous organic carbon and inorganic nutrients from riverine run-off, especially from the Yangtze River. In June and August 2003, two cruises sampled at stations that covered almost the entire ECS to exam temporal and spatial variations in hypoxic events, and their potential causes. Results showed that, in August, an area estimated at greater than 2 x 104 km2, with DO concentrations < 2-3 mg l-1, extended from the Yangtze River plume ca. 400 km offshore and ca. 300 km southward along the coast. Vertically in the water column, DO became deficient below the mixed layer, ca. 10-20 m, to the bottom in this hypoxic area. In June however, DO concentration in the water column was higher than 4 mg l-1 in most of the shelf region. When the hypoxic zone was compared between these two months, Chl a concentration in the euphotic zone was much higher in June (6.1 mg Chl m-3) than that in August (0.8 mg Chl m-3). Similar to the Chl a comparison, dissolved inorganic nitrogen was also higher in June than in August with values of 9.13 Μ M and 7.48 Μ M, respectively. Physically, a very sharp density gradient was observed under the mixed layer in August. Results suggest that hypoxia in the ECS might due to water column stratification, restricting vertical re-aeration, and increasing rates of respiration and bio-degradation of residual organic material, which result in a warmer sea floor water temperature.
OS11A-03 0830h
210Po Enrichment Relative to 210Pb in the Planktons Collected from the Northern South China Sea
Plankton samples have been collected by net-towing within the top 100m layer from the northern South China Sea (SCS) mainly during a summer (July, 2003) and a winter (February, 2004) cruises. Each of the plankton samples was totally digested with a known 209Po spike and stable Pb as yield tracer, and Po isotopes were auto-plated onto a silver disc for alpha assay of its 210Po activity. The sample was then purified for 210Pb determination by either a second alpha assay of its in-grown 210Po or counting its in-grown 210Bi activity, the energetic beta emitter of 210Pb daughter. 210Po activity was decay-corrected to the sampling date based on the 210Pb activity. The results show significant temporal and spatial variations of 210Po and 210Pb, especially the former. 210Po in the planktons range from about 100 to 350 dpm/g, while 210Pb varies generally between 10 and 20 dpm/g, indicating large enrichment of 210Po relative to 210Pb with 210Po/210Pb ratios being generally about 10 to 15. This is due to a close affiliation of 210Po with the planktons and a repulsion of 210Pb by the planktons. At some of the locations where the particulate 210Po and 210Pb were measured from the filtered samples whose organic matter (POM) contents were estimated (25 to 35 percent), the partition of 210Po and 210Pb in both the planktons and non-planktons (inorganic or aluminosilicate materials) can be evaluated. The 210Po activity ratio between planktons and non-organic matters varies from about 1 to 3, while the corresponding 210Pb ratio is about 0.03 to 0.05. This suggests that 210Po is preferentially enriched in the planktons by a factor up to 3 but 210Pb is strongly repulsed by the planktons, i.e. only 3 to 5 percent of the particulate 210Pb is associated with the planktons. The planktons collected from the area southwest off Taiwan show that the 210Po/210Pb activity ratio is higher, at about 30, with much lower 210Po and 210Pb activities, i.e. 45 and 1.6 dpm/g, respectively. In this area, 210Po in the planktons is about 5 to 8 times high than in the non-organic particulates, but 210Pb in the planktons remains at about 4 percent of the non-organic particulates. Large variation of 210Po and 210Pb activities observed on the planktons collected from different areas may be due to different types and amounts of the planktons and varying particulate matter properties, such as the abundance and size distribution.
OS11A-04 0830h
228Ra and 226Ra Profiles from the Northern South China Sea
We previously reported the distributions of 228Ra and 226Ra in the northern South China Sea (SCS) which showed that both nuclides in surface waters were much higher than those in the open oceans because the SCS was enclosed mostly by landmasses which are known as sources of these nuclides. Large temporal and spectial variations were also observed probably due to the monsoons and intrusion of the Kuroshio Current. During a recent cruise conducted in the northern SCS in February, 2004, three vertical 228Ra profiles were measured by gamma spectrometry on the Ra isotopes which were concentrated first by the MnO2-impregnated acrylic fiber and then acid-washed as sample solution for counting. The two deep water 228Ra profiles are remarkably similar, showing high values in the surface layer and fairly uniform at about 10 to 13 dpm/100L below 200m depth but with a clear increase toward the bottom due to input from the underlying sediments. The shallow water profile on the shelf shows higher 228Ra values due to both vertical and horizontal mixing of the shelf water with additional source from the shore zone. Additional 228Ra profiles measured on samples from earlier cruises show that the deep water values may differ significantly (up to 5 dpm/100L) at the same location in different seasons or cruises. The associated 226Ra profiles are also variable but quite comparable to those in the northwest Pacific in deep water. 226Ra activities in the shallow water (less than 1000m depth) are higher in the SCS than in the open oceans. The 228Ra/226Ra activity ratios vary mostly from about 0.3 to 0.5 in the deep water. These values are much higher than those in the open oceans which are generally less than 0.1.
OS11A-05 0830h
Particulate and Dissolved 210Po and 210Pb in Water Columns from the East and West of the Luzon Strait
The Luzon Strait between Taiwan and the Luzon Island is where the Kuroshio intrudes into the South China Sea (SCS). Its deep sill depth (about 2200m) also allows transport and exchanges of the deep water and its properties between the SCS and the West Philippine Sea (WPS). During a recent cruise (October, 2004) conducted for biogeochemical studies in the Luzon Strait area, water samples were collected and filtered on board for measurements of 210Po and 210Pb in both the particulate and dissolved phases in attempt to decipher the transport and exchange processes via the distribution of these and other radionuclides, including 226Ra and 228Ra. This paper is to focus on the 210Po and 210Pb distributions at a location east of the Strait (St. 5) along the Kuroshio path and a location west of the Strait (St. F) in the northeast SCS. 210Po was determined by alpha counting of the auto-plated Po isotopes, including a known 209Po spike and the 210Po from the sample. 210Pb was then determined on the same sample by re-purifying for either alpha counting of the re-grown 210Po or beta counting of 210Bi, the energetic beta emitter of 210Pb daughter. Preliminary results show that the dissolved 210Po is higher at St. F than at St. 5 within the upper 1000m layer with a maximum at both stations around a few hundred meters depth, suggesting a contribution from dissolution on decomposition of sinking organic particulates. Below 1000m depth the dissolved 210Po is lower at St. F than at St. 5, but its values are fairly constant with depth at each station. The particulate 210Po at St. F is fairly uniform at about 8 dpm/100kg which is much higher than any open ocean deep water values and is also higher than our previous results (about 5 dpm/100kg). As the particulate concentrations vary between about 0.1 and 0.3 mg/kg, the specific 210Po activity of these particulates may range from 270 to over 1000 dpm/g. Results on 210Pb in both phases of these samples will be presented together with the corrected 210Po data for the discussion of their status of radioactive disequilibrium at the Joint Assembly Meeting.
OS11A-06 0830h
210Pb and Mass Flux Imbalance Between the Settling Particulates and Sediments at a Sediment Trap
In order to evaluate the mass balance problems between the settling particulates and the underlying sediments, sediment traps were deployed at M1 site (21o32'N, 119o28'E; 2948m) where a box core was taken recently. The sediment trap results were previously published (Chung et al., 2004: Continental Shelf Research). We report here the 210Pb profile measured on this box core by determining its daughter, 210Po, with alpha spectrometry, assuming the paired nuclides are in radioactive equilibrium. The box core (33 cm long, taken in June, 2004) was analyzed for the distributions of its water content, loss on ignition (LOI), and 210Pb. The water content decreases with depth and averages about 55 percent; the LOI as a measure of the total organic matter (TOM) is fairly constant at about 12 percent. The 210Pb profile shows a general exponential decrease with depth, yielding a maximum sedimentation rate of 0.26cm/y, applying a constant flux and constant sedimentation rate model. This rate translates to a mass flux of 0.31g/cm2/y or about 8.5g/m2/d with a dry bulk density of 1.2g/cm3. Based on the excess 210Pb inventory integrated over the core length, the 210Pb flux is estimated at about 19.2dpm/cm2/y or 526dpm/cm2/d assuming at steady state. The 210Pb and mass fluxes obtained from the deepest trap at M1 were only 129dpm/m2/d and 0.55g/m2/d, respectively. The large "excess" of the 210Pb and mass fluxes in the sediments over those measured from the sediment trap (4 times in 210Pb flux and 14 times in mass flux) suggests either the sedimentation rate was overestimated due to neglect of the mixing effect, and/or large additional particulates, as resuspended sediments which contain less 210Pb, have been transported laterally near the bottom from elsewhere.
OS11A-07 0830h
Lava Morphology Classification of a Fast-Spreading Ridge Using Deep-Towed Sonar Data: East Pacific Rise
Classification of lava morphology on a regional scale contributes to the understanding of the distribution and extent of lava flows at a mid-ocean ridge. Seafloor classification is essential to understand the regional undersea environment at midocean ridges. In this study, the development of a classification scheme is found to identify and extract textural patterns of different lava morphologies along the East Pacific Rise using DSL-120 side-scan and ARGO camera imagery. Application of an accurate image classification technique to side-scan sonar allows us to expand upon the locally available visual ground reference data to make the first comprehensive regional maps of small-scale lava morphology present at a mid-ocean ridge. The submarine lava morphologies focused upon in this study; sheet flows, lobate flows, and pillow flows; have unique textures. Several algorithms were applied to the sonar backscatter intensity images to produce multiple textural image layers useful in distinguishing the different lava morphologies. The intensity and spatially enhanced images were then combined and applied to a hybrid classification technique. The hybrid classification involves two integrated classifiers, a rule-based expert system classifier and a machine learning classifier. The complementary capabilities of the two integrated classifiers provided a higher accuracy of regional seafloor classification compared to using either classifier alone. Once trained, the hybrid classifier can then be applied to classify neighboring images with relative ease. This classification technique has been used to map the lava morphology distribution and infer spatial variability of lava effusion rates along two segments of the East Pacific Rise, 17 deg S and 9 deg N. Future use of this technique may also be useful for attaining temporal information. Repeated documentation of morphology classification in this dynamic environment can be compared to detect regional seafloor change.
OS11A-08 0830h
Distribution and Mass Inventories of p,p'-DDE and o,p'-DDE in the Water Column of the Southern California Bight, California
As part of the Southern California Bight(SCB) 2003 Regional Marine Monitoring Survey (Bight' 03), we examined the concentrations and distribution patterns of p,p'-DDE and o,p'-DDE in the water columns at randomly-selected sites throughout SCB using solid phase microextraction (SPME) technique. The technique involves deployment of polydimethylsiloxane (PDMS)-coated fibers at certain depths of the water column for a prolonged time (3 weeks or longer). The fibers were then retrieved and analyzed by GC-MS, and water column concentrations of DDTs were calculated based on an equilibrium partitioning between water and PDMS coating. Our results showed that p,p'-DDE concentrations were in the range of 0.05-0.22ng/L, and 0.005-0.036 for o,p'-DDE. For p,p'-DDE, there is a general increasing trend toward the sediment-water interface. Santa Monica Bay contains the highest concentrations of both p,p'-DDE and o,p'-DDE, indicative of the influence of `hot spots' at Palos Verdes Shelf. Concentrations of DDTs were lower in San Pedro Shelf and Santa Barbara Basin, and were mostly under detection limit in other areas. The results showed that water column DDT concentrations have decreased significantly compared with previous investigations, but a constant source of DDTs from sediment to the overlying water column is still significant. The spatial distribution patterns of water column p,p'-DDE and o,p'-DDE suggest that the Palos Verdes Shelf remains a dominant source of DDT contamination to the SCB. Based on the spatial data, we estimated that the total mass of DDTs in the entire SCB is rather low ( under 1 kg). The annual loss of DDTs from SCB to the open ocean should be about tens of kg. Based on the historical trend of the DDT input into SCB since the 1970s, SCB should have contributed considerable amount of DDTs to the open ocean.
OS11A-09 0830h
Temporal Variability of Gulf Stream Warm and Cold Ring Formation (1975-2004)
Weekly maps and satellite images of Gulf Stream position were obtained and analyzed to compile a record of Gulf Stream ring formation. The record begins in 1975 and the 30 year dataset consists of nearly 400 warm and cold core rings. Warm core rings were tracked to document their lifespan and the location they were reabsorbed by the Gulf Stream. Cold core rings could not be tracked since their locations are often lost on the sea surface temperature images as the rings drift far from the Gulf Stream. Observations indicate a decrease in the number of rings formed over the last decade. Statistical analysis indicates this decrease could be part of a 12-14 year cycle in ring formations.
OS11A-10 0830h
Monthly Variation of Taiwan Strait Through-flow Transports and Associated Water Masses
Through-flow transports and associated water masses are analyzed using current data measured by bottom-mounted and ship-board ADCP (1999-2001) across the central Taiwan Strait and strait-wide hydrographic data acquired from 79 CTD survey cruises (1986-2003). The East Asian monsoon, from southwest in July to August and northeast in October to March, controls the transport fluctuation which peaks in August (2.34 Sv northward), is hampered by the northeast monsoon after September and diminishes to the minimum (0.26 Sv southward) in December. The standard deviation of the calculated transport ranges from 0.56 to 1.05 Sv during northeast monsoon months and is relatively small in other months. A cluster analysis together with conventional T-S diagrams identifies the saline and warm Kuroshio Branch Water (KBW), the less saline South China Sea Surface Water (SCSSW), the brackish and cold China Coastal Water (CCW), the saline Subsurface Water (SW) (depth > 100 m) and the Diluted Coastal Water (DCW). The majority of the northward transport in summer carries the SCSSW to the East China Sea. Meanwhile, the DCW appears off the northwest bank of the strait and the SW resides in the bottom layer of a deep trench in the southeastern strait. The onset of the northeast monsoon in September drives the CCW from the Yangtze river mouth to the northern strait. In the southern strait, the northward-moving KBW replaces the SCSSW and meets the southward-intruding CCW in the middle strait during November to April.
OS11A-11 0830h
Development & Application of the SEDCON Index for Resource and Risk Assessment of Coral Ecosystems
Resource managers need inexpensive bioindicators to evaluate the health of coral ecosystems and to guide decisions on when and where to utilize more expensive assessment techniques. Following USEPA Guidelines for Evaluating Ecological Indicators, we are developing a rapid-assessment protocol, termed the SEDCON Index, that utilizes reef sediment constituents to assess the integrity of coral-reef communities. The underlying assumption of the index is that community structure is reflected by proportions of recognizable remnants of calcareous shells and skeletal remains of autotrophic (calcareous and coralline algae), mixotrophic (zooxanthellate corals and larger foraminifers) and heterotrophic (e.g., bryozoans, molluscs, smaller foraminifers) benthic organisms, and of unrecognizable debris as a proxy for bioerosion. Implementation and assessment of this diagnostic tool is currently underway for the Florida Middle Grounds (FMG) and the Coral Reef Monitoring Program (CRMP) sites in the Florida Keys National Marine Sanctuary (FKNMS). We are calibrating SI data with benthic community data from CRMP. Data from samples collected in FKNMS between 2001 and 2004 indicate dominance of the sediments by bioerosional debris, while samples collected in the early 1980s from the FKNMS and the FMG in 2003 contained higher proportions of identifiable mixotrophic components. Application of the SI provides an assessment of ecosystem condition on scales of years and will be used to supply reef baseline data for detecting changes associated with the Comprehensive Everglades Restoration Plan.
OS11A-12 0830h
Finding Persistent Residual Eddies in Complex Tidal Channels
Natural tidal channels are often complicated and as a result eddies may form at different locations within the channels. With a strong tidal forcing, such eddies may change periodically at tidal frequencies. When the tidal forcing dominates other forces that can generate similar eddies, the tidally averaged flow field will appear to have persistent eddies that are apparently associated with the goemorphology of the channels. These tidally induced residual eddies are practically results of asymmetric flow conditions at different locations within the channels such that the flood and ebb flows are sharply different. In a series of surveys conducted in Wassaw Sound with a complicated tidal creek/channel system, we have tried to resolve the tidally induced residual eddies and explain their existence in relation to their locations. These surveys, though only cover one tidal cycle each time, show that the major eddies are quite persistent in their general characteristics, despite the fact that some fine structures may vary from survey to survey. These circulation conditions should have significant implications regarding the transport of waterborn materials, water quality, and ecology.
OS11A-13 0830h
Observational Evidence of Alternating Zonal Jets in the Ocean
Two-dimensional turbulence freely evolving on a rotating sphere is predicted by theory to form zonally oriented alternating jets. Such jets are well seen in atmospheres of Jupiter and Saturn as bands of clouds, but it is not certain whether they exist in the Earth's ocean with its complex forcing, boundary conditions and baroclinicity. Interpretation of quasi-periodic in latitude alternating currents on many meridional sections is complicated because of the lack of information about zonal extent of the structures. It is only recently that the newest global eddy-resolving numerical models of ocean circulation provided multi-year outputs suitable for the study of long-term evolution of realistic eddy field. Their data do reveal persistent, slowly varying zonal jets in the deep ocean. In this study, we succeeded to extract the signal of time-varying alternating zonal jets from maps of the Aviso sea level anomaly based on satellite altimeter data. The jets are ubiquitous and demonstrate complex spatial distribution of their characteristics and interesting tendencies of their temporal evolution. Data of the OGCM for the Earth Simulator (OFES) forced for 50 years by climatological fluxes are used to prove that jets' velocity is highly coherent in vertical and that the signal extracted from the satellite altimeter data is directly relevant to the deep jets. Results of preliminary comparison with the theory are discussed.
OS11A-14 0830h
Marked Shore-Parallel Variability in Holocene Sediment Thickness Offshore La Jolla, California
New CHIRP seismic data indicate that the bathymetry and subsurface structure in the nearshore are not well correlated offshore La Jolla, CA. For example, the thickness of the Holocene transgressive sequence varies along strike in the region from less than 1 m thick offshore Torrey Pines State Reserve in the north to greater than 20 m farther south between the Scripps and La Jolla Canyons. This thickness variability appears to be structurally controlled (i.e., by relief on the transgressive surface). Existing theory and observations show longshore transport of sand in the Oceanside littoral cell occurs from north to south. In the littoral cell sediment is transported by waves and currents and exits the nearshore via the La Jolla submarine canyon system. However, the observed variability in shore-parallel sediment thickness cannot simply be explained by longshore transport. The seismic data, collected in April and November 2002, reveals three areas of relatively thick Holocene sediments: 1) between La Jolla and Scripps canyons, 2) directly north of Scripps Canyon, and 3) within the La Jolla Canyon system on small "inter-canyon highs" or shoals between branches of the canyon. The sediment thickness on the inner shelf systematically thins toward the north away from the canyons. In fact, the observed variability of sediment thickness in the shore-parallel direction (south - north) is larger than that observed cross-shelf (east - west). The spatial correlation of the shoaling transgressive surface and the offshore bathymetric high, together with a left-lateral jog in the right-lateral Rose Canyon Fault in this region, suggest a tectonic control on the shelf morphology that in turn controls Holocene sediment thickness offshore of La Jolla. Convergence as a result of fault geometry engenders uplift and shoaling of the transgressive surface, which also has a bathymetric expression offshore. If correct, this would imply that processes such as wave climate control sediment transport and short-term deposition, but long-term accumulation is tectonically controlled by strain accommodation along the Rose Canyon Fault system. Mount Soledad to the south is another example of pop-up structure associated with a left-lateral jog along the Rose Canyon Fault. We will use the depth to the transgressive surface and the overlying Holocene sediment thickness as inputs to our model to determine the relative importance of the multiple forcing mechanisms on nearshore sediment transport and distribution. We ultimately intend to use this modeling to illuminate the relationship between short-term processes and strata formation.
OS11A-15 0830h
Clay Mineralogy of a Pleistocene Barrier Island, Skidaway Island, GA
A dense blue clay underlies various marsh localities near the Skidaway Institute of Oceanography (SKIO). SKIO is located on a Pleistocene barrier island in Georgia. The clay is found in two cores drilled at the high marsh/maritime forest transition zone. It was recovered at a depth of one meter, below unconsolidated marsh sediment. A total of 12 clay samples were collected at 10cm intervals downcore beginning at 100cm, through the clay layer. The thickness of the clay layer is unknown due to core refusal at a heavily iron oxide mottled zone. Samples were examined for the presence of diatoms and foraminifera, neither of which were found to be present, thereby suggesting a non-marine environment, although dissolution may be a factor. Geochemical data also lacks a definite marine signature, thereby confirming the micropaleontological conclusions. Smectite, illite, and kaolinite were found throughout both cores with halloysite present only below a depth of roughly 120cm. These particular clay minerals have been identified in blue clays analyzed in the Carolina Bays of southern North Carolina. Halloysite is typically a metastable phase forming from the weathering of illite to kaolinite, thus the location of the halloysite found in these cores indicates more recent weathering of the parent illite at depth. This is unusual as weathering is normally thought to be more intense closer to the surface. Vermiculite, which is also part of the general weathering scheme of mica, was not identified in the cores based on the behavior of the samples after solvation in ethylene glycol and potassium-saturation. The observed clay assemblage is similar to that of North Carolina Carolina Bays. Carolina Bays are found from Virginia to Georgia, and are found to contain blue clays (Ingram et al., 1959). Morphologically, Carolina Bays are typically elliptical and while the study area is more rounded, this could be due to its modification by an active tidal system.
OS11A-16 0830h
A Complementary Error Model for a Finite Difference Linear Barotropic Ocean Model
We construct an error model to complement a finite difference barotropic ocean model. We discuss and illustrate how this error model augments the simulation to provide a more comprehensive interpretation of the model results. Our method of construction works for systems of partial differential equations numerically approximated with finite differences. We transform the truncated portions of fully dimensional Taylor Series term expansions to frequency space. The transformation replaces the higher order, unknown and un-modelled partial derivatives in physical space with harmonic components in frequency space whose impact can be understood within the approximation space associated with a given numerical grid. The result is a complementary error model for a given numerical approximation running on a given grid. The construction of the error model requires the transformation of the function itself, which may be estimated from data sources. Alternatively a test function may be used. The technique works whenever a Taylor Series representation of a function is available. We note that the Taylor Series remainder term may also be used to produce an error bound for our error model.
OS11A-17 0830h
Impact of Mean SSH on Synthetic Profiles and Data Assimilation: Currents and Transport in Global NCOM
Four year-long parallel experiments were performed on variations of the Naval Research Laboratory's (NRL) global ocean modeling system. This system has three main components: the global NRL Layered Ocean Model (NLOM), the global Navy Coastal Ocean Model (NCOM), and the Modular Ocean Data Assimilation System (MODAS). By targeting the strengths of each of these elements, the combined system has improved coverage, resolution and skill to provide global analyses, forecasts and boundary conditions. Our experiments examine changes in the derivation of synthetic temperature and salinity profiles for data assimilation. We evaluate the global impact of these variations on model simulations by comparison with unassimilated vertical temperature profiles and WOCE drifter trajectories. We demonstrate that improved resolution of sea surface height (SSH) and better treatment of the differences between MODAS and NLOM mean SSH leads to more accurate NCOM model results.
http://www7320.nrlssc.navy.mil/global_ncom/
OS11A-18 0830h
Directional Wavelet Analysis of Remotely Sensed Surface Wave Fields
Modern measurement techniques such as Aerial Laser Scanning allow for rapid determination of the spatial variation of sea surface elevation. Wave fields obtained from such data show a level of inhomogeneity associated with the presence of wave groups. As such spatial variability cannot be adequately represented by a Fourier wavenumber spectrum, a method based on 2-dimensional directional wavelet analysis is described by which such inhomogeneity can be characterised in the spatial and wavenumber domains. The directional wavelet method has been applied to Aerial Laser Scanning measurements of nearshore wave conditions off the east coast of New Zealand's South Island. A high level of spatial variability was observed, with evidence of ensembles of wave group envelopes of quasi-Gaussian form.
OS11A-19 0830h
Intermediate-Depth Currents Estimated From Float Measurements in the Gulf of Mexico
Data from 17 PALACE floats set in the Gulf of Mexico sampling the intermediate-depth (~ 900 db) flow from April 1998 to February 2002 indicate a mean cyclonic circulation along the northern, western and southwestern edges of the Gulf of Mexico. This flow intensified into a ~ 0.10 m/s current in the western and southern Bay of Campeche and was deflected around a topographic feature, called here the Campeche Bay Bump, in the southern Bay of Campeche. Associated with this intensified flow was a small cyclonic gyre in the southwestern Bay of Campeche. Floats launched in the eastern Gulf of Mexico tended to stay there and those launched in the western Gulf of Mexico tended to stay in the western Gulf of Mexico suggesting restricted connection at depth between the eastern and western Gulf of Mexico. The current estimates made neglecting non-900 db depth drifts before first-surface fix and drifts after last-surface fix were 10% larger than those which took into account these drifts. Most of this (8%) was due to neglect of the surface drift before first and after last fix. Except for stronger flow below the Loop Current and Loop Current warm-core rings, no other pattern was seen between the intermediate depth flow and the surface flow.
OS11A-20 0830h
A Late Holocene Record of Human Impact in the Tropical Lowlands of the Mexican Gulf Coast: Lago Verde.
Lago Verde is a maar lake (18 36 43 N; 95 20 52 W) located on the Gulf Coast of Mexico in "Los Tuxtlas" region. The area was cover by tropical rain forest and is part of the core area of the earliest Mesoamerican cultures. A 6 m sediment core was obtained in order to document vegetation and lake level history of this area. Lago Verde is a shallow, eutrophic lake (max. 4 m), the natural vegetation has been removed and grasslands with some tropical trees such Bursera grows around the lake. According with the radiocarbon chronology the sequence covers the last 2500 yr BP. At the base of the sequence low abundance of tropical trees is record, with intermediate lake levels. A sudden change in the pollen stratigraphy occurs at ca. 2000 yr BP, with important presence of Poaceae, Ambrosia and Cheno.-Am. along with Zea mays indicating human activity in the area. This is associated with a change in limnological conditions, recording turbid, shallow environments. This pollen signals correlates with dry phases in Yucatan, suggesting that this dry climatic signal probably had effect on an ample area of Mexico. However, at 1200 yr BP, no more Zea mays pollen is recovered suggesting the abandonment of the area. Lake levels recover as well as the tropical forest. The last 150 yr BP is characterized by the reduction in the pollen of tropical forest trees, presence of Zea mays, increased erosion rates, turbidity and eutrophication in the lake, all related to deforestation.
OS11A-21 0830h
In the Land of the Sky: Recent Paleoenvironmental Research From Coastal Oaxaca, Mexico
The Lower RÃo Verde Valley of Oaxaca has had a long and complex cultural history in part shaped by significant landscape change that ocurred 2300 years ago, when the RÃo Verde river changed morphology from a meandering to a braided form. These changes were precipitated by anthropogenic landuse impacts in the highland valleys of Oaxaca over 125 linear km to the north. While the lower valley's geomorphic history is well studied, little is known of its paleoecology. In order to reconstruct the history of vegetation, climate, and associated land use change, sediment cores were raised from several sites throughout the region. We present stratigraphical, palynological, and charcoal data from three sites in the region. The lower drainage basin consists entirely of the Verde's coastal valley. The climate of the lower RÃo Verde Valley is hot and humid with mean annual rainfall of 1000 mm to 2000 mm and average temperatures range from 25°C to 28°C. We discuss the pollen and stratigraphic record from Laguna PastorÃa which is a brackish estuary protected from the Pacific Ocean by a roughly east-west trending bay barrier. The bay barrier is about 500 m wide and 2 to 4 m high. Low scrub vegetation (cacti, thorny bushes, small trees and palms) grow on the barrier. The lagoon itself is approximately 9 km long and varies in depth with a 3-4 m maximum. Tides are microtidal (1 m). The lagoon supports a diverse array of mangroves (Rhizophora mangle, Laguncularia racemosa, Conocarpus erectus and Avicennia germinans). Two sediment cores were raised from the lagoon (LP1 and LP2) and provide a record of hurricane strikes and possible changes in the frequency of El Niño's. The LP1 core covers approximately the last 5000 yrs. Preliminary pollen analysis indicates that pollen is in excellent condition and is diverse (>60 taxa). Zea mays pollen was identified from sediments dating to the early Formative period (~ 3600 yr ago). The charcoal records analyzed from 2 paleomeanders of the RÃo Verde are also discussed. The sediments from these sites are dominated by clays and have a low organic content, pollen preservation was poor. However, microscopic and macroscopic charcoal are well preserved. These data provide a long term record of changing fire history for the region as far back as the late Archaic Period.
OS11A-22 0830h
Interaction of Finite Volume Gravity Currents With a Two-layer Stratified Interface
An experimental study of two-dimensional gravity currents impinging on a stratified interface in a two-layer stratified environment has been conducted. The gravity currents are created by the release of a fixed volume of dense fluid along a 6° inclined boundary. The effect of the stratified interface on the entrainment and mixing processes is quantified by the use of Planar Laser Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV). The instantaneous velocity and vorticity fields are quantified and averages are computed over 0.2 seconds. For both experimental techniques, the laser sheet is positioned at mid-span and extends in the streamwise direction. This allows for the measurements to be centered on the impact region between the gravity current and the stratified interface. We have previously determined the entrainment rate and studied the internal structure of a gravity current created by a continuous source with similar experimental conditions (for the slope and the ambient stratification) as in the current study. A thorough comparison of the two cases is provided.