Evan B. Forde

Satellite Estimates of Wind Speed and Latent Heat Flux over the Global Oceans

Surface fluxes of momentum, freshwater, and energy across the air‐sea interface determine oceanic circulation and its variability at all timescales. The goal of this paper is to estimate and examine some ocean surface flux variables using satellite measurements. The remotely sensed data come from the European Remote Sensing (ERS) satellite scatterometer on ERS-2, NASA scatterometer (NSCAT), and several Defense Meteorological Satellite Program (DMSP) radiometers [Special Sensor Microwave Imager (SSM/I)] on board the satellites F10‐ F14. The sea surface temperature comes from daily analysis calculated from Advanced Very High Resolution Radiometer (AVHRR) measurements. This study focuses on the 9-month period (October 1996‐June 1997) of the NSCAT mission. To ensure high quality of the merged surface parameter fields, comparisons between different satellite estimates for the same variable have been performed, and bias corrections have been applied so that they are compatible with each other. The satellite flux fields are compared to in situ observations from buoys and ships globally and in different regions of the ocean. It is found that the root-mean-square (rms) difference with weekly averaged wind speeds is less than 2.5 m s21 and the correlation coefficient is higher than 0.8. For weekly latent heat flux, the rms difference between satellite and buoys does not exceed 30 W m 22. The comparisons with weekly ship latent heat flux estimates gives an rms difference approaching 40 W m 22. Comparisons are also made between satellite fields and atmospheric analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) and reanalyses from the National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR). The wind speeds and latent heat fluxes from these atmospheric analyses compare reasonably well with the satellite estimates. The main discrepancies are found in regions and seasons of large air‐sea temperature difference and high wind speed, such as the Gulf Stream during the winter season.

Reconnaissance in DSRV Alvin of a “fluvial-like” meander system in Wilmington Canyon and slump features in South Wilmington Canyon

Three dives in DSRV Alvin on the Atlantic Continental Rise in Wilmington and South Wilmington Canyons, off the east coast of the United States, allowed examination and sampling of morphological features, in water depths of 2,300 to 2,400 m, that were observed in midrange sidescan sonar data. In Wilmington Canyon, a “fluvial-like” meandering system was confirmed. The meandering channel had steep undercut outer banks and gently sloping inner banks. Localized slumping is inferred from many steplike depressions on the steep outer banks. Although the meander system is as much as 500 m wide and has many characteristics typical of a fluvial system, extreme depth and evidence of episodic ongoing sedimentary processes preclude a true fluvial origin. Currents of unknown origin—that is, downcanyon turbidity flows—appear to be the only agent capable of sculpturing the observed features. A meander system was not observed in South Wilmington Canyon. Channel-floor features, including deformed and displaced sediment, support a previous suggestion of large-scale slumping in the area. These sediments include upturned clay beds, disaggregated gravels, loosely bound gravel conglomerates in a reddish-brown matrix, and a tubular structure, resembling a tree-root cast, within a thinly bedded, reddish-brown sandstone.

Piezometer Probes for Assessing Effective Stress and Stability in Submarine Sediments

Multisensor piezometer probes were deployed at four different sites in the Mississippi Delta in water depths ranging from 13.5 to 43.6 m with sensor penetration depths of up to 15.6 meters. Absolute and differential pressure sensors were used to measure pore water pressure and excess pressures, respectively. The free water column pressure was measured with absolute pressure sensors. Pore pressures induced by probe insertion were determined as well as ambient excess pore pressures following the time-dependent decay of induced pressures. Significant differences in the pore pressures and related geotechnical properties were found between East Bay and Main Pass sediments. Generally higher probe insertion pressures and lower ambient excess pore pressures were characteristic of Main Pass compared to East Bay. Probe insertion pressures (Ui) were found to correlate well with the undrained shear strength (Su) of the sediments, indicating reasonably good agreement with the predicted relation: Ui = 6Su as suggested by an earlier study42. Using this relationship undrained shear strengths were calculated and compared with measured values.

Evolution of Veatch, Washington, and Norfolk Submarine Canyons: Inferences from strata and morphology

Abstract Investigations of Veatch, Washington, and Norfolk Submarine Canyons, on the eastern continental margin of the United States, resulted in a detailed assessment of their past and present geologic processes. Norfolk and Washington Canyons share such characteristics as similar levee geometry, stratigraphy, and mineralogy; Veatch Canyon differs from Norfolk and Washington Canyons in these respects. Seismic reflection profiles of all three submarine canyon heads indicate an erosional origin. Seismic profiles of Veatch Canyons lower continental slope indicate a formation by depositional processes; in contrast, the continental-slope profiles of Washington and Norfolk Canyons reveal a history of alternating episodes of deposition and erosion. Cored sediments from Washington and Norfolk Canyons disclose recent, intermittent, down-canyon transport of sand to the continental rise. Bathymetric and seismic profiles suggest Veatch, Washington, and Norfolk Canyons have all been subjected to a minimum of two periods of development. Similar axial trends of Washington and Norfolk Canyons imply a common structural influence.

Sediment transport in Washington and Norfolk submarine canyons

Abstract A sediment study suggests that Washington and Norfolk canyons off the Mid-Atlantic States are not inactive, but have served periodically since the Late Pleistocene as conduits of sediment originating on the adjacent shelf and upper slope. Large quantities of sand occur in the canyon heads as thin beds and laminae, and on the continental slope as mixtures of sand (to >40% ), silt and clay that are extensively reworked by burrowing organisms. Sandy turbidites occur in the canyons on the rise. Basinward dispersal, from the outer shelf and uppermost slope, is recorded by heavy mineral suites and bioclastic components, primarily foraminifera of shallow marine origin, in the lower slope and upper continental rise canyon cores. The down-axis movement of material, presumably episodic, in the Holocene to recent results from offshelf spillover into canyon heads, failure on the steep walls bordering canyons on the slope, and resuspension by bottom currents.

Analysis of Slope Stability, Wilmington to Lindenkohl Canyons, US Mid-Atlantic Margin

The continental slope gradient in the study area averages 7–8°. Many valleys, canyons and occasionally large sediment slumped masses occur. Moderate to steep slopes (19–27°) as well as very steep to precipitous slopes (> 27°) are abundant and occupy about 7% of the investigated area.

Biological disturbance and camouflage of sedimentary features on the Northeast United States slope and rise

Evidence from over 200 sediment cores, numerous submersible dives, and bottom photographs prove that bioturbation and bioerosion are ongoing processes affecting northeastern U.S. continental slope and rise sedimentation. Evidence of biological activity was found in greater than 95% of the cores examined. Submersible dive observations reveal that the results of biological activity often dominate sea-floor microtopography. Bioturbation can disturb sediments several centimeters deep in a matter of seconds and is in some areas the primary sediment transport mechanism. Many cores with sandy intervals were profoundly disturbed by bioturbation. Biologically camouflaged sand-rich intervals can easily be missed by visual observation.

Variability of Sedimentary Textures and Processes on Continental Margin North of Wilmington Canyon: ABSTRACT

Sedimentary properties and processes of a 7,500-km2 corridor seaward of the Baltimore Canyon Trough off New Jersey were studied in detail using over 100 bottom samples consisting of grab samples and box, hydroplastic, and piston cores. The sediments in both canyon and intercanyon areas are primarily bioturbated, olive-gray, sandy silts with local features indicative of gravity-induced mass sediment movements (i.e., graded sequences and load structures). C14-based sediment accumulation rates vary by a factor of three in cores separated by as little as 6 km. These variations seem to be a function of shelf-edge spillover rates. Detailed analyses of the sand grain-size distribution throughout the corridor reveal that the sand component of the slope and rise sediments contains a high percentage of material currently present on the adjacent shelf. The relatively large percentage of sandy sediment in the upper parts of cores from Spencer Canyon suggests its recent role in transporting shelf sediments seaward. Sandy intervals in other slope cores are commonly obscured by intense bioturbation. Cores from the continental rise show an upward decrease in the number of sand layers and lenses. Active transport of sediment to the slope and rise occurred during the late Pleistocene and, although the intensity has declined, the slope is presently the site of deposition for both fine and coarse sediment. End_of_Article - Last_Page 1662------------

Sedimentologic Processes in Wilmington and South Wilmington Submarine Canyons as Indicated from DSRV Alvin: ABSTRACT

End_Page 1671------------------------------Three submersible dives in DSRV Alvin were made on the continental rise seaward of Atlantic City, New Jersey: two in Wilmington Canyon and one in South Wilmington Canyon. The dives, made on features observed in midrange sidescan sonar records reveal a sharp contrast in physiography between the two canyons and provide an insight to the sedimentologic processes associated with canyon development. On the upper rise, Wilmington Canyon is characterized by a well-developed meander pattern with alternating steep and gently sloping side walls. The steep channel walls correlate with the outside (concave) part of a meander bend and are segmented by numerous steep-walled, steplike depressions. Therefore, undercutting at the base of the channel wall, accompanied by localized slumping, is inferred. These depressions are missing on the gently sloping channel walls. Unconsolidated sediment, in excess of 1.3 m thick, is present in the channel axis but is generally missing in the depressions on the channel wall. The high quartz sand content (30%) in the channel sediments and the paucity of unconsolidated sediment in the depressions indicate recent activity. In South Wilmington Canyon various deformed and displaced(?) sediments support a previous suggestion of large-scale slumping. These features include upturned clay horizons at the base of the wall, stratigraphically overlain by disaggregated gravels, a loosely bound gravel conglomerate, and oxidized sandstone horizons. A tubular object 1 m long, tentatively interpreted as a tree root cast, was recovered from the sandstone. A meander pattern was not found in South Wilmington Canyon. End_of_Article - Last_Page 1672------------