Timothy R. Keen

A coupled hydrodynamic−bottom boundary layer model of Ekman flow on stratified continental shelves

Abstract This paper describes a hydrodynamic model with turbulent energy closure that uses a simplified wave-current interaction model of the bottom boundary layer to compute bed drag coefficients. The coupled model is used to investigate the interaction of the upper and lower boundary layers with the geostrophic core flow for simple shelf geometry and forcing, and to evaluate the effects of increased bottom friction on coastal hydrodynamics for summer and winter stratification. The thickness of the bottom boundary layer predicted by the model ranges from 10 to 35 m and is consistent with observations from the California shelf. The increased bottom friction calculated by the coupled model in intermediate water depths increases bottom Ekman veering (leftward in the Northern Hemisphere) by as much as 10° if stratification is strong, thus enhancing downwelling and upwelling. Currents along isobaths in shallow water are uniformly decreased by as much as 25% in the coupled model for both summer and winter init...

The interaction of southwest monsoon upwelling, advection and primary production in the northwest Arabian Sea

Abstract A biological model comprising phytoplankton, Zooplankton, detritus, and nitrogen pools has been coupled to a 3-layer reduced-gravity ocean circulation model of the Arabian Sea through the vertical mixing and horizontal transport fields. Coupled biophysical interactions during the southwest monsoon have been investigated using experiments with progressively more physical forcing: (1) initial-value biological calculations with no forcing; (2) upwelling simulations for the southwest monsoon that incorporate vertical forcing only; and (3) coupled physical-biological model experiments that use both upwelling and horizontal advection, and are forced by monthly varying climatological wind stresses and density profiles for the Arabian Sea. The initial-value and upwelling experiments indicate the roles of grazing pressure and remineralization of detritus when physical forcing is present. The coupled model experiments demonstrate the importance of coastal upwelling and offshore advection in determining patterns of total nitrogen in surface waters, both near the coast and offshore. Offshore jets are significant as sources of both transported nutrients and biomass in open waters far from coastal upwelling areas.

A Coupled Hydrodynamic–Bottom Boundary Layer Model of Storm and Tidal Flow in the Middle Atlantic Bight of North America

Abstract The effects of increased friction and tides on circulation in the Middle Atlantic Bight (MAB) during the SWADE storm of 25–28 October 1990 have been investigated using a three-dimensional hydrodynamic model coupled to a bottom boundary layer model that calculates combined wave–current bottom drag coefficients. Winds were initially parallel to the coast (downwelling favorable) throughout the MAB, first shifting to offshore within the central MAB and then in the northern MAB, while remaining parallel to the coast within the southern MAB. The wind-driven circulation was approximately alongshore, with an onshore component at the surface and an offshore component at depth associated with downwelling. Compared to model runs with a pure current bottom friction formulation, the additional bottom friction in the coupled model decreased currents uniformly in shallow water and caused slight offshore rotation during downwelling circulation, but the effects were limited because of the persistent stratificatio...

The origin and preservation of a major hurricane event bed in the northern Gulf of Mexico: Hurricane Camille, 1969

Cores collected from Mississippi Sound, in the Gulf of Mexico, were studied using 210 Pbxs and 137 Cs geochronology, X-radiography, granulometry, and a multi-sensor core logger. Our results indicate the presence of a widespread sandy event layer that we attribute to Hurricane Camille (1969). An initial thickness of more than 10 cm is estimated from the cores, which is large compared to the time-averaged apparent accumulation rate of 0.29^0.47 cm y 31 . Physical and biological post-depositional processes have reworked the sandy layer, producing a regional discontinuity and localized truncation, and resulting in an imperfect and biased record of sedimentary processes during the storm. The oceanographic and sedimentological processes that would have produced an event bed during Hurricane Camille are simulated using a series of numerical models, i.e. (1) a parametric cyclone wind model, (2) the SWAN third-generation wave model, (3) the ADCIRC 2D finite-element hydrodynamic model, and (4) a wave^ current bottom boundary layer model that is coupled to transport and bed conservation equations (TRANS98). The simulated bed ranges from 5 cm to over 100 cm within a tidal channel near the barrier islands. Seaward of the islands, the bed is more than 10 cm in thickness with local variability. The magnitude and local variability of the storm bed thickness are consistent with the observed stratigraphy and geochronology on both the landward and seaward sides of the barriers. @ 2002 Elsevier Science B.V. All rights reserved.

Shallow water currents during Hurricane Andrew

Oceanographic measurements are used in combination with a numerical model to examine the influence of stratification on shallow water currents during the directly forced stage of a tropical cyclone (Hurricane Andrew) on the continental shelf. The following stratification-dependent coastal processes are examined: (1) turbulent mixing, (2) coastally trapped waves, (3) near-inertial oscillations, and (4) upwelling and downwelling. Turbulent mixing was strong within 1 Rw (radius of maximum winds) of the storm track, and stratification was nearly destroyed. Turbulent mixing was weak at distances greater than 2 Rw. The dominant coastal wave was a barotropic Kelvin wave generated as the storm surge relaxed after landfall. Baroclinic near-inertial oscillations were dominant at the shelf break and occurred along with a barotropic response on the middle shelf. Downwelling-favorable flow developed east of the track prior to the storm peak, and upwelling-favorable flow evolved west of the track as the eye crossed the shelf. The idealized storm flow was modified by local barotropic and baroclinic pressure gradients on the shelf. Ocean circulation during Hurricane Andrew was hindcast using both stratified and unstratified three-dimensional numerical models. For areas within 1 Rw of the storm track, the unstratified model matched the observed currents better than the stratified model, partly because of errors in the initial stratification. At distances greater than 2 Rw the influence of stratification increases, and the unstratified model does not reproduce the observed upwelling-favorable flow.

A numerical study of sediment transport and event bed genesis during tropical storm Delia

Some event beds (tempestites) are thought to be emplaced on shallow marine shelves by the combined action of strong coastal currents and high waves during fairly short-lived storms. To test this hypothesis, a storm sedimentation system has been constructed from six numerical models describing a cyclonic wind field, three-dimensional coastal circulation, wind waves generated over the continental shelf, the combined effect of steady and oscillatory currents within the benthic boundary layer, suspended and bed load transport of sediment, and conservation of the seafloor. This model system is used to hindcast winds, currents, waves, and resulting sedimentation during Tropical Storm Delia, which passed over the Texas-Louisiana shelf on September 3–4, 1973. Sensitivity to the initial substrate is investigated in four experiments using uniform silt, uniform sand, a mud line at the 20-m isobath, and a simplified modern sediment distribution. Modeled coastal currents are vertically uniform and do not reveal the structure predicted by the mid-latitude geostrophic storm circulation model, because the predicted depth of the wind-mixed layer is greater than the water depth over the shelf. Shelf currents in excess of 2 m/s flow predominantly along the coast to the southwest during most of the storm, driven by the wind stress and the trapped coastal wave which peaks at 180 cm near Galveston. Significant wave heights reach 8 m on the outer shelf but are less than 4 m over the inner shelf. These waves combine with steady currents to produce bed shear velocities which locally exceed 20 cm/s. The region of highest stresses always lies to the right of the storm track (viewed down the path) and moves across the shelf with the eye of the storm. Three general sediment transport paths are evident: (1) onshore transport of finer sediment over the outer shelf to the right of the storm track, (2) westward-directed along-shelf transport of predominantly fine sediment between approximately 40-m and 20-m water depths, and (3) minor offshore transport of sand from the shoreface to depths less than 30m. The resulting event bed has a ragged appearance with a maximum thickness of about 20 cm in region 1, and covers an area of approximately 3×104 km2 to the right of the storm track. Aside from local transport associated with finer sediments, these results are relatively insensitive to initial sediment type. Comparison of model results to observed data from Buccaneer platform shows that the different models performed adequately during the peak of the storm, except for a significant underprediction of the significant wave height by the wind sea model. The estimated uncertainty in the calculated combined shear stresses u*, based on errors produced by the individual model components, is most dependent on the wave bottom orbital amplitude. The total uncertainty in u* is estimated to be approximately 7%.

Potential transport pathways of terrigenous material in the Gulf of Papua

[t] This work discusses potential transport pathways of plume water may infrequently enter the Torres Straits terrigenous material in the Gulf of Papua (GoP), New because of sub-tidal water elevation differences between Guinea, using Lagrangian tracers as proxies for clay the GoP and the Arafura Sea. Modeling studies have been minerals and finer particles. The tracers are transported by used in conjunction with measurement programs to elucicurrents from the East Asia Seas implementation of the date dispersal mechanisms for noncohesive sediments in the Navy Coastal Ocean Model. The results suggest that clay GoP [Wolanski el al., 1984, 1995

Building the Holocene clinothem in the Gulf of Papua: An ocean circulation study

Hemer et al., 2004]. minerals input from rivers along the northwest coast of the [4] This study complements earlier work by examining GoP accumulate on the inner shelf with high concentrations the dispersal of dissolved and fine-grained suspended ternear the mouth of the Fly and Purari Rivers. Finer particles, rigenous material within the GoP for a year-long interval which are also representative of dissolved metals, are using a combination of hydrodynamic and Lagrangian transported eastward along the GoP coast into the Solomon tracer modeling. Furthermore, because of the large model Sea as well as into the Torres Strait to the west. The results domain used in this study, we are able to study the impact of also suggest that some finer particles are entrained by eddies large-scale flows on transport patterns without imposing within the northern Coral Sea. These results are in artificial boundary conditions. The use of tracers to examine qualitative agreement with observations from the region. complex multidisciplinary problems in oceanography has Citation: Keen, T. R., D. S. Ko, R. L. Slingerland, S. Riedlinger, led to understanding the behavior of both suspended and and P. Flynn (2006), Potential transport pathways of terrigenous dissolved constituents in a range of ocean environments. material in the Gulf of Papua, Geophys. Res. Lett., 33, L04608, [Nakano and Povinec, 2003; McManus and Prandle, 1997;

The generation of internal waves on the continental shelf by Hurricane Andrew

Received 31 August 2006; revised 21 August 2007; accepted 15 November 2007; published 28 March 2008. [1] This paper investigates the role that tidal and wind-driven flows and buoyant river plumes play in the development of the Holocene clinothem in the Gulf of Papua. Time series data from bottom tripods and a mooring were obtained at four locations near the mouth of the Fly River during portions of 2003 and 2004. Flows in the Gulf of Papua during calendar year 2003 were hindcast every 3 h using the Navy Coastal Ocean Model (NCOM) with boundary conditions from the Navy Atmospheric Prediction System, the east Asian seas implementation of NCOM, and the OTIS Tidal Inversion System. Results show that tidal flows on the modern clinoform are strong and are landward and seaward directed. Peak spring tidal velocities can provide the shear stresses necessary to keep sediment up to sand size in motion as the wind-driven and baroclinic currents distribute it from the river mouths across and along the shelf in two circulation states. During the monsoon season, the clinoform topset is swept by a seaward surface flow and landward bottom flow, reflecting river plumes and coastal upwelling. Seaward, this structure evolves into a SW directed surface current over the clinothem foreset with accompanying landward directed near-bed currents that trend obliquely up the foreset to the WSW over much of the clinothem. During the trade wind season, the inner and outer topset are swept by NE directed, contour-parallel surface currents, underneath which lie obliquely landward near-bed currents. These modeled flows and complex gyres in shallow water coupled with wave- and current-supported gravity flows or river floods can explain the form, internal clinoform shapes, and mineralogy of the modern Gulf of Papua clinothem.

Suspended minerogenic particle distributions in high‐energy coastal environments: Optical implications

Observed currents, temperature, and salinity from moored instruments on the Louisiana continental slope and shelf reveal multiple baroclinic oscillations during Hurricane Andrew in August 1992. These measurements are supplemented by numerical models in order to identify possible internal wave generation mechanisms. The Princeton Ocean Model is run with realistic topography, stratification, and wind forcing to extend the observations to Mississippi Canyon and other areas on the shelf. A two-layer isopycnal model is used with idealized topography and spatially uniform winds to isolate internal waves generated in and around the canyon. The combination of the observations and the results from the numerical models indicates several possible mechanisms for generating long internal waves: (1) near-inertial internal waves were generated across the slope and shelf by dislocation of the thermocline by the wind stress; (2) interaction of inertial flow with topography generated internal waves along the shelf break, which bifurcated into landward and seaward propagating phases; (3) downwelling along the coast depressed the thermocline; after downwelling relaxes, an internal wave front propagates as a Kelvin wave; and (4) Poincare waves generated within Mississippi Canyon propagate seaward while being advected westward over the continental slope. These processes interact to produce a three-dimensional internal wave field, which was only partly captured by the observations.