Kathleen Edwards

Form Drag and Mixing Due to Tidal Flow past a Sharp Point

Barotropic tidal currents flowing over rough topography may be slowed by two bottom boundary‐related processes: tangential stress of the bottom boundary layer, which is generally well represented by a quadratic drag law, and normal stress from bottom pressure, known as form drag. Form drag is rarely estimated from oceanic observations because it is difficult to measure the bottom pressure over a large spatial domain. The ‘‘external’’ and ‘‘internal’’ components of the form drag are associated, respectively, with sea surface and isopycnals deformations. This study presents model and observational estimates of the components of drag for Three Tree Point, a sloping ridge projecting 1 km into Puget Sound, Washington. Internal form drag was integrated from repeat microstructure sections and exceeded the net drag due to bottom friction by a factor of 10‐50 during maximum flood. In observations and numerical simulations, form drag was produced by a lee wave, as well as by horizontal flow separation in the model. The external form drag was not measured, but in numerical simulations was found to be comparable to the internal form drag. Form drag appears to be the primary mechanism for extracting energy from the barotropic tide. Turbulent buoyancy flux is strongest near the ridge in both observations and model results.

Adjustment of the Marine Atmospheric Boundary Layer to a Coastal Cape

Abstract During summer, significant changes in marine atmospheric boundary layer (MABL) speed and depth occur over small spatial scales (<100 km) downstream from topographic features along the California coast. In June and July 1996, the Coastal Waves 96 project collected observations of such changes at capes with an instrumented aircraft. This paper presents observations from the 7 June flight, when the layer-averaged speed increased 9 m s−1 and depth decreased by 500 m over a 75-km downwind from Cape Mendocino, accompanied by enhanced surface fluxes and local cloud clearing. The acceleration and thinning are reproduced when the flow is modeled as a shallow transcritical layer of fluid impinging the bends of a coastal wall, leading to the interpretation that they are produced by an expansion fan. Model runs were produced with different coastlines and imposed pressure gradients, with the best match provided by a coastline in which the cape protruded into the flow and forced a response in the subcritical r...

A revised control network for Mercury

An improved control net for Mercury has been completed by utilizing images acquired during the three Mariner 10 flybys in 1974–1975. Relative positional errors within the net are ∼1 km on average, and absolute locations are estimated to be better than 25 km. The analytical triangulation resulted in new values for focal lengths (Camera A: 1493.6 mm; Camera B: 1500.1 mm), W0 (329.548°), and camera orientation angles for 811 images acquired during all three flybys of Mercury.

Frontogenesis and Frontal Progression of a Trapping-Generated Estuarine Convergence Front and Its Influence on Mixing and Stratification

Estuarine fronts are well known to influence transport of waterborne constituents such as phytoplankton and sediment, yet due to their ephemeral nature, capturing the physical driving mechanisms and their influence on stratification and mixing is difficult. We investigate a repetitive estuarine frontal feature in the Snohomish River Estuary that results from complex bathymetric shoal/channel interactions. In particular, we highlight a trapping mechanism by which mid-density water trapped over intertidal mudflats converges with dense water in the main channel forming a sharp front. The frontal density interface is maintained via convergent transverse circulation driven by the competition of lateral baroclinic and centrifugal forcing. The frontal presence and propagation give rise to spatial and temporal variations in stratification and vertical mixing. Importantly, this front leads to enhanced stratification and suppressed vertical mixing at the end of the large flood tide, in contrast to what is found in many estuarine systems. The observed mechanism fits within the broader context of frontogenesis mechanisms in which varying bathymetry drives lateral convergence and baroclinic forcing. We expect similar trapping-generated fronts may occur in a wide variety of estuaries with shoal/channel morphology and/or braided channels and will similarly influence stratification, mixing, and transport.

A Seasonal Heat Budget across the Extent of the California Current

Abstract A seasonal heat budget is based on observations that span the broad California Current (CC) region. Budget terms are estimated from satellite data (oceanic heat advection), repeat ship transects (heat storage rate), and the Comprehensive Ocean–Atmosphere Data Set (COADS) (surface heat flux). The balance between terms differs with distance from shore. Offshore, a local balance between the heat storage rate and net heat flux (Q0) holds; the latter is dominated by its shortwave component QSW. Shoreward of ∼500 km, oceanic heat advection shifts the phase of the heat storage rate to earlier in the year and partially offsets an increase in Q0 due to cloud clearing. During the summer maximum of Q0, the ∼500-km-wide CC region loses heat to alongshore geostrophic transport, offshore Ekman transport, and, to a lesser degree, cross-shore geostrophic transport and eddy transport. The advective heat loss is neither uniform in space nor temporal phase; instead, the region of geostrophic and eddy heat loss expa...

Circulation in Carr Inlet, Puget Sound, During Spring 2003

The relatively slow flow and exchange of Carr Inlet water with the main basin of Puget Sound, Washington, favor eutrophication. To study Carr Inlet’s circulation, the Model-measurement Integration Experiment in Estuary Dynamics (MIXED) was conducted in March–May 2003, spanning the spring bloom. From observations and numerical simulations the circulation was decomposed into tidal and subtidal components; the former was dominated by the M2 tide, the latter by atmospheric forcing. Near the surface, the subtidal velocity was correlated with wind. At mid depths, the subtidal velocity was organized into vertical bands arising from internal waves excited by wind forcing of the water surface. The tidal flow was more strongly steered by local bathymetry and weaker in peak magnitudes than the subtidal flow, yet it contributed more mechanical energy to the inlet. Tidal eddies reduce exchange of water through the inlet’s entrances. Numerical simulations with the Princeton Ocean Model recreated many observed features, including the three-layer vertical structure of outflow at the surface and bottom and inflow at mid depth, the mid-depth subtidal response to the wind, and characteristics of the tide. While the model produced greater subtidal flow magnitudes at depth and differences in the phase of the M2 tide compared to observations, overall the case study provided support for more comprehensive simulations of Puget Sound in the future.

Evolution of Tidal Vorticity in Stratified Coastal Flow

The lifespan of a tidal eddy generated by flow around a coastal headland is examined. Field observations of a tidal headland eddy at Three Tree Point, WA (USA) are presented that examine the temporal evolution of the flood tide separation eddy from its generation, through the eddy release at the turn of the tide, until its dissipation during subsequent tidal cycles. Ship-based acoustic profiling examines the vertical structure of the velocity field and subsurface drogued drifters are used to track the horizontal motion of the flow structure. Drifter tracks from successive days at similar phases of the tide indicate that flow structure is repeatable. The combined set of drifter tracks is used to obtain an estimate of eddy lifetime. Time scales for vorticity decay of less than a tidal period are significantly shorter than simple estimates using boundary friction would imply. This finding suggests that the internal wave response of the stratified flow over the sloping headland plays a significant role in the dissipation of vorticity. Field observations are compared with results from numerical modeling that also suggest that baroclinic effects are significant.Copyright