Geno Pawlak

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.

Stratified Flow along a Corrugated Slope: Separation Drag and Wave Drag

Abstract Lee wave generation and horizontal flow separation in stratified flow along a slope, with corrugations or a ridge running directly downslope, are explored using analytical and numerical methods. Both of these processes are important to the drag on alongslope currents. The analytical solution for steady wave generation by stratified flow along a corrugated slope is extended to the evanescent flow regimes. There are two evanescent regimes, having intrinsic frequencies either above the buoyancy frequency N (fast flow), or below N sin(a) (slow flow), for nonrotating fluid and slope angle, a. Streamlines of the low speed evanescent solution tend to follow isobaths, while those of wave solutions tend to flow up over ridges and down in canyons. An analytical expression is developed for the wave drag felt by an isolated ridge on a slope. For a Gaussian ridge of alongslope length L, the drag becomes small when U/LN > 1 (the fast flow regime), or when U/(LN sin a) < 1/2 (the slow flow regime). Numerical ex...

Form Drag due to Flow Separation at a Headland

Abstract Observational and model estimates of the form drag on Three Tree Point, a headland located in a tidal channel of Puget Sound, Washington, are presented. Subsurface, Three Tree Point is a sloping ridge. Tidal flow over this ridge gives rise to internal lee waves that lead to wave drag and enhanced mixing. At the same time, horizontal flow separation produces a headland eddy that distorts the surface height field in the lee of the point. Two observational methods for estimating the portion of the form drag associated with deformation of the surface height field, referred to here as the “external” form drag, are also introduced. Drogued drifters and ship-mounted acoustic current profiles from different days are used to indirectly map the flood-tide surface height field. Data are derived from a depth shallow enough that baroclinic pressure gradient forcing may be neglected, and yet deep enough that wind stress may also be ignored. This leaves an approximate balance between the acceleration and surfac...

ADCP observations of edge waves off Oahu in the wake of the November 2006 Kuril Islands tsunami

Received 13 September 2007; revised 5 October 2007; accepted 8 November 2007; published 15 December 2007. [1] During the 2006 Kuril Islands tsunami, edge waves propagating along Oahu’s south shore were observed via depth-averaged ADCP velocity and pressure data acquired in real-time by a coastal observatory at 12 m depth and 400 m offshore. Time-varying rotary-component velocity spectra obtained via wavelet analysis agree with the phase lag observed between pressure and each Cartesian velocity component, in indicating the directions of rotation and travel of progressive edge waves. Furthermore, the theoretical ratios between power in the free surface elevation and in each velocity component of edge waves, agree with those observed, and a nonlinear shallow-water model shows edge waves of various modes propagating along the shore near our observation location. Importantly, the maximum surge in sea level occurred at a time when edge waves of all constituent frequencies were superposed. Citation: Bricker, J. D., S. Munger, C. Pequignet, J. R. Wells, G. Pawlak, and K. F. Cheung (2007), ADCP observations of edge waves off Oahu in the wake of the November 2006 Kuril Islands tsunami, Geophys. Res. Lett., 34, L23617, doi:10.1029/ 2007GL032015.

Tilted Baroclinic Tidal Vortices

The structure of baroclinic vortices generated by horizontal flow separation past a sloping headland in deep, stably stratified waters is investigated. The most distinctive feature of these eddies is that their cores are strongly tilted with respect to the stratification, yet their velocity field remains quasi-horizontal. Field observations and numerical simulations are used to explore the consequences of the strong tilt on the eddy baroclinic structure. It is found that the background density field is altered in such a way as to maintain a pressure minimum in the tilted vortex cores. This adjustment results in a fundamental asymmetry of the density field. Isopycnals are deflected upward on the shoreward side and downward on the opposite side of the eddy center. The resulting pattern closely resembles the asymmetries of azimuthal wavenumber one that develop when tropical cyclones become tilted by an environmental shear.The authors provide a simple analytical model that suggests this structure is obtained via a balance between the centrifugal force and the horizontal pressure gradient. As the eddies release from the boundary, adjust, and decay, their tilt as well as the associated density perturbation decrease and lose coherence. It is suggested that this may lead to a conversion of potential energy into kinetic energy.

Diurnal cross‐shore thermal exchange on a tropical forereef

Observations of the velocity structure at the Kilo Nalu Observatory on the south shore of Oahu, Hawaii show that thermally driven baroclinic exchange is a dominant mechanism for cross-shore transport for this tropical forereef environment. Estimates of the exchange and net volume fluxes are comparable and show that the average residence time for the zone shoreward of the 12 m isobath is generally much less than 1 day. Although cross-shore wind stress influences the diurnal cross-shore exchange, surface heat flux is identified as the primary forcing mechanism from the phase relationships and from analysis of momentum and buoyancy balances for the record-averaged diurnal structure. Dynamic flow regimes are characterized based on a two-dimensional theoretical framework and the observations of the thermal structure at Kilo Nalu are shown to be in the unsteady temperature regime. Diurnal phasing and the cross-shore momentum balance suggest that turbulent stress divergence is an important driver of the baroclinic exchange. While the thermally driven exchange has a robust diurnal profile in the long term, there is high temporal variability on shorter time scales. Ensemble-averaged diurnal profiles indicate that the exchange is strongly modulated by surface heat flux, wind speed/direction, and alongshore velocity direction. The latter highlights the role of alongshore variability in the thermally driven exchange. Analysis of the thermal balance in the nearshore region indicates that the cross-shore exchange accounts for roughly 38% of the advective heat transport on a daily basis.

High frequency temperature variability reduces the risk of coral bleaching

Coral bleaching is the detrimental expulsion of algal symbionts from their cnidarian hosts, and predominantly occurs when corals are exposed to thermal stress. The incidence and severity of bleaching is often spatially heterogeneous within reef-scales (<1 km), and is therefore not predictable using conventional remote sensing products. Here, we systematically assess the relationship between in situ measurements of 20 environmental variables, along with seven remotely sensed SST thermal stress metrics, and 81 observed bleaching events at coral reef locations spanning five major reef regions globally. We find that high-frequency temperature variability (i.e., daily temperature range) was the most influential factor in predicting bleaching prevalence and had a mitigating effect, such that a 1 °C increase in daily temperature range would reduce the odds of more severe bleaching by a factor of 33. Our findings suggest that reefs with greater high-frequency temperature variability may represent particularly important opportunities to conserve coral ecosystems against the major threat posed by warming ocean temperatures.Coral bleaching is often predicted via remote sensing of ocean temperatures at large scales, obscuring important reef-scale drivers and biological responses. Here, the authors use in- situ data to show that bleaching is lower globally at reef habitats with greater diurnal temperature variability.

Three-dimensional vortex dynamics in oscillatory flow separation

The dynamics of coherent columnar vortices and their interactions in an oscillatory flow past an obstacle are examined experimentally. The main focus is on the low Keulegan―Carpenter number range (0.2 < KC < 2), where KC is the ratio between the fluid particle excursion during half an oscillation cycle and the obstacle size, and for moderate Reynolds numbers (700 < Re v < 7500). For this parameter range, a periodic unidirectional vortex pair ejection regime is observed, in which the direction of vortex propagation is set by the initial conditions of the oscillations. These vortex pairs provide a direct mechanism for the transfer of momentum and enstrophy to the outer region of rough oscillating boundary layers. Vortices are observed to be short-lived relative to the oscillation time scale, which limits their propagation distance from the boundary. The instability mechanisms leading to vortex decay are elucidated via flow visualizations and digital particle image velocimetry (DPIV). Dye visualizations reveal complex three-dimensional vortex interactions resulting in rapid vortex destruction. These visualizations suggest that one of the instabilities affecting the spanwise vortices is an elliptical instability of the strained vortex cores. This is supported by DPIV measurements which identify the spatial structure of the perturbations associated with the elliptical instability in the divergence field. We also identify regions in the periphery of the vortex cores which are unstable to the centrifugal instability. Vortex longevity is quantified via a vortex decay time scale, and the results indicate that vortex pair lifetimes are of the order of an oscillation period T.

AUV-based observations of rough bed hydrodynamics

For highly irregular boundaries such as coral reefs, the choice of a characteristic roughness scale for use in hy-drodynamic modeling is not straightforward. As an initial step in relating measurable physical scales to hydrodynamic roughness, we explore the use of a combination of sidescan, high-resolution altimetry, and water velocity measurements collected with a REMUS-100 (Remote Environmental Monitoring UnitS, Hydroid, Inc) AUV in the vicinity of the Kilo Nalu Nearshore Reef Observatory (Oahu, HI). Different substrate classes (i.e. sand, coral, mixed) are identified using principal component analysis of variables derived from sidescan backscatter. Each bottom class is then connected to physical roughness measurements made with a high-resolution altimeter. Within the sampled region, sandy areas show a spectral root mean square (RMS) height of less than 3cm, while coral patches show RMS heights between 8cm to 14cm. To gain insight into the hydrodynamic response to broad-band roughness, we conducted a series of tests over the reef using the AUV mounted DVLs. In order to validate these hydrodynamic measurements, along-shore water velocities are sampled with spatial averaging of 50-100m and compared with two nearby bottom-fixed ADCPs. The results show that the REMUS DVLs can be effective in resolving steady boundary layer structure, which integrate the effects of roughness and implicitly reflect the response of wave motion to the boundary. Ongoing efforts are aimed to correlate bottom type, roughness RMS and hydrodynamic response at reef scales.

Observations of Nonlinear Internal Wave Runup to the Surfzone

AbstractThe cross-shore evolution of nonlinear internal waves (NLIWs) from 8-m depth to shore was observed by a dense thermistor array and ADCP. Isotherm oscillations spanned much of the water column at a variety of periods. At times, NLIWs propagated into the surfzone, decreasing temperature by ≈1°C in 5 min. When stratification was strong, temperature variability was strong and coherent from 18- to 6-m depth at semidiurnal and harmonic periods. When stratification weakened, temperature variability decreased and was incoherent between 18- and 6-m depth at all frequencies. At 8-m depth, onshore coherently propagating NLIW events had associated rapid temperature drops (ΔT) up to 1.7°C, front velocity between 1.4 and 7.4 cm s−1, and incidence angles between −5° and 23°. Front position, ΔT, and two-layer equivalent height zIW of four events were tracked upslope until propagation terminated. Front position was quadratic in time, and normalized ΔT and zIW both decreased, collapsing as a linearly decaying funct...