Search Results

You are looking at 1 - 10 of 12 items for

  • Author or Editor: Peter E. Holloway x
  • Refine by Access: All Content x
Clear All Modify Search
Peter E. Holloway

Abstract

A nonlinear, primitive equation, finite-difference numerical model is applied to the problem of the generation, propagation, and dissipation of internal tides over a cross section of the continental slope and shelf topography of a region on the Australian North West Shelf. The model is forced through the specification of the offshore tidal elevation and as such the full tidal field is modeled for the M2 constituent. An energetic internal tide is produced in the model with results showing sensitivity to changes in both stratification and bathymetry. The ratio of the slope of the internal wave characteristics to the bathymetry is generally less than or close to one, producing subcritical and approximately critical conditions. Model results are compared to previously reported observations and show reasonable agreement in terms of wave structure, propagation direction, and regions of generation and energy dissipation.

The model shows a high degree of spatial variability in the amplitude and phase of internal wave currents and vertical displacements with motion tending to propagate along characteristic paths as beams of signal. However, dissipation prevents the beams from radiating large distances from the generation regions. The energy flux of the internal tide propagates both onshore and offshore and the magnitude of the flux is strongly dependent on the slope of the bathymetry with largest values occurring for steepest topography. The internal wave amplitude and hence energy flux is also found to be dependent on the magnitude of the vertical and horizontal mixing of momentum with maximum values achieved under conditions of no mixing.

Full access
Peter E. Holloway

Abstract

An analysis of current meter data and cross-shelf temperature measurements from the Australian North-West Shelf shows the existence of internal waves of semi-diurnal period. Vertical displacements of density interface am seen to reach ∼30 m, a value nearly half the water depth. Baroclinic currents are isolated from the measurements and have amplitudes reaching 0.2 m s−1 in the cross-shelf direction.

The baroclinic motion is shown to be consistent with a first-mode internal wave propagating in the onshore direction at phase speed of approximately 0.4 m s−1 with wavelength of 20 km and being rapidly damped in amplitude while propagating across the shelf. The dissipation of the internal tide across the shelf appears to result from the influence of turbulent mixing processes. There is little coherence between the barotropic tide and the internal tide and it is suggested that this results from the internal tide being generated over a large section of the shelf slope down to a depth of approximately 900 m. There is no obvious point-source of generation for the internal tide.

Full access
Peter E. Holloway

Abstract

The vertical structure of the semidiurnal internal tide is calculated from current meter data, for three locations of varying bathymetry, on the southern part of the Australian North West Shelf. These results are compared to results from a previous study at a fourth location. Each site is characterized by the ratio α/c, where α is the slope of the bathymetry and c = [(σ2f 2)/(N 2 − σ2)]½ is the slope of the internal Wave characteristics, where σ is the wave frequency, f the inertial frequency and N) the buoyancy frequency. The observations then fall into three categories, subcritical (α/c<1), near-critical (α/c∼1), and super-critical (α/c>1) bottom slopes. The results at near-critical bottom slope (α/c = 1.2) show a strong intensification of the near-bottom baroclinic currents. Observations at the super-critical slope (α/c = 1.9) show a weak bottom intensification, and at the subcritical slopes (α/c = 0.2 and 0.5), no bottom intensification of currents is observed. The observations agree qualitatively with Wunsch's solutions for freely propagating internal waves in a wedge. except for the case α/c = 0.5, where the weak intensification predicted is not observed.

Full access
Peter E. Holloway

Abstract

The properties of the semidiurnal internal tide, in the region of the shelf-break, at a location on the Australian North West Shelf are discussed. Information is derived from an analysis of thermistor chain and current meter data, collected over six months at the shelf-break and slope locations. The work is an extension of an earlier study.

The internal tide is described in terms of modes, finding that the firm mode dominates, propagating onshore at an angle of ∼30° from normal to the bathymetry, with a rapid decay in amplitude of nearly five times from the shelf-slope to the shelf-break, a distance of 22.5 km or approximately one wavelength. The loss of energy flux from this decay gives rise to vertical mixing with a vertical eddy viscosity of 1.4 × 10−4 m2 s−1. The amplitude at the M2 tidal frequency dominates over the S2 amplitude giving an S2/M2 amplitude ratio significantly smaller than for the barotropic tidal motion. The internal tide appears to have a three-dimensional structure at the shelf-slope location with the ratio of alongshore over onshore wavenumbers ∼0.8. There are large variations in amplitude of the motion with time, showing a gradual buildup in amplitude over summer, reaching a maximum at the M2 frequency of 25 m (an average over a 14½-day segment) in 123 m depth of water. The analysis results support the suggestion by Holloway that this measurement region is not the generation site for the internal tides. A region with steep bathymetric features to the northwest is suggested as a likely site.

Full access
Peter E. Holloway

Abstract

The onset of thermal stratification in an isohaline, wind-mixed water body is shown, by a simple model and observations, to be determined by the parameter u * 3/hB′, where u * is the friction velocity of the air just above the water surface, h the water depth and B′ a buoyancy flux. Defined as B′= gα(ρ Cp)−1 × [Q 0 – 2Q 1(ch)−1], where g is gravitational acceleration, α the coefficient of thermal expansion, ρ the density of water, Cp, the specific heat of water at constant pressure, Q 0 the not surface heat input, Q 1 the solar radiation that penetrates the water column and c the extinction coefficient for Q1, in the water, this buoyancy flux is the net buoyancy input to the water, less an amount due to solar radiation penetrating the water column. The transition from the well-mixed to stratified regime occurs when u * 3/hB′ falls below a value of approximately 6700. This is supported by observations from a lagoon 3 m deep where the complete formation and breakdown cycle of thermal stratification occurs over several hours. A value of 1.8 is found for the ratio of the rate of increase in potential energy of the water column due to wind mixing, over νv*, where v is the surface wind stress and ν* the friction velocity in the water near the air-water interface. The value of this ratio was obtained from measurements made in the lagoon where the effects of water beating were considered, as well as wind mixing, on changing the potential energy. The development of the simple stratification criterion allows some predictions to be made of the influence of turbidity on the thermal structure of a water body.

Full access
Peter E. Holloway

Abstract

The propagation of internal tides (12-h period internal waves) is observed over a continental shelf break region on the Australian North West Shelf. The observed phase speeds of the waves, calculated from thermistor chain and current meter data, are shown to agree closely with simple linear theory for phase velocity Of freely propagating waves. Even over the relatively steep bathymetry of a continental shelf break region (ratio of bathymetric slope to slope of internal wave characteristics reaching 0.27), a simple modal model for phase speed is found to be accurate, providing that slowly varying depth is taken into account. The direction of wave propagation is determined from the orientation of baroclinic current ellipses derived from current meter data; a two-dimensional, horizontal field of phase information; and visual observations of surface slicks. 0bservations also demonstrate that substantial temporal variations in phase are a result of variations in the thermal stratification of the water column. Both the direction of internal tide propagation and the phase values, when averaged over some months of data, are consistent from year to year. Orientation of the wave fronts is relatively constant over an area of approximately 60 × 60 km2, centered around the shelf break, and they are aligned approximately 10° of parallel to the local bathymetric contours.

Full access
Peter E. Holloway, Paul G. Chatwin, and Peter Craig

Abstract

Observations are presented of the internal tide over the continental shelf and slope from a cross section on the Australian North West Shelf. Data collected from moored instruments and repeated profile measurements during the summer months of 1995 show an energetic, large amplitude, shoreward propagating semidiurnal internal tide. Multiple generation sites are suggested, coinciding with near-critical bottom slopes. In water less than approximately 200 m deep, the vertical structure of the internal tide is predominantly a first vertical mode, whereas in deeper water over the slope the vertical structure is more complicated with currents and vertical displacements intensified in the lower part of the water column. The internal tide is largely confined to a region approximately 100 km wide, from water depths between 70 and 1000 m. Strong generation and dissipation of the internal tide energy is observed over this region and there is evidence that the dissipated energy impacts on the vertical mixing of the density field, particularly near the shelf break and upper continental slope. Even though the diurnal barotropic tidal currents are weak, a diurnal internal tide is observed and appears to be generated over a section of the continental slope that is at the critical slope for the K 1 tidal frequency. The M 4 harmonic is also observed and this results from nonlinear interactions of the M 2 baroclinic tide.

Full access
Peter E. Holloway, Efim Pelinovsky, Tatyana Talipova, and Belinda Barnes

Abstract

A numerical solution to the generalized Korteweg-de Vries (K-dV) equation, including horizontal variability and dissipation, is used to model the evolution of an initially sinusoidal long internal wave, representing an internal tide. The model shows the development of the waveform to the formation of shocks and solitons as it propagates shoreward over the continental slope and shelf. The model is run using observed hydrographic conditions from the Australian North West Shelf and results are compared to current meter and thermistor observations from the shelf-break region. It is found from observations that the coefficient of nonlinearity in the K-dV equation changes sign from negative in deep water to positive in shallow water, and this plays a major role in determining the form of the internal tide transformation. On the shelf there is strong temporal variability in the nonlinear coefficient due to both background shear flow and the large amplitude of the internal tide, which distorts the density profile over a wave period. Both the model and observations show the formation of an initial shock on the leading face of the internal tide. In shallow water, the change in sign of the coefficient of nonlinearity causes the shock to evolve into a tail of short period sinusoidal waves. After further propagation a second shock forms on the back face of the wave, followed by a packet of solitons. The inclusion of bottom friction in the model is investigated along with the dependance on initial wave amplitude and variability in the coefficients of nonlinearity and dispersion. Friction is found to be important in limiting the amplitudes of the evolving waves.

Full access
Mohd Fadzil Firdzaus Mohd Nor, Christopher E. Holloway, and Peter M. Inness

Abstract

Severe rainfall events are common in western Peninsular Malaysia. They are usually short and intense, and occasionally cause flash floods and landslides. Forecasting these local events is difficult and understanding the mechanisms of the rainfall events is vital for the advancement of tropical weather forecasting. This study investigates the mechanisms responsible for a local heavy rainfall event on 2 May 2012 that caused flash floods and landslides using both observations and simulations with the limited-area high-resolution Met Office Unified Model (MetUM). Results suggest that previous day rainfalls over Peninsular Malaysia and Sumatra Island influenced the development of overnight rainfall over the Strait of Malacca by low-level flow convergence. Afternoon convection over the Titiwangsa Mountains over Peninsular Malaysia then induced rainfall development and the combination of these two events influenced the development of severe convective storm over western Peninsular Malaysia. Additionally, anomalously strong low-level northwesterlies also contributed to this event. Sensitivity studies were carried out to investigate the influence of the local orography on this event. Flattened Peninsular Malaysia orography causes a lack of rainfall over the central part of Peninsular Malaysia and Sumatra Island and produces a weaker overnight rainfall over the Strait of Malacca. By removing Sumatra Island in the final experiment, the western and inland parts of Peninsular Malaysia would receive more rainfall, as this region is more influenced by the westerly wind from the Indian Ocean. These results suggest the importance of the interaction between landmasses, orography, low-level flow, and the diurnal cycle on the development of heavy rainfall events.

Free access
Craig M. Lee, Thomas B. Sanford, Eric Kunze, Jonathan D. Nash, Mark A. Merrifield, and Peter E. Holloway

Abstract

Full-depth velocity and density profiles taken along the 3000-m isobath characterize the semidiurnal internal tide and bottom-intensified turbulence along the Hawaiian Ridge. Observations reveal baroclinic energy fluxes of 21 ± 5 kW m−1 radiating from French Frigate Shoals, 17 ± 2.5 kW m−1 from Kauai Channel west of Oahu, and 13 ± 3.5 kW m−1 from west of Nihoa Island. Weaker fluxes of 1–4 ± 2 kW m−1 radiate from the region near Necker Island and east of Nihoa Island. Observed off-ridge energy fluxes generally agree to within a factor of 2 with those produced by a tidally forced numerical model. Average turbulent diapycnal diffusivity K is (0.5–1) × 10−4 m2 s–1 above 2000 m, increasing exponentially to 20 × 10−4 m2 s–1 near the bottom. Microstructure values agree well with those inferred from a finescale internal wave-based parameterization. A linear relationship between the vertically integrated energy flux and vertically integrated turbulent dissipation rate implies that dissipative length scales for the radiating internal tide exceed 1000 km.

Full access