Vertical heat flux and lateral mass transport in nonlinear internal waves

Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 37 (2010): L08601, doi:10.1029/2010GL042715.Comprehensive observations of velocity, density, and turbulent dissipation permit quantification of the nonlinear internal wave (NLIW) contribution to vertical heat flux and lateral mass transport over New Jersey's shelf. The effect of NLIWs on the shelf heat budget was significant. On average, heat flux in NLIWs was 10 times larger than background at the pycnocline depth. NLIWs were present at midshelf <10% of the time, yet we estimate that they contributed roughly one−half the heat flux across the pycnocline during the observation period, which was characterized by weak to moderate winds. Lateral transport distances due to the leading 3 waves in NLIW packets were typically inline equation(100 m) but ranged several kilometers. The month-averaged daily onshore transport (per unit alongshelf dimension) by NLIWs is estimated as 0.3 m2s−1. This is comparable to a weak downwelling wind, but sustained over an entire month.This work was funded by the Office of Naval Research

Local ocean response to a multiphase westerly wind burst: 1. Dynamic response

The dynamic response to a westerly wind burst which occurred during the Coupled Ocean Atmosphere Response Experiment in the warm pool of the equatorial Pacific Ocean is described using velocity, hydrography, and microstructure measurements. Turbulent fluxes distributed momentum input from the wind over a near‐surface layer of variable thickness. Coriolis and pressure gradient terms combined to induce a wavelike response whose frequency was close to the local inertial frequency. Wind stress variations on near‐inertial timescales interfered both constructively and destructively with the wave response, exerting considerable influence on the observed currents

Nonlinear internal waves over New Jersey's continental shelf

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 116 (2011): C03022, doi:10.1029/2010JC006332.Ship and mooring data collected off the coast of New Jersey are used to describe the nonlinear internal wave (NLIW) field and the background oceanographic conditions that formed the waveguide on the shelf. The subinertial, inertial, and tidal circulation are described in detail, and the background fluid state is characterized using the coefficients of the extended Korteweg–de Vries equation. The utility of this type of analysis is demonstrated in description of an amplitude-limited, flat wave. NLIWs observed over most of the month had typical displacements of −8 m, but waves observed from 17–21 August were almost twice as large with displacements near −15 m. During most of the month, wave packets occurred irregularly at a fixed location, and often more than one packet was observed per M2 tidal period. In contrast, the arrival times of the large-amplitude wave groups observed over 17–21 August were more closely phased with the barotropic tide. The time span in which the largest NLIWs were observed corresponded to neap barotropic conditions, but when the shoreward baroclinic energy flux was elevated. During the time of large NLIWs, near-inertial waves were a dominate contributor to the internal motions on the shelf and apparently regulated wave formation, as destructive/constructive modulation of the M2 internal tide by the inertial wavefield at the shelf break corresponded to stronger/weaker NLIWs on the shelf.This work was funded by the Office of Naval Research

Mode 2 waves on the continental shelf : ephemeral components of the nonlinear internal wavefield

Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 115 (2010): C07001, doi:10.1029/2009JC005605.Shoreward propagating, mode 2 nonlinear waves appear sporadically in mooring records obtained off the coast of New Jersey in the summer of 2006. Individual mode 2 packets were tracked between two moorings separated by 1 km; however, packets could not be tracked between moorings separated by greater distances from one another (∼10 km). The inability to track individual packets large distances through the mooring array combined with detailed observations from a ship suggest that these waves are short lived. The evolution of the ship-tracked wave group was recorded using acoustic backscatter, acoustic Doppler current profilers, and turbulence profiling. The leading mode 2 wave quickly changed form and developed a tail of short, small-amplitude mode 1 waves. The wavelength of the mode 1 oscillations agreed with that expected for a copropagating tail on the basis of linear theory. Turbulent dissipation in the mixed layer and radiation of the short mode 1 waves contributed to rapid energy loss in the leading mode 2 wave, consistent with the observed decay rate and short life span of only a few hours. The energy in the leading mode 2 wave was 10–100 times smaller than the energy of mode 1 nonlinear internal waves observed during the experiment; however, the magnitudes of wave-localized turbulent dissipation were similar.This work was funded by the Office of Naval Research

Structure and Generation of Turbulence at Interfaces Strained by Internal Solitary Waves Propagating Shoreward over the Continental Shelf

Detailed observations of the structure within internal solitary waves propagating shoreward over Oregon\u27s continental shelf reveal the evolving nature of interfaces as they become unstable and break, creating turbulent flow. A persistent feature is high acoustic backscatter beginning in the vicinity of the wave trough and continuing through its trailing edge and wake. This is demonstrated to be due to enhanced density microstructure. Increased small-scale strain ahead of the wave trough compresses select density interfaces, thereby locally increasing stratification. This is followed by a sequence of overturning, high-density microstructure, and turbulence at the interface, which is coincident with the high acoustic backscatter. The Richardson number estimated from observations is larger than 1/4, indicating that the interface is stable. However, density profiles reveal these preturbulent interfaces to be O(10 cm) thick, much thinner than can be resolved with shipboard velocity measurements. By assuming that streamlines parallel isopycnals ahead of the wave trough, a velocity profile is inferred in which the shear is sufficiently high to create explosively growing, small wavelength shear instabilities. It is argued that this is the generation mechanism for the observed turbulence and hence the persistent structure of high acoustic backscatter in these internal solitary waves

Seafloor Pressure Measurements of Nonlinear Internal Waves

Highly resolved pressure measurements on the seafloor over New Jersey’s continental shelf reveal the pressure signature of nonlinear internal waves of depression as negative pressure perturbations. The sign of the perturbation is determined by the dominance of the internal hydrostatic pressure (p⁰Wh) due to isopycnal displacement over the contributions of external hydrostatic pressure (ρ₀gηH; ηH is surface displacement) and nonhydrostatic pressure (p₀nh), each of opposite sign to p⁰Wh. This measurement represents experimental confirmation of the wave-induced pressure signal inferred in a previous study by Moum and Smyth.There was a press error in Fig. 10 of Moum and Nash (2008). The legend in Fig. 10a was shown in black and white instead of color. The correct figure and caption as they were meant to appear are shown below. The staff of the Journal of Physical Oceanography regrets any inconvenience this error may have caused.Keywords: Pressure, Internal waves, Continental shelf, Waves\, oceani

Topographically Induced Drag and Mixing at a Small Bank on the Continental Shelf

Recent turbulence measurements over a small bank on the continental shelf off Oregon reveal a previously undetected site for intense mixing of the coastal ocean. The flow is hydraulically controlled and turbulence diffusivities over the bank are more than 100 times greater than estimates made on the shelf away from topography. The total drag exerted by the bank on the flow field is a combination of bottom friction plus form drag (analogous to mountain drag) and is comparable to the Coriolis force. This drag is sufficient to decelerate the flow over the bank in a matter of hours