ALBERT

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2003

Description: Oceanic observations indicate that abyssal mixing is localized in regions of rough topography. How locally mixed fluid interacts with the ambient fluid is an open question. Laboratory experiments explore the interaction of mechanically induced boundary mixing and an interior body of linearly stratified rotating fluid. A single oscillating bar produces a small region of turbulence along the wall at middepth. Mixed fluid quickly reaches a steady state height set by a turbulent-buoyant balance, independent of rotation. Initially, the bar is exposed on three sides. Mixed fluid intrudes directly into the interior rather than forming a boundary current. The circulation patterns suggest a model of unmixed fluid being laterally entrained into the turbulent zone. In accord with the model, observed outflux is constant, independent of stratification and restricted by rotation. Later the bar is laterally confines between two walls, which form a channel opening into the basin. A small percentage of mixed fluid enters a boundary current, which exits the channel. The bulk forms a cyclonic circulation in front of the bar, which blocks the channel and restricts horizontal entrainment. In the confined case, the volume flux of mixed fluid decays with time.

Description: This work was supported by the Ocean Ventures Fund, the Westcott Fund and the WHOI Education Office. Financial support was also provided by the National Science Foundation through grant OCE-9616949.

Description: Author Posting. © American Geophysical Union, 2006. 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 111 (2006): C12004, doi:10.1029/2006JC003667.

Description: Observations indicate that oceanic mixing is intensified near the head of submarine canyons. How the presence of canyon walls affects the local production and distribution of mixed fluid is an open question. These dynamics are addressed through rotating tank experiments which impose mixing at middepth at the closed end of a channel open to a larger body of water. Turbulence is generated in a linearly stratified fluid with initial buoyancy frequency N by means of a single bar oscillated with frequency ω. The mixed fluid quickly reaches a steady state height h ∼ (ω/N)1/2 independent of the Coriolis frequency f and collapses into the channel interior. A small percentage of the fluid exported from the turbulent zone enters a boundary current. The bulk forms a cyclonic circulation in front of the bar. As the recirculation cell expands to fill the channel, it restricts horizontal entrainment into the turbulent zone. Mixed fluid flux decays with time as t inline equation and is dependent on the size of the mixing zone and the balance between turbulence, rotation, and stratification. The recirculation cell is confined within the channel, and export of mixed fluid into the basin is restricted to the weak boundary current. As horizontal entrainment is shut down, long-term production of mixed fluid relies more on vertical entrainment. However, the scalings indicate that short-term dynamics are the most applicable to oceanic conditions.

Description: This work was supported by the Ocean Ventures Fund, the Westcott Fund, and the WHOI Academic Programs Office. Financial support was also provided by the National Science Foundation through grant OCE-9616949.

Description: Author Posting. © Cambridge University Press, 2004. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 516 (2004): 83-113, doi:10.1017/S0022112004000473.

Description: Oceanic observations indicate that abyssal mixing tends to be localized to regions of rough topography. How localized mixing interacts with the ambient fluid in a stratified, rotating system is an open question. To gain insight into this complicated process laboratory experiments are used to explore the interaction of mechanically induced boundary mixing and an interior body of linearly stratified rotating fluid. Turbulence is generated by a single vertically oscillating horizontal bar of finite horizontal extent, located at mid-depth along the tank wall. The turbulence forms a region of mixed fluid which quickly reaches a steady-state height and collapses into the interior. The mixed-layer thickness, $h_m\,{\sim}\,\gamma ({\omega}/{N})^{1/2}$, is spatially uniform and independent of the Coriolis frequency $f$. $N$ is the initial buoyancy frequency, $\omega$ is the bar oscillation frequency, and $\gamma\,{\approx}\,1$ cm is an empirical constant determined by the bar geometry. Surprisingly, the export of mixed fluid does not occur as a boundary current along the tank perimeter. Rather, mixed fluid intrudes directly into the interior as a radial front of uniform height, advancing with a speed comparable to a gravity current. The volume of mixed fluid grows linearly with time, $V\,{\propto}\,({N}/{f})^{3/2}h_m^3 \textit{ft}$, and is independent of the lateral extent of the mixing bar. Entrainment into the turbulent zone occurs principally through horizontal flows at the level of the mixing that appear to eliminate export by a geostrophic boundary flow. The circulation patterns suggest a model of unmixed fluid laterally entrained at velocity $u_e \,{\sim}\,Nh_m $ into the open sides of a turbulent zone with height $h_{m}$ and a length, perpendicular to the boundary, proportional to $L_f \,{\equiv}\,\gamma ({\omega}/{f})^{1/2}$. Here $L_{f}$ is an equilibrium length scale associated with rotational control of bar-generated turbulence. The model flux of exported mixed fluid $Q\,{\sim}\,h_m L_f u_e$ is constant and in agreement with the experiments.

Description: This work was supported by the Ocean Ventures Fund, the Westcott Fund and the WHOI Academic Programs Office. Financial support was also provided by the National Science Foundation through grant OCE-9616949.

Description: Author Posting. © Cambridge University Press, 2001. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 437 (2001): 301-323, doi:10.1017/S0022112001004402.

Description: Laboratory and numerical experiments are used to study flow of a uniform-density fluid on the [beta]-plane around a thin zonally elongated island (or ridge segment in the abyss). This orientation is chosen specifically to highlight the roles of the zonal boundary layer dynamics in controlling the circulation around the island. There are examples of deep ocean topography that fall into this category which make the work directly applicable to oceanic flows. Linear theory for the transport around the island and the flow structure is based on a modification of the Island Rule (Pedlosky et al. 1997; Pratt & Pedlosky 1999). The linear solution gives a north–south symmetric flow around the island with novel features, including stagnation points which divide the zonal boundary layers into eastward and westward flowing zones, and a western boundary layer of vanishing length, and zonal jets. Laboratory experiments agree with the linear theory for small degrees of nonlinearity, as measured by the ratio of the inertial to Munk boundary layer scales. With increasing nonlinearity the north–south symmetry is broken. The southern stagnation point (for anticyclonic forcing) moves to the eastern tip of the island. The flow rounding the eastern tip from the northern side of the island now separates from the island. Time-dependence emerges and recirculation cells develop on the northern side of the island. Mean transport around the island is relatively unaffected by nonlinearity and given to within 20% by the modified Island Rule. Numerical solutions of the shallow water equations are in close agreement with the laboratory results. The transition from zonal to meridional island orientation occurs for island inclinations from zonal greater than about 20°.

Description: This work was supported by the National Science Foundation (Grant Number OCE96-16949).

Description: During extended deployment at an ocean observatory off the coast of New Jersey, a bottom-mounted five-beam acoustic Doppler current profiler measured large-scale velocity structures that we interpret as Langmuir circulations filling the entire water column. These circulations are the large-eddy structures of wind-wave-driven turbulent flows that occur episodically when a shallow water column experiences prolonged strong wind forcing. Many observational characteristics agree with former descriptions of Langmuir circulations in deep water. The three-dimensional velocity field reveals quasi-organized structures consisting of pairs of surface-intensified counter-rotating vortices, aligned approximately downwind. Maximum downward velocities are stronger than upward velocities, and the downwelling region of each cell, defined as a pair of vortices, is narrower than the upwelling region. Maximum downward vertical velocity occurs at or above mid-depth, and scales approximately with wind speed. The estimated crosswind scale of cells is roughly 3–6 times their vertical scale, set under these conditions by water depth. The long axis of the cells appears to lie at an angle ∼10°–20° to the right of the wind. A major difference from deep-water observations is strong near-bottom intensification of the downwind ‘jets’ found typically centred over downwelling regions. Accessible observational features such as cell morphology and profiles of mean velocities, turbulent velocity variances, and shear stress components are compared with the results of associated large-eddy simulations (reported in Part 2) of shallow water flows driven by surface stress and the Craik–Leibovich vortex forcing generally used to represent generation of Langmuir cells. A particularly sensitive diagnostic for identification of Langmuir circulations as the energy-containing eddies of the turbulent flow is the depth trajectory of invariants of the turbulent stress tensor, plotted in the Lumley ‘triangle’ corresponding to realizable turbulent flows. When Langmuir structures are present in the observations, the Lumley map is distinctly different from that of surface-stress-driven Couette flow, again in agreement with the large-eddy simulations (LES). Unlike the LES, observed velocity fields contain two distinct and significant scales of variability, documented by wavelet analysis of observational records of vertical velocity. Variability with periods of many minutes is that expected from Langmuir cells drifting past the instrument at the slowly time-varying crosswind velocity. Shorter period variability, of the order of 1–2 min, has roughly the observed periodicity of surface wave groups, suggesting a connection with the wave groups themselves and/or the wave breaking associated with them in high wind conditions.

Description: The rate of coastal sea level change in the northeast Pacific (NEP) has decreased in recent decades. The relative contributions to the decreased rate from remote equatorial wind stress, local longshore wind stress, and local windstress curl are examined. Regressions of sea level onto wind stress time series and comparisons between NEP and Fremantle sea levels suggest that the decreased rate in the NEP is primarily due to oceanic adjustment to strengthened trade winds along the equatorial and coastal waveguides. When taking care to account for correlations between the various wind stress time series, the roles of longshore wind stress and local windstress curl are found to be of minor importance in comparison to equatorial forcing. The predictability of decadal sea level change rates along the NEP coastline is therefore largely determined by tropical variability. In addition, the importance of accounting for regional, wind-driven sea level variations when attempting to calculate accelerations in the long-term rate of sea level rise is demonstrated.