David F. Hill

Oceanography of Glacier Bay, Alaska: Implications for Biological Patterns in a Glacial Fjord Estuary

Alaska, U.S.A, is one of the few remaining locations in the world that has fjords that contain temperate idewater glaciers. Studying such estuarine systems provides vital information on how deglaciation affects oceanographic onditions of fjords and surrounding coastal waters. The oceanographic system of Glacier Bay, Alaska, is of particular interest ue to the rapid deglaciation of the Bay and the resulting changes in the estuarine environment, the relatively high oncentrations of marine mammals, seabirds, fishes, and invertebrates, and the Bay’s status as a national park, where ommercial fisheries are being phased out. We describe the first comprehensive broad-scale analysis of physical and iological oceanographic conditions within Glacier Bay based on CTD measurements at 24 stations from 1993 to 2002. easonal patterns of near-surface salinity, temperature, stratification, turbidity, and euphotic depth suggest that freshwater nput was highest in summer, emphasizing the critical role of glacier and snowmelt to this system. Strong and persistent tratification of surface waters driven by freshwater input occurred from spring through fall. After accounting for seasonal nd spatial variation, several of the external physical factors (i.e., air temperature, precipitation, day length) explained a large mount of variation in the physical properties of the surface waters. Spatial patterns of phytoplankton biomass varied hroughout the year and were related to stratification levels, euphotic depth, and day length. We observed hydrographic atterns indicative of strong competing forces influencing water column stability within Glacier Bay: high levels of freshwater ischarge promoted stratification in the upper fjord, while strong tidal currents over the Bay’s shallow entrance sill enhanced ertical mixing. Where these two processes met in the central deep basins there were optimal conditions of intermediate tratification, higher light levels, and potential nutrient renewal. These conditions were associated with high and sustained hlorophylla levels observed from spring through fall in these zones of the Bay and provide a framework for understanding he abundance patterns of higher trophic levels within this estuarine system.

Climate Divisions for Alaska Based on Objective Methods

AbstractAlaska encompasses several climate types because of its vast size, high-latitude location, proximity to oceans, and complex topography. There is a great need to understand how climate varies regionally for climatic research and forecasting applications. Although climate-type zones have been established for Alaska on the basis of seasonal climatological mean behavior, there has been little attempt to construct climate divisions that identify regions with consistently homogeneous climatic variability. In this study, cluster analysis was applied to monthly-average temperature data from 1977 to 2010 at a robust set of weather stations to develop climate divisions for the state. Mean-adjusted Advanced Very High Resolution Radiometer surface temperature estimates were employed to fill in missing temperature data when possible. Thirteen climate divisions were identified on the basis of the cluster analysis and were subsequently refined using local expert knowledge. Divisional boundary lines were drawn th...

Integrated experimental and computational approach to simulation of flow in a stirred tank

The stirred tank reactor is one of the most commonly used devices in industry for achieving mixing and reaction. We consider a combined experimental=computational approach for the simulation of oerent r–z planes phase locked at 0

Transient and steady-state amplitudes of forced waves in rectangular basins

A weakly-nonlinear analysis of the transient evolution of two-dimensional, standing waves in a rectangular basin is presented. The waves are resonated by periodic oscillation along an axis aligned with the wavenumber vector. The amplitude of oscillation is assumed to be small with respect to the basin dimensions. The effects of detuning, viscous damping, and cubic nonlinearity are all simultaneously considered. Moreover, the analysis is formulated in water of general depth. Multiple-scales analysis is used in order to derive an evolution equation for the complex amplitude of the resonated wave. From this equation, the maximum transient and steady-state amplitudes of the wave are determined. It is shown that steady-state analysis will underestimate the maximum response of a basin set into motion from rest. Amplitude response diagrams demonstrate good agreement with previous experimental investigations. The analysis is invalid in the vicinity of the “critical depth” and in the shallow-water limit. A separat...

Spatial and temporal variability of freshwater discharge into the Gulf of Alaska

A study of the freshwater discharge into the Gulf of Alaska (GOA) has been carried out. Using available streamgage data, regression equations were developed for monthly flows. These equations express discharge as a function of basin physical characteristics such as area, mean elevation, and land cover, and of basin meteorological characteristics such as temperature, precipitation, and accumulated water year precipitation. To provide the necessary input meteorological data, temperature and precipitation data for a 40 year hind-cast period were developed on high-spatial-resolution grids using weather station data, PRISM climatologies, and statistical downscaling methods. Runoff predictions from the equations were found to agree well with observations. Once developed, the regression equations were applied to a network of delineated watersheds spanning the entire GOA drainage basin. The region was divided into a northern region, ranging from the Aleutian Chain to the Alaska/Canada border in the southeast panhandle, and a southern region, ranging from there to the Fraser River. The mean annual runoff volume into the northern GOA region was found to be 792 ± 120 km3 yr−1. A water balance using MODIS-based evapotranspiration rates yielded seasonal storage volumes that were consistent with GRACE satellite-based estimates. The GRACE data suggest that an additional 57 ± 11 km3 yr−1 be added to the runoff from the northern region, due to glacier volume loss (GVL) in recent years. This yields a total value of 849 ± 121 km3 yr−1. The ease of application of the derived regression equations provides an accessible tool for quantifying mean annual values, seasonal variation, and interannual variability of runoff in any ungaged basin of interest.

SUBHARMONIC RESONANCE OF OBLIQUE INTERFACIAL WAVES BY A PROGRESSIVE SURFACE WAVE

Experimental and theoretical investigations into the generation of internal gravity waves by monochromatic progressive surface waves are presented. Using the method of nonlinear resonant interactions, a triad consisting of a single surface wave and two oblique internal waves in a two–layer model is considered. A multiple scales analysis is adopted and the boundary value problem is expanded in a power series of the surface–wave steepness. At the leading order, the linear harmonics are obtained and the conditions for resonance are determined. A second–order analysis is then used to derive temporal evolution equations for the internal–wave amplitudes. As a consequence of having a single generating train of the surface waves, two oblique trains of internal waves of much shorter wavelength are found to be resonated exponentially in time. Both linear and nonlinear bounds on surface–wave frequency, density ratio and interaction angle are found, demonstrating that the instability is highly narrow banded. It is found that the internal waves grow most rapidly at the linear cut–off values. Experimental evidence is presented and demonstrates good agreement with the theoretical results. Discussion of an application of the theory to the nonlinear energy transfer between very–low–frequency waves in the deep ocean is then provided.

Subharmonic resonance of short internal standing waves by progressive surface waves

Experimental evidence and a theoretical formulation describing the interaction between a progressive surface wave and a nearly standing subharmonic internal wave in a two-layer system are presented. Laboratory investigations into the dynamics of an interface between water and a fluidized sediment bed reveal that progressive surface waves can excite short standing waves at this interface. The corresponding theoretical analysis is second order and specifically considers the case where the internal wave, composed of two oppositely travelling harmonics, is much shorter than the surface wave. Furthermore, the analysis is limited to the case where the internal waves are small, so that only the initial growth is described. Approximate solution to the nonlinear boundary value problem is facilitated through a perturbation expansion in surface wave steepness. When certain resonance conditions are imposed, quadratic interactions between any two of the harmonics are in phase with the third, yielding a resonant triad. At the second order, evolution equations are derived for the internal wave amplitudes. Solution of these equations in the inviscid limit reveals that, at this order, the growth rates for the internal waves are purely imaginary. The introduction of viscosity into the analysis has the effect of modifying the evolution equations so that the growth rates are complex. As a result, the amplitudes of the internal waves are found to grow exponentially in time. Physically, the viscosity has the effect of adjusting the phase of the pressure so that there is net work done on the internal waves. The growth rates are, in addition, shown to be functions of the density ratio of the two fluids, the fluid layer depths, and the surface wave conditions.

Stirring up Trouble? Resuspension of Bottom Sediments by Recreational Watercraft

ABSTRACT An experimental and theoretical study of the hydrodynamic impacts of recreational watercraft in shallow water bodies is presented. Of particular interest is the ability of turbulent prop or jet wash to resuspend bottom sediments. Intuition suggests, and the experiments confirm, that this ability is a strong function of boat speed and water depth. The results of this study demonstrate that boats operating at high speed have no greater impact on the lake bed than boats travelling at idle speeds. The greatest impact is seen when boats are travelling at ‘near-plane’ speeds. This critical speed is a function of boat size and water depth. To increase the usefulness of the observations, a theoretical model of the flow underneath a passing boat was developed and validated with the data. Relying on only a few input parameters, the model can be used to estimate, for example, the minimum operating depth required for a given boat to prevent sediment resuspension. Discussion of the relevance of this work in the context of setting use restrictions for watercraft is provided.

High-resolution modeling of coastal freshwater discharge and glacier mass balance in the Gulf of Alaska watershed

This is the publisher’s final pdf. The article is copyrighted by the American Geophysical Union and published by John Wiley & Sons, Inc. It can be found at: http://sites.agu.org/

The Faraday resonance of interfacial waves in weakly viscous fluids

The Faraday resonance of interfacial waves in a two-layer, weakly-viscous system in a rectangular domain is presented. A perturbation analysis is pursued and, at the second-order, the scaling of the viscosity results in boundary layer corrections at the solid walls and at the interface. Special attention is paid to the damping in the meniscus region where the interface contacts the side-walls. As a result of the presence of both destabilizing effects (vertical oscillation) and stabilizing effects (viscosity), a threshold condition for instability is determined. The derived analytic results are quite general and prove useful in elucidating the influences of the various boundary layers, as well as the threshold for growth. In an effort to describe the maximum amplitude attained by the resonated wave, a third-order analysis is then presented for the idealized case of equal-depth, inviscid layers, with a rigid-lid condition at the free surface. A balance between cubic nonlinearity and the vertical shaking yields a Landau-type equation for the interfacial wave amplitude. Comparisons with some existing experimental data are made at both orders and indicate very good agreement.