Estimating the Mean Ocean-Bay Exchange Rate of the Chesapeake Bay

A model of the salt balance in the Chesapeake Bay is discussed, which takes into account only time-dependent riverine input and mean ocean-bay exchange. Estimates of (spatial) mean bay salinity are made using two different data sources: a 16 year record of surveys taken by the Environmental Protection Agency\u27s Chesapeake Bay Program, and a 10 year record of hydrographic sections taken in the lower bay by the Center for Coastal Physical Oceanography at Old Dominion University. Using United States Geological Survey river flow data to force the model, both data sets are consistent with this simple model and both imply a mean oceanic exchange (e-folding) timescale of 90 days, which is equivalent to an effective exchange rate of roughly 8 x 103 m3 s-1

Wind-driven circulation on a shallow, stratified shelf

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 August 1998Data from the Coastal Ocean Processes Inner Shelf Study are analyzed to determine atmospheric forcing characteristics and the heat balance of the inner shelf, and are used as motivation for a numerical study of inner shelf circulation during upwelling and downwelling. Variation in meteorological forcing on the North Carolina Inner shelf is shown to be dominated by synoptic weather systems. The structure of cold fronts, which are the dominant synoptic feature, and the local meteorological conditions they produce result in a strong correlation between the surface heat flux and the wind orientation. This has implications for the heat balance of the inner shelf, which is considered next. During stratified conditions (observed during August 1994), cross-shelf heat fluxes due to Ekman dynamics dominate variation in heat content of the inner shelf, while during weakly-stratified conditions (observed during October 1994), the surface heat flux dominated variation in heat content. Both processes are correlated with the alongshelf wind, implying that the heat balance of the inner shelf can be modeled largely in terms of the alongshelf wind. The dominance of cross-shelf processes during stratified conditions motivated numerical studies of upwelling and downwelling. It was found that the feedback between mixing and stratification played a role in determining the strength of the circulation on the inner shelf, which differed between upwelling and downwelling. During upwelling, dense water is brought onto the inner shelf from below the pycnocline, producing vertical stratification, lowering eddy viscosities, and enhancing the inner shelf circulation. In contrast, during downwelling, circulation was weakened by the presence of stratification. These circulation patterns are discussed in the context of coastal observations, and the implications for cross-shelf transport and exchange processes are considered.My first two years were supported by a Graduate Fellowship from the National Science Foundation, which gave me the freedom to explore different topics and settle on something I really enjoyed. For the last four years I have been supported by an Office of Naval Research AASERT grant (N00014-93-1-1154), and by a National Science Foundation grant (OCE-9633025)

Variation in the Position of the Upwelling Front on the Oregon Shelf

As part of an experiment to study wind-driven coastal circulation, 17 hydrographic surveys of the middle to inner shelf region off the coast of Newport, OR (44.65°N, from roughly the 90 m isobath to the 10 m isobath) were performed during Summer 1999 with a small, towed, undulating vehicle. The cross-shelf survey data were combined with data from several other surveys at the same latitude to study the relationship between upwelling intensity and wind stress field. A measure of upwelling intensity based on the position of the permanent pycnocline is developed. This measure is designed so as to be insensitive to density-modifying surface processes such as heating, cooling, buoyancy plumes, and wind mixing. It is highly correlated with an upwelling index formed by taking an exponentially weighted running mean of the alongshore wind stress. This analysis suggests that the front relaxes to a dynamic (geostrophic) equilibrium on a timescale of roughly 8 days, consistent with a similar analysis of moored hydrographic observations. This relationship allows the amount of time the pycnocline is outcropped to be estimated and could be used with historical wind records to better quantify interannual cycles in upwelling

Variation in the position of the upwelling front on the Oregon shelf

As part of an experiment to study wind-driven coastal circulation, 17 hydrographic surveys of the middle to inner shelf region off the coast of Newport, OR (44.65°N, from roughly the 90 m isobath to the 10 m isobath) were performed during Summer 1999 with a small, towed, undulating vehicle. The cross-shelf survey data were combined with data from several other surveys at the same latitude to study the relationship between upwelling intensity and wind stress field. A measure of upwelling intensity based on the position of the permanent pycnocline is developed. This measure is designed so as to be insensitive to density-modifying surface processes such as heating, cooling, buoyancy plumes, and wind mixing. It is highly correlated with an upwelling index formed by taking an exponentially weighted running mean of the alongshore wind stress. This analysis suggests that the front relaxes to a dynamic (geostrophic) equilibrium on a timescale of roughly 8 days, consistent with a similar analysis of moored hydrographic observations. This relationship allows the amount of time the pycnocline is outcropped to be estimated and could be used with historical wind records to better quantify interannual cycles in upwelling

Drifter Behavior on the Oregon–Washington Shelf during Downwelling-Favorable Winds

Drifters released offshore of Oregon during predominantly downwelling favorable alongshore winds during three different deployments (October 1994, January 1998, and September 1998) display similar behavior: after being advected around in the offshore eddy field, they move onshore to a particular isobath and are advected poleward alongshore, without coming ashore. Numerical modeling results suggest that this may be due to downwelling circulation creating a marginally stable density gradient on the shelf inshore of the downwelling front, thereby increasing the vertical eddy diffusivity, which reduces the effective cross-shelf Ekman transport to nearly zero. The downwelling front itself is accompanied by a poleward jet, which carries drifters rapidly to the north. This behavior is consistent with previous modeling results

A Preliminary Examination of an in situ Dual Dye Approach to Measuring Light Fluxes in Lotic Systems

Light is a critical parameter in aquatic ecosystems, affecting primary production and in situ photochemistry. However, measuring light exposure for suspended particles or dissolved components in a dynamic water column can be challenging with existing Eulerian approaches. Here, we assess the simultaneous deployment of two dyes differing in photolability (rhodamine WT and fluorescein) as a Lagrangian measure of sunlight exposure in a lotic system. Fluorescein is sensitive to light exposure; rhodamine WT is relatively photostable. We examined dye fluorescence at various pH, salinity, and temperature conditions. We also tested dye photolability as a function of pH and wavelength range. In conjunction with this laboratory work, we performed initial field testing of the dual-dye approach in a stream on the north shore of Lake Superior, USA. Irradiation of the dyes using long-pass filters identified wavelengths \u3e= 420 nm as responsible for the vast majority of the loss of fluorescein fluorescence, with rhodamine appearing relatively photostable in these short-term studies across the wavelength ranges tested. Dye response to irradiation is pH-sensitive; the dual-dye approach will require additional calibration for acidic or basic waters and should be used with caution in aquatic systems undergoing strong (several pH unit) changes in pH. Field testing showed that the fluorescein to rhodamine WT ratio decreased approximately linearly with light exposure. The dual-dye methodology shows promise as an in situ light sensor applicable to water column species in lotic systems if temperature is recorded, and the pH range is measured and relatively stable (e.g., varies by \u3c 1 unit)

Coastal ocean processes inner-shelf study : coastal and moored physical oceanographic measurements

To improve our understanding of the physical and biological processes influencing plantonic larval distributions over the inner shelf, an interdisciplinary field program funded by the National Science Foundation's Coastal Ocean Processes program (CoOP) was conducted near Duck, North Carolina in the southern porton of the Middle Atlantic Bight. The field program took place from August to December, 1994 and included both moored and shipboard measurements of physical, biological and sedimentological variables. This report summarizes the observations from one component of this field program, a moored array of physical oceanographic and meteorological instruments. This component of the field program consisted of a cross-shelf array of three surface/subsurface mooring pairs in 13 m, 20 m and 25 m of water supporting instruments to measure currents, temperature and conductivity, a suite of meteorological instruments on surface buoys at the 20 -m and 25 -m site, and an along-shelf array of temperature, conductivity and bottom pressure sensors mounted on jetted pipes along the 5-m isobath and on moorings along the 20-m isobath. The report includes descriptions of the cross-shelf and along-shelf arrays, the four types of instruments used (VAWRs, VMCMs, SeaCats, and SeaGauges), and the data return from the field program. Statistical and graphical summaries of the atmospheric (wind, air temperature, barometric pressure, relative humidity, short- and long-wave radiation), and oceanic (current, water temperature, conductivity and bottom pressure) measurements are presented.Funding was provided by the National Science Foundation through Grant No. OCE-9221615

The Changing Face of Winter: Lessons and Questions From the Laurentian Great Lakes

Among its many impacts, climate warming is leading to increasing winter air temperatures, decreasing ice cover extent, and changing winter precipitation patterns over the Laurentian Great Lakes and their watershed. Understanding and predicting the consequences of these changes is impeded by a shortage of winter-period studies on most aspects of Great Lake limnology. In this review, we summarize what is known about the Great Lakes during their 3–6 months of winter and identify key open questions about the physics, chemistry, and biology of the Laurentian Great Lakes and other large, seasonally frozen lakes. Existing studies show that winter conditions have important effects on physical, biogeochemical, and biological processes, not only during winter but in subsequent seasons as well. Ice cover, the extent of which fluctuates dramatically among years and the five lakes, emerges as a key variable that controls many aspects of the functioning of the Great Lakes ecosystem. Studies on the properties and formation of Great Lakes ice, its effect on vertical and horizontal mixing, light conditions, and biota, along with winter measurements of fundamental state and rate parameters in the lakes and their watersheds are needed to close the winter knowledge gap. Overcoming the formidable logistical challenges of winter research on these large and dynamic ecosystems may require investment in new, specialized research infrastructure. Perhaps more importantly, it will demand broader recognition of the value of such work and collaboration between physicists, geochemists, and biologists working on the world\u27s seasonally freezing lakes and seas

Small-boat hydrographic surveys of the Oregon mid- to inner shelf : May-September 1999 : a component of the Prediction of Wind-Driven Coastal Circulation Project

During Spring and Summer 1999, as part of the Oregon State University National Oceanographic Partnership Program (OSU-NOPP) field program, 20 successful hy­drographic surveys were undertaken on the Oregon mid- to inner shelf, all near Yaquina Bay. These consisted of 17 cross-shelf sections along the Newport Hydro­graphic line ( extending approximately 30 km offshore), two "radiator" -type pat terns, and one 40-km long alongshore section. These surveys were made with a Guildline "MiniBAT" (Figure 1 ), a small towed vehicle equipped with a Sea-Bird Electronics SBE-25 Conductivity-Temperature-Depth (CTD) instrument and a WetLabs trans­missometer and fluorometer. This package was towed on a 200-m cable behind the R/V Sacajawea (Figure 2), OSU's 10-meter research boat. This report summarizes the instrumentation used, the cruise procedure, specific cruise summaries and details, the data processing procedure, and the data collected