The Impact of Episodic Events on the Nearshore-Offshore Transport in the Great Lakes:
Physical Oceanography Observations

J. Saylor, G. Miller, and M. McCormick

This proposal is written in response to the NSF/OCE and NOAA/COP Announcement of Opportunity for Coastal Studies in the Great Lakes. It is part of a multi-proposal, multidisciplinary program on the impact of episodic events on the coastal ecosystem in the Great Lakes. This group proposal focuses on observing and quantifying the offshore volume flux of water and suspended materials due to the coupling of a storm generated coastal turbidity plume and a two-gyre vorticity wave in southern Lake Michigan.
It is our hypothesis that the forced, two-gyre vorticity wave response of the lake to episodic wind events, occasionally modified by stratification, is a major mechanism for nearshore-offshore transport in the Great Lakes.
Our program is designed to test this hypothesis through the analysis of a carefully planned field program and the applicaton of numerical models by Schwab and Beletsky as outlined in their circulation modeling proposal. Our specific objectives are:
(1) To identify and quantify the physical processes generating nearshore- offshore transport of biogeochemically important materials during winter/spring episodic events; (2) To establish an observation strategy for generating maximum resolution of all the dominant time and space scales; (3) To estimate the velocity field along and across the plume as it evolves; (4) To determine the coastal energetics of the two-gyre vorticity wave under barotropic conditions and its response to increasing baroclinicity; (5) To provide a Lagrangian platform for studying biogeochemical processes in collaboration with other group proposals; (6) To estimate particle dispersion from Lagrangian statistics and compare with circulation model based estimates to assess model adequacy; and (7) To establish a database of all of our observations and make them easily available to other program participants.
The observation strategy will consist of three components: (a) moored instruments, (b) Lagrangian measurements, and (c) shipboard surveys. Optimal data coverage also requires data from AVHRR imagery and multifrequency HF radar observations that are proposed by colleagues to complement this work. Three years of field activities are planned with the first year being a pilot study. (a) Moored Instruments. To observe the cross-shore and along- shore physical environment, current velocities and water temperatures will be measured on five coastal cross sections oriented normal to the shelf bathymetry in the coastal area near St. Joseph, Michigan. A total of 17 moorings are planned to be deployed. During the pilot study approximately half of the moorings will be deployed. Alongshore and cross-shore correlations will be studied to ensure that transect spacing during the two intensive field years is adequate to describe both the mean flow structure and the propagation of vorticity or coastally - trapped Kelvin waves. (b) Lagrangian Measurements. This portion of the observational program is designed to make quasi-Lagrangian current measurements with satellite-tracked drifters. Our objective is to provide a detailed description of the flow field (the currents, vorticity and convergence patterns and the net offshore transport of water in the coastal plume) and understanding of its dynamics during the winter/spring transition in southern Lake Michigan. (c) Shipboard Surveys. Shipboard surveys will be limited to CTD surveys and toyoing data on temperature and transparency made with a V-Fin and Optical Plankton Counter (OPC).
This program in collaboration with hydrodynamic modeling and HF radar observation proposals and all of the other proposed joint efforts provides an opportunity for an intensive examination of a potentially important transport mechanism that is unique to enclosed basins.