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.