James H. Saylor

Mean Circulation in the Great Lakes

Abstract In this paper new maps are presented of mean circulation in the Great Lakes, employing long-term current observations from about 100 Great Lakes moorings during the 1960s to 1980s. Knowledge of the mean circulation in the Great Lakes is important for ecological and management issues because it provides an indication of transport pathways of nutrients and contaminants on longer time scales. Based on the availability of data, summer circulation patterns in all of the Great Lakes, winter circulation patterns in all of the Great Lakes except Lake Superior, and annual circulation patterns in Lakes Erie, Michigan, and Ontario were derived. Winter currents are generally stronger than summer currents, and, therefore, annual circulation closely resembles winter circulation. Circulation patterns tend to be cyclonic (counterclockwise) in the larger lakes (Lake Huron, Lake Michigan, and Lake Superior) with increased cyclonic circulation in winter. In the smaller lakes (Lake Erie and Lake Ontario), winter circulation is characterized by a two-gyre circulation pattern. Summer circulation in the smaller lakes is different; predominantly cyclonic in Lake Ontario and anticyclonic in Lake Erie.

Seasonal Thermal Cycle of Lake Erie

A summary of the seasonal water temperature characteristics of Lake Erie and the 1979 and 1980 thermal structure in the central basin is described. Ice cover extends over 90% of Lake Erie most winters. Minimum surface temperature usually occurs in February (0.1° C) but fully mixed conditions at 1°C or less occur in January with isothermal conditions at (1°C) occurring from mid-February to mid-March. The thermal bar advance lasts about 5 to 6 weeks from April to mid-May and permanent stratification usually begins in mid-June with maximum heat storage in mid-August and overturn in mid-September. The central basin thermocline position varies significantly from year to year, the variability of the upper and lower mesolimnion boundaries being as large as 10 m. Thermocline position shows some dependence on prevailing meteorological conditions and has implications to the development of central basin anoxia. Temperature increases and decreases depicted on isotherm plots for stations in the central basin show correspondence with peak wind stress events. During fragile stability conditions, even moderate wind stresses of less than 0.5 dynes/cm2 are capable of producing upper layer deepening. Episodes of complete vertical mixing in response to high wind stresses of 3 dynes/cm2 during storm periods are observed. Double thermoclines are evident at several locations within the basin and temperature changes resulting from an influx of hypolimnetic water from the Pennsylvania Ridge is documented. Periods of hypolimnetic entrainment are clearly observed along with thermocline tilting of 1 to 2 meters toward the south.

Vortex Modes in Southern Lake Michigan

Abstract Current velocities and water temperatures were observed in southern Lake Michigan with an array of AMF vector-averaging current meters during late spring, summer and fall 1976. Analyses of the recorded current data have revealed that persistent oscillations of nearly 4 days in period were at least as energetic as inertial oscillations in the kinetic energy spectra and current hodographs. The 4-day oscillations were present at all stations, including a very clear signal at stations near the center of the lake basin. This lake-wide oscillation was present during both stratified and unstratified seasons and current vectors rotated cyclonically near the center of the lake and anticyclonically elsewhere. The observed rotational oscillations closely fit the characteristics of barotropic second-class motions of a basin with variable depth first described by Lamb (1932). While such topographic vortex modes are of the same class as low-frequency shelf waves, their kinematic properties and natural period a...

Modeling wind‐driven circulation during the March 1998 sediment resuspension event in Lake Michigan

[1] A three-dimensional primitive equation numerical ocean model was applied to Lake Michigan to simulate hydrodynamic conditions during the March 1998 sediment resuspension event in southern Lake Michigan caused by a storm with winds up to 20 m/s. The hydrodynamic model is driven with surface winds derived from observed meteorological conditions at 18 land stations and a meteorological buoy and also with surface winds calculated using a mesoscale meteorological model. Current observations from 11 subsurface moorings showed that the model driven with observed winds was able to qualitatively simulate wind-driven currents but underestimated current speeds during the most significant wind event. In addition, a pronounced offshore flow in the area of observations was also underestimated. Hydrodynamic model results using the meteorological model winds as the forcing function showed significant improvement over model results which were based on observed winds proving the importance of mesoscale winds for current modeling in large lakes.

Studies of Large-Scale Currents in Lake Erie, 1979–80☆

Abstract Currents and water temperatures were recorded at a large-scale grid of fixed moorings in Lake Erie from May 1979 through June 1980. Currents measured in the lower half of the central basin water column were mostly return flows (beneath the surface wind drift) driven by the surface pressure gradient. Often observed was a complex system of Lake Erie circulation gyres as predicted by models. Another common occurrence was for one of the central basin gyres to become dominant and envelop the whole basin in either uniform clockwise or anticlockwise flow. It is not fully certain why one of the circulation cells grows as opposed to the others, but the curl of the wind stress had influence. The currents were more barotropic than predicted by full Ekman layer current models. Tidal-like currents driven by the longitudinal seiches of Lake Erie dominate the island-filled passages between the western and central Lake Erie basins, with currents across the whole island chain very closely in phase. Processes of hypolimnion volume entrainment are suggested from the central basin temperature recordings. Large volume water exchanges between the central and eastern basins occurred after the water mass in the vicinity of the shallow ridge that separates them had become unstratified. These and other topics are discussed as the large data set generated from the experiment is explored.

Hatching, Dispersal, and Bathymetric Distribution of Age-0 Wild Lake Trout at the Gull Island Shoal Complex, Lake Superior

We studied age-0 lake trout (Salvelinus namaycush) associated with spawning and nursery areas of the Gull Island Shoal complex in western Lake Superior. Post-emergent age-0 lake trout were captured on rocky spawning substrate with a 3-m beam trawl and at the nursery area with a bottom trawl from June to September 1990 and June to August 1991. Catch data suggested that age-0 lake trout move distances of 7–11 km to the nursery area over a 3-month period. Water currents, measured at Gull Island Shoal, may be a part of the transport mechanism. Examination of daily-growth increments on the sagittae and back-calculation from the date of capture revealed that most fish hatched between 6 June and 19 July in 1990 and between 30 April and 30 May in 1991. The duration of the hatch was 100 days in 1990 and 120 days in 1991, and the estimated incubation period is about 7 months for lake trout eggs at this site. Similar hatch-date distributions of age-0 captured on different sampling dates suggested that natural mortality was low.

Currents and Temperatures in Green Bay, Lake Michigan☆

Abstract Current velocities and water temperatures were measured in the four main passages between Green Bay and Lake Michigan and at several sites within the bay during summer and fall 1977. Monthly resultant currents indicate there is anticlockwise circulation in the bay during dominant southwesterly wind and a reversal of this pattern during episodes of northeasterly wind. It is common for two layers to flow through the mouth of the bay in opposite directions during the stratified season. Cold hypolimnetic lake water entering through the mouth and extending far into the bay maintains stratification and promotes flushing. The effects of resonance of forced and free long wave disturbances are prominent in current records; these oscillations are coherent and in phase across the mouth.

PROPERTIES OF LONGSHORE BARS IN THE GREAT LAKES

The wave-induced longshore current variation across the surf zone is described for a simplified model The basic assumptions are that the conditions are steady, the bottom contours are straight and parallel but allow for an arbitrary bottom profile, the waves are adequately described by linear theory, and that spilling breakers exist across the surf zone Conservation equations of mass, momentum, and energy, separated into the steady and unsteady components, are used to describe second order-wave-induced phenomena of shoaling waves approaching at an angle to the beach An expression for the longshore current is developed, based on the alongshore component of excess momentum flux due to the presence of unsteady wave motion Wave set-down and set-up have been included in the formulation Emphasis in the analysis is placed on formulating usable predictive equations for engineering practice Comparison with experimental results from the laboratory and field show that if the assumed conditions are approximately fulfilled, the predicted results compare quite favorably

Measurements of the Summer Currents in Saginaw Bay, Michigan☆

A combination of Lagrangian measurements and fixed current meter moorings were used during the summer of 1974 to determine the circulation patterns of Saginaw Bay. Because the bay is shallow, the water responds rapidly to wind changes. Distinct circulation patterns were determined for a southwest wind and a northeast wind. Speeds measured in the inner bay were of the order of 7 cm s−1, whereas in the outer bay the speeds averaged closer to 11 cm s−1. A typical water exchange rate between the inner and outer bay for winds parallel to the axis of the bay is 3700 m3s−1, but winds perpendicular to the axis of the bay cause little water to be exchanged. A comparison with the results of a numerical model for the bay indicates there is good agreement between the data and the simulation of the circulation in the inner bay. However, the agreement is poor in the outer bay, where specification of proper boundary conditions at the open mouth of the bay is important for meaningful model simulations.

Observation of Ekman Veering at the Bottom of Lake Michigan

An experiment in 100 m of water off the east coast of Lake Michigan during 1984 provided continuous current velocity recordings at four levels located from 1 m to 9 m above the lake bottom and also at 50 m. Near-inertial-period current oscillations were prominent and superimposed on longer-period velocity variations. Current speed at 1 m elevation varied from less than the threshold speed of the Savonious rotor sensor to 13 cm s−1. Profiles of current speed were close to logarithmic during many episodes of measurable currents over a 4-month-long recording interval. Over 50% of hourly-averaged profiles were in the nearly-logarithmic category (R > 0.987) during the month of September 1984. Veering averaging 11° was observed between low-pass-filtered current velocities measured at elevations of 1 m and at 9 m. Boundary layer theories developed from ocean studies are surveyed and the Lake Michigan data are examined from that perspective.