The currents of Lakes Michigan and Huron / John C. Ayers.

7

The water transport direction in the Straits appears to be the key to the
whole current pattern in Lake Michigan. When the effective wind direction (resulting from the preceding few days' winds) is from too far north or east the
accompanying water transport direction is headed back into Lake Michigan and
surface outflow to Lake Huron is stopped. At such times the outflow current
turns down the east shore of the lake and reinforces the southward current
which is being formed by the T-arrow water transport forces. At such times,
also, Lake Huron water is pushed into Lake Michigan through the Straits. The
levels of the two lakes are kept in balance at such times by a subsurface flow
of water from Lake Michigan into Lake Huron.
In Chart 11, on the second day of the easterly blow and before the full
effects of the blow had developed, northeast effective winds with westward water
transport were pushing the currents away from shore in the Grand Rapids area.
In the Chicago region they were drawing the east-shore south current over to
the city. At Milwaukee they were moving water toward shore but its effect
would not be fully expressed until the next day. At Escanaba the water transport direction was such as to draw water in through the mouth of Green Bay
(and probably produce strong eastward current in Sturgeon Bay Canal).
In Chart 12, after two full days of easterly winds (three at Chicago) the
east-shore south current was wide and extended out to the center of the lake.
The large clockwise eddy normally against the Michigan shore in the southern
part of the lake had been pushed over against the Illinois shore in the region
between Wilmette and the Wisconsin boundary. At Milwaukee, water had been
transported onto shore until it made a wind set-up, or hill of water, and on
the slope of this set-up current ran south as it should under such conditions.
The outflow current was broken at latitude 43~ 45'. The southern part flowed
eastward below Milwaukee to join the east-shore south current; the remainder
formed a large and flattened eddy off the mouth of Green Bay. Except for
Grand Traverse Bay and the eddy off Naubinway, the eddies in the northern
half of the lake were reversed in current direction from their normal condition.
Lake Huron, Winds North of Normal
In Chart 13 is shown the current pattern of Lake Huron on 27 July 1954.
This survey followed winds (Table 2) which, in the Straits region, were from
the "normal" west northwest direction during the days before the survey. Consequently, the currents in the upper end of the lake were essentially as in the
normal pattern given in Chart 5. A small eddy in South Channel was the only
well-substantiated difference. A scanty amount of evidence suggested that there
might have been eastward current along the shores of Drummond, Cockburn,
and Manitoulin Islands, but there was little certainty of it.
In the central part of the lake the winds during the week prior to 27 July
had been from the north for three consecutive days (Table 2) and the currents
appeared to be recovering from the southwestward distortion that such winds
would have imposed upon them. The recovery was not being materially helped
by northwest winds just before the survey. North winds blow south but the rotation of the earth converts the wind energy into a water-moving (water transport) force about 45 to the right of the wind movement, i. e., a north wind
tends to move water to the southwest. In the same fashion northwest winds
blow southeast but move water south.
The flow-through current off Thunder Bay was just off the Michigan shore,
but still made a westward turn to come shoreward in the region from Sturgeon