Sloshing And Surfing On Lake Erie
June 27, 2015
Introduction
The marine forecast for Lake Erie on Saturday, June 27, 2015 suggested conditions were going to be unusually nautical for late June. Sustained northeasterly winds were likely to reach 35 knots, while gusts approaching 45 knots were possible. Significant wave heights were expected to reach 11 feet (click here for the wave forecast valid at 2 pm).
When strong winds persist for many hours, the force of the wind pushes water from the windward towards the leeward shore of the Lake. Under favorable conditions, this decrease and corresponding increase in water levels can reach several feet or more. The strong winds on June 27 were expected to produce a drop of 3.5 feet at Buffalo and an increase of nearly the same amount at Toledo (click here for forecast).
Displacements and Seiches
The most significant and long-lasting water level fluctuations,
referred to as displacements on the Great Lakes, occur in
response to strong, long-lasting winds blowing across the surface of the
lake. In the case of Lake Erie, this is particularly likely when the wind direction is aligned with the long axis of the Lake, either southwesterly or northeasterly.
The process of displacement begins on a very small scale as capillary waves (wavelengths less than 1.7 centimeters) form in response to the transfer of wind energy to the water's surface. Although capillary waves form, scientists are uncertain as to the precise dynamics of how these small ripples form on an otherwise smooth surface.
If the wind persists, particularly if its speed increases, form and frictional drag start to promote the development of larger waves. In contrast to the small capillary waves which are restrained by the surface tension of the water, the restoring force of waves having larger wavelengths is gravity.
As shown in Figure 4, the height and steepness of the waves both promote, and increase in response to, the development of localized areas of low and high pressure in the wave's trough. The area of high pressure on the windward side helps to lift the leading edge of the wave's crest, while the area of low pressure on the leeward side works to deepen the trough. These pressure patterns in the wave's trough help sustain and increase wave heights.
Of course, the overall height of waves is restricted by the combined
interaction of wind speed, duration of the wind, depth of the water in
which the waves form and distance over which the wind and water
interact, known as fetch. If these wave development
variables favorably interact for a sufficient period, a considerable
volume of water can be moved from one end of the lake to the other.
A seiche (pronounced "saysh") is defined by the National Weather Service (NWS) as a "standing wave oscillation of water in large lakes usually created by strong winds and/or a large barometric pressure gradient." In the Great Lakes, the term seiche is used generically to describe an increase in water level along a shore, referred to here as a displacement. While this article adheres to the strict NWS definitions, seiches and displacements are closely related.
Observations
The strong northeast winds were the result of an increasing barometric pressure gradient between
a building area of high pressure over New England and a deepening low pressure system (Figure 6) approaching Lake Erie from the south (click here for the suface weather map).
The sustained winds at buoy 45005 in Lake Erie's western basin (Figure 5) were east-northeasterly to northeasterly and ranged between 30 and 40 knots for most of the day before abruptly decreasing during the evening. The peak gust associated with the event -- 51.4 knots -- was recorded by the buoy at 11:37 am. Approximately 30 minutes later, a peak wave height of 8.9 feet was observed.
As expected, the strong winds produced a significant displacement of water on Lake Erie on June 27th. As shown in Figure 7, the water level dropped a little more than 3 feet at Buffalo and increased by a similar amount at Toledo (click here for an annotated graph). As the low pressure system moved towards the northeast, a decrease in the barometric pressure gradient prompted a decrease in the wind speeds over the Lake.
As the force of displacement-creating wind relaxed, its ability to maintain the unnatural distribution of water in the lake faltered and gravity stepped in to restore order. However, the level of the lake did not immediately reach equilibrium, instead it sloshed back and forth - a seiche -- while gravity progressively dampened the oscillations. Figure 8 shows the seiche persisted for nearly two days after the wind relaxed (click here for an annotated graph).
Surf's Up!
While it was no day for sailing, I paid a visit to Osprey to make sure she was weathering the storm. As I pulled into the marina, I was surprised to see several surfers taking advantage of the gnarly waves from the summer nor'easter.
