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Thanks Frank

Looks like our e-mails crossed in cyberspace.

It is interesting to watch the 24-hour animation of water levels (and
currents, max 1.8 kts in the past two days). It looks like about a 2" slosh
as you stated.  Seems to have piled up to the NE of the lake, as well as
leaving the south end of Green Bay and Grand Traverse bay low, then sloshed
back down in an irregular manner.

Will keep an eye on that and wind over the next few days and see if I can
perceive a pattern.

Bill


> Although Lake Michigan has natural frequencies of oscillation like  any lake,
> the oscillations you're seeing right now are primarily lake effect  weather.
> This time of year, when the weather is mild, there is a strong tendency  for
> the wind to pick up out of the west in the morning and blow from Illinois
> across towards Michigan. The lake follows the wind. Later in the day, as the
> wind
> dies, there is often a cool "lake effect" breeze that blows back towards  the
> west from the lake. Late this afternoon in Chicago, there was thick fog
> within one mile of the lake and it was about five degrees cooler. You can
> watch
> the water flow back into the lagoons over the course of a few hours, raising
> the  local water level by two inches or so. Then the whole cycle will repeat
> tomorrow. In this case, the lake is a passive partner. The cycle is in the
> local
> atmosphere.
>
> That said, yes, there are seiches in Lake Michigan. The  primary oscillation
> is a north-south motion. There is some confusion over the  use of the word
> "seiche" in the Great Lakes. A seiche properly refers to a  tide-like
> oscillation
> at a natural frequency, e.g. the 14 hour periodic  pseudo-tide on Lake Erie.
> This is usually intiated or "driven" by weather  systems. Some people apply
> the term "seiche" to something like a tidal bore that  results when a major
> difference in lake elevation has been created by a strong  storm or a strong
> pressure differential. These "bores", which can be deadly, are  often followed
> by
> periodic seiches lasting for several  days.
>
> Incidentally, Lake Michigan does have real tides --tides driven by  the
> combined gravity of the Sun and Moon, that is. They have a maximum range of
> about
> 2cm, which is just barely detectable in lake level statistics. Approximate
> tidal constants for the southern tip of Lake Michigan are:
> M2:   0.750cm   37.4deg
> S2:  0.310cm   48.3deg
> In small  bodies of water (Lake Michigan is small by the standards of the
> gravitational  tides), the tide basically tracks the Moon's position in the
> sky
> with a small  inertial lag, which is why the phase angles are low. The
> luni-solar tides in the  other great lakes are quite a bit smaller.

   

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