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fxop
Joined: 13 Jun 1998 Posts: 203
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Posted: Wed Feb 21, 2024 2:07 pm Post subject: Maui's Weak High Tides in Winter |
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Last year user "jse" asked on the west coast forum:
"Why is there a seasonal pattern of highest tides early in the day (midnight to noon) on Maui during winter months and late in the day (after noon) during summer months."
jse mentioned that Google searches yielded nothing. The University of Hawaii website was searched -- nothing. Chatgpt was consulted, but it just made stuff up. Thus began a journey, which began with trying to answer the question, is it true?:
Yes, there are noticeable "holes" in the plot of high tides. In the first figure below, the strongest tides are blue, and we don't get those tides in the windsport window of 12-5pm until April. The next figure is another look at the data, where blue dots show time points where the tide is above 1.5ft. White space means shallow waters, and there is a lot of white space in Jan-Feb.
The lack of published information motivated a serious effort. In the process several surprises emerged:
* Full moon high tides are just as strong as new moon high tides for Maui and the few other ports that were analyzed, contrary to intuition about the moon and sun "pulling together" on new moon tides
* For Maui, there are more max (king) tides around Aug 1 than Jan 1.
* The Moon doesn't "lift" the waters, it sweeps or gathers the tidal waters horizontally to create the tidal bulge
* The moon being overhead doesn't mean a high tide is happening right now. In Maui there is a variable 0 to 4 hour lag between a zenith transit and the associated high tide.
* NOAA tide predictions are not done from mathematical first principles, they are done with least squares fits to a 19 year set of observations
The "holes" happen when the tide temporarily transitions from semi-diurnal (two high tides per day) to diurnal. The morning high tide disappears for a few days, reappearing as an early afternoon weak high tide in the winter. There is no simple astronomical answer as to why that happens in Maui and not San Francisco. A blog was created for people who would like to see the details including explanations for the "surprises" above:
https://tidepatterns.blogspot.com/
The plots and analysis were facilitated by an oceanography package written by Dan Kelley, professor of oceanography at Dalhousie University, Halifax, Canada. Here is Dan's practical answer -- the sloshing is complicated:
"I suspect surfers might want a "why" answer that is along the lines of "the sun is more nearly overhead at certain times of the year, owing to the tilt of the earth's rotation axis to the plane of the earth's orbit around the sun" but that is a bit made-up. Which constituents are dominant in any given area depend on the shape of the ocean there, nearby, and far away. Think of a bowl of soup sloshing as you walk across a food court, and now imagine that the bowl is not symmetric, but instead has deep and shallow portions that make for complicated sloshing."
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Last edited by fxop on Wed Feb 21, 2024 6:26 pm; edited 1 time in total |
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jse
Joined: 17 Apr 1995 Posts: 1460 Location: Maui
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Posted: Wed Feb 21, 2024 5:06 pm Post subject: Re: Maui's Weak High Tides in Winter |
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Cool. Thanks for this
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