Interpreting tide charts
I've been investigating the tide charts at NOAA (https://tidesandcurrents.noaa.gov/tide_predictions.html) and have noticed something..... that seems odd to me.
For locations on the west coast of the US (Hawaii, AK, CA for three that I've seen), there appears to be a "stutter step" sort of pattern in the tidal patterns.... high, low, medium high, medium low, high......
whereas on the east coast and in the Gulf of Mexico, it's a smoother "sine wave" shape.
Why the difference in the patterns?
Comments
Not an expert, but... I think it's difference between a mixed tide (west coast) and a semidiurnal tide (east coast).
NOAA has a map showing where you will find the different tide types: https://oceanservice.noaa.gov/education/tutorial_tides/media/supp_tide07b.html
More info from sciencing.com:
@evmo --
I think you're correct.
For anyone who might like to see what the difference is......
For instance, in the Gulf of Mexico -- diurnal -- only a single high/low tide per 24 hr 50 min period.
(https://tidesandcurrents.noaa.gov/noaatidepredictions.html?id=8735180&units=standard&bdate=20201201&edate=20201231&timezone=LST/LDT&clock=12hour&datum=MLLW&interval=hilo&action=monthlychart) Note the long look of the sine wave in the chart.
VA Beach, though, is semidiurnal -- two hi/low tides per 24 hr 50 min period.
(https://tidesandcurrents.noaa.gov/noaatidepredictions.html?id=8639168&units=standard&bdate=20201201&edate=20201231&timezone=LST/LDT&clock=12hour&datum=MLLW&interval=hilo&action=monthlychart) Note the more compressed look of the sine wave in the chart.
Honolulu, HI.... is mixed.... two hi/low tides, but they aren't the same height. The article evmo linked to says that mixed tides are what makes surfing so great on west coasts.
(https://tidesandcurrents.noaa.gov/noaatidepredictions.html?id=1612340&units=standard&bdate=20201201&edate=20201231&timezone=LST&clock=12hour&datum=MLLW&interval=hilo&action=monthlychart) See the stutter step in the chart?
The tide in Adelaide South Australia is sometimes sinusoidalish and sometimes the stutter step type waveform.
https://www.tide-forecast.com/locations/Port-Adelaide-Australia/tides/latest
Yes the forces interacting that creates the exact tides appears quite complex.
The mathematicians who devise the forecasts must understand it fully.
They are very complex: a summation of harmonic constituents taking into account Moon position, Sun position, relative position, corrections for swallow waters... Each hydrographic service use different formulas: in Spain we use 68 harmonics; in the USA, 37; in France, 105; and in Australia, 115. And these are only the main constituents!
But they are also very regular, so predictions are easy and accurate - when based on data series taken for many many many years. Cycles repeat every 18 years (meaning that one specific tide will be identical to another one, the same day in 18 years time).
Exact forecasts can only be get in specific locations, where tide gauge stations have been installed for long enough (less than 2000 stations throughout the world); those are the ones published in tables. For other locations, calculations need to be done (nowadays, easily done with computers).
Tides and their streams are an amazing phenomena. I wrote an article about them: https://dieciseisgrados.com/2020/02/02/el-abece-de-las-corrientes-de-marea/ . It's in Spanish, though; but you'll find a lot of links with my sources.
@loneswimmer wrote one in 2 chapters, very worth a read: https://loneswimmer.com/2010/08/23/tides-for-swimmers-part-1-theory/
dieciseisgrados.com/
Huh! This is the first time I've seen this aspect of tides described. Fascinating! I love learning new stuff from smart people!
Thanks for the info!