Interpreting tide charts

Sara_Wolf

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?

evmo

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:

What Causes a Mixed Tide?
A mixed tide occurs in locations where there is a combination of a diriurnal and semidiurnal tide. Since a diurnal tide only has one high tide and one low tide during the typical tidal cycle and a semidiurnal tide has two high tides and two low tides during the same tidal cycle the combination of these two types of tides creates what is known as a mixed tide. This combination does not create even high tides or even low tides. There will be one high tide higher than the next high tide and one low tide that is lower than the following low tide.

Sara_Wolf

@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?

teknik

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.

bruno

@teknik said:
Yes the forces interacting that creates the exact tides appears quite complex.

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/

Sara_Wolf

@bruno said:

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).

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!