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Created by jn1 > 9 months ago, 24 Apr 2017

Google has 11,100,00 results. Here is just one with drawings

www.enchantedlearning.com/subjects/ocean/Tides.shtml

Winter pattern

Summer pattern

Jacko, I read your link. It did not answer my question.

Another explanation here. oceanservice.noaa.gov/education/kits/tides/tides04_angle.html

kdalton: Interesting theory KD, but tides are published without the effects of storm surge. So, the effect you are talking about can't be explained by looking at tide charts. (How could BOM know when a low is going to go over Adelaide in 6 months time ?. They can barely predict a seabreeze ! )

The effect I am talking about is a phase shift that occurs twice a year. I think it's because we have a mixed tide (as you implied SJ). The Sun's changing declination (going from plus to minus and then minus to plus at the equinoxes) and our gulf's bathymetry (ie: shape) will cause the winter and summer tides to be out of phase. There will be multiple water current vectors flowing into/out of our gulf. Added up will give a tide at a particular point. These vectors in our gulf (vector = magnitude value + phase value) will go out of phase to each other during the equinoxes. I'm pretty sure some clever researcher has simulated and published this effect

PS/ What I've always wondered. Imagine if we got big tides in summer afternoons. How much stronger would our seabreezes be ?. Could they be on par with Perth ?

Select to expand quote

your sample is too small - look at over a longer period and you will see it just cycles slowly. if you wait long enuf the paternt will be reversed.

The Principal lunar semidiurnal ("M2"): This force is due to the moon spinning around the earth and causes ocean facing the moon to bulge towards to moon, and ocean the other side of the earth bulge way from the moon. So, this constituent gives us the two high tides and two low tides over a period of 24 hours.

The bulge on the other side of the earth (in line with the moon) is due to the centrifugal forces of the earth swaying about the earth-moon barycentre (a bit like an discus thrower spinning a discus - the discus thrower is going to wobble as he spins the discus around)

Principal solar semidiurnal ("S2"): Same effect as the earth-moon except it's the sun-earth - so the sun's gravity is causing it. It will also cause it's own bulge, giving two high tides and two low tides. Period is 12hrs exactly (half a day).. or 2 high tides and 2 low tides in 24 hours. This force is far weaker than M2... approximately 1/3 of M2.

Lunar diurnal ("K1"): I've got a sketchy understanding of this force. I believe it's latitude dependant, and to do with a location having a big high tide and then 12 hours later having low or no high tide due to the location being outside of the "bulge" (see diagram above - Points "Y" and "X"). It's period is 23.93hrs, meaning that this force causes 1 high tide and 1 low tide per day (hence diurnal). This constituent has a 365 day beat frequency that gives us this post's titled effect (read on..).

So, to calculate the Outer Harbour tides, you add these constituents up (M2 + S2 + K1)... but due to the shape of our gulf (and KI getting in the way), each constituent will first need to be amplified (scaled) and advanced/retarded (phase shifted) before the summation.

The tide constituent that causes big high tide/little high tide in summer and little high tide/big high tide in winter is K1. With this force removed, our tides will look different, but the mentioned effect will also be removed. If you look carefully at the tide graphs I posted in my last post, you'll see that the heights of the two daily high tides switch sizes at the equinoxes (March 20 and September 23). This is K1 at work !

Some light reading:

en.wikipedia.org/wiki/Arthur_Thomas_Doodson#Doodson_numbers

tidesandcurrents.noaa.gov/restles3.html

en.wikipedia.org/wiki/Theory_of_tides#Higher_harmonics

en.wikipedia.org/wiki/Earth_tide

eprints.soton.ac.uk/19157/1/sea-level.pdf

If anyone thinks I could be misinterpreting BOM's explanation (they were brief !), then I can PM you their response. I won't post it here.