Forums > Windsurfing   South Australia

Big tide, little tide, little tide, big tide

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Created by jn1 > 9 months ago, 24 Apr 2017
jn1
SA, 1730 posts
24 Apr 2017 5:44PM
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I asked this question many years ago, but never got a straight answer. In summer, why do we get big tides in the morning, then little tides in the afternoon, and in winter, get little tides in the morning, and big tides in the arv ?. Sea temperature ?, less windier conditions ?, gravitational effects of the sun ?

Jacko51
SA, 160 posts
24 Apr 2017 10:17PM
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The position of the Moon.
Google has 11,100,00 results. Here is just one with drawings
www.enchantedlearning.com/subjects/ocean/Tides.shtml

Von
SA, 91 posts
25 Apr 2017 12:03AM
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Would have to be tha angle of tha dangle

Slim Jim
SA, 186 posts
25 Apr 2017 10:13AM
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The "Dodge" Tide is probably affecting your perception of tide. It's quite remarkable and only happens in a 2-3 places in the world. It's caused by KI blocking the water inflow/outflow at certain phases of the moon.

kdalton
SA, 72 posts
25 Apr 2017 3:28PM
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its only called a dodge tide in sa .other places call them neap tides.the water still moves in and out but very slow.goolwa spencer gulf yorkes have dodge or neap tides so ki has nothing to do with it

jn1
SA, 1730 posts
25 Apr 2017 8:53PM
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Thanks for the reply guys. Just to clarify what I am talking about:

Winter pattern

Summer pattern


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

Jacko51
SA, 160 posts
26 Apr 2017 12:04PM
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The Earth's axis in relation to the Sun ( Southern hemisphere closer in Summer further away in Winter) and the time the Moon revolves around the Earth ( 24 hours 50 minutes ) add to the equation of tide calculations.
Another explanation here. oceanservice.noaa.gov/education/kits/tides/tides04_angle.html

kdalton
SA, 72 posts
26 Apr 2017 1:48PM
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this is my theory.in summer the lows are further out to sea and the highs are on the water.in winter the lows move closer to land and the highs move inland.the highs have higher barometric pressure which dosnt push down on the water so hard so thats why we get .1m low tides in summer.and in winter the lows have a lower barometric pressure pushing down on the water harder so we only get .4m low tides in winter.the high tides in summer in the morning are bigger than the arv are caused by atmospheric pressure or heating of the land.at night time the land cools down in summer so the barometric can push against the atmospheric pressure because the lands cooled down so we get bigger tides in the morning in summer. in the arv the land heats up and the barometric pressure cant push against the atmospheric pressure because the lands heated up. so the high tides arnt as big in the arv.in winter the high tides in the arv are caused by low barometric pressure pushing down on the water harder and when the land cools down the atmospheric pressure drops and the tides are higher.and a bit of swell and a sw wind the tides get even bigger.so its all got to with barometric and atmospheric pressure.too much pressure time for a beer.

jn1
SA, 1730 posts
26 Apr 2017 11:38PM
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Slim Jim: if a perceived/actual phase shift is happening every dodge tide, then why don't we see a phase shift every fortnight ? (both in summer and winter)

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 ?

kdalton
SA, 72 posts
28 Apr 2017 10:32AM
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ye my theory was aload of cr#p.maybe its gots to do with the suns closer to the earth in summer and further away in winter.and the moon rotating above the below the equator in different seasons.if u work it out jn let me know because I have been wondering aswell.

jn1
SA, 1730 posts
29 Apr 2017 12:20AM
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I contacted Lachlan at the BOM. He has given me a broad explanation and some online resources to read. Very interesting. Give me a few days to digest the data so I can explain it.

Roar
NSW, 470 posts
29 Apr 2017 12:56PM
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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.

jn1
SA, 1730 posts
29 Apr 2017 10:50PM
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Select to expand quote
Roar said..
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.




jn1
SA, 1730 posts
30 Apr 2017 10:38PM
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Ok, I've had a crack at understanding the info BOM given to me. This is an academic field, and I can't understand it over a few nights reading. I'm also missing some data that would allow me to calculate the Outer Harbor tides. This data isn't available on the net. So I'm going to leave it there and give you the info BOM explained to me. Basically, our gulf is subjected to three primary tidal "constituents": M2, S2 and K1.

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.



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"Big tide, little tide, little tide, big tide" started by jn1