Tides

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Super Anarchist
1,043
416
43 south
I have a reasonable grasp of weather systems and the forces behind tides, but I'm struggling to get my head around  90 minutes (and 1cm) between high and low tomorrow (Thursday Dec 3). This is Hobart, I heard somewhere what we share an unusual tidal system to Cape Horne, due to the Antarctic Circumpolar Current... I don't know how much water that explanation holds- pardon the pun.

Can the brains trust offer an explanation? 

http://www.bom.gov.au/australia/tides/#!/tas-hobart

image.png

 

Mid

Super Anarchist
There are three prohable reasons for this failure.

The first reason lies in the smallness of the tidal range and the cons~qupnt relative amount of conection to be applied for such factors as change of barometric pressure, prevailing wincl, &c.

Another reason is th(; lack of economic necessity fOl' such an accurate prediction.

Due to the depth of water in the harbour, a difference of a few feet is of little importance.

The third reason lies in the fact that there is a double entry for the tides at Hobart.

https://core.ac.uk/download/pdf/33323347.pdf

bold mine 

 

Pipe Dream

Anarchist
597
86
Aust.
There are three prohable reasons for this failure.

The first reason lies in the smallness of the tidal range and the cons~qupnt relative amount of conection to be applied for such factors as change of barometric pressure, prevailing wincl, &c.

Another reason is th(; lack of economic necessity fOl' such an accurate prediction.

Due to the depth of water in the harbour, a difference of a few feet is of little importance.

The third reason lies in the fact that there is a double entry for the tides at Hobart.

https://core.ac.uk/download/pdf/33323347.pdf

bold mine 
hmmm not so sure about some of this:

- as best as I know, prevailing wind and barometric pressure are not factors in tide prediction. 

- there has been a tide gauge in Hobart since 1889 (I think) so the economic necessity/accuracy argument is not valid

- double entry? Do you mean an actual mistake in the tide tables?

Hobart tide gauge info

http://www.bom.gov.au/ntc/IDO50000/IDO50000_61220.pdf

http://www.bom.gov.au/ntc/IDO70000/IDO70000_61220_SLD.shtml

This is a good thing to read too ("Dodge Tide" may possibly explain Hobart??)

http://www.bom.gov.au/oceanography/projects/ntc/NTC_glossary.pdf

I can't give an answer to the original question but I have learnt something so thank you  :)

 
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Not sure why but I do have an oceanographer friend working at Tidetech which is based in Hobart. 
I will ask him and get an answer, maybe 

www.tidetech.org

 
From my oceanographer friend at Tidetech. 
 
Attached is a timeseries of 20-min predictions for Hobart this week. As you can see on a daily basis the shape of the tide varies from the almost once a day occurrence that  you have noticed to a more regular nearly twice a day tide. This kind of tide is called 'mixed' for obvious reasons. Why? Because there is competition between the once a day tidal forces and the twice a day tidal forces. Oceans are affected by both these tidal frequencies but the shape (and depth) of the ocean offers more favourable response to one set or the other; the pacific favours once a day (diurnal), the Indian ocean (and the southern ocean) favour the twice a day (semi-diurnal) frequency. As Hobart is caught in the middle of 2 oceans it experiences both more or less equally. If we look at the amplitude of the different tidal frequencies we see the semi-diurnal values are (M2=0.254m, S2=0.011m), diurnal values are (K1=0.222, O1=0.150) [M2 and K1 relate to the Moon, S2 and O1 relate to the Sun), so as S2 is very small there is almost no Neap-Spring cycle variation, but we see that the combined diurnal amplitudes are almost equal to the M2 amplitude, and this introduces a monthly variation instead of the 2-week neap-spring usually seen.







hobart_tide_level.png 






 






 

The Q

Super Anarchist
hmmm not so sure about some of this:

- as best as I know, prevailing wind and barometric pressure are not factors in tide prediction. 
Tide level certainly can be affected  by prevailing wind and barometric pressure.

If we get a northerly /  north easterly wind the southern North Sea gets a higher tide.

If there is a low pressure going over we get a higher tide..

if it's spring tides as well we get flooding all along the coast.

I normally sail about 20 miles inland, wind direction, air pressure  both can delay or advance the time the tide reaches us.. Our Nominal tide time is 4 hours after high tide at the coast.

Rarely it can occur  with westerlies / southerlies, high air pressure, and low water, it just keeps going out and we don't see a high tide at all.. 

That's all affected by the state of the tides along the coast, wind direction and air pressure..

 
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Panoramix

Super Anarchist
From my oceanographer friend at Tidetech. 
 
Attached is a timeseries of 20-min predictions for Hobart this week. As you can see on a daily basis the shape of the tide varies from the almost once a day occurrence that  you have noticed to a more regular nearly twice a day tide. This kind of tide is called 'mixed' for obvious reasons. Why? Because there is competition between the once a day tidal forces and the twice a day tidal forces. Oceans are affected by both these tidal frequencies but the shape (and depth) of the ocean offers more favourable response to one set or the other; the pacific favours once a day (diurnal), the Indian ocean (and the southern ocean) favour the twice a day (semi-diurnal) frequency. As Hobart is caught in the middle of 2 oceans it experiences both more or less equally. If we look at the amplitude of the different tidal frequencies we see the semi-diurnal values are (M2=0.254m, S2=0.011m), diurnal values are (K1=0.222, O1=0.150) [M2 and K1 relate to the Moon, S2 and O1 relate to the Sun), so as S2 is very small there is almost no Neap-Spring cycle variation, but we see that the combined diurnal amplitudes are almost equal to the M2 amplitude, and this introduces a monthly variation instead of the 2-week neap-spring usually seen.







hobart_tide_level.png 






 
I love this place!

In the Solent they also have weird tides but I think that it is for a different reason (complex coast). In Brittany we are blessed with nearly perfectly sinusoidal tides that a 1980s pocket calculator can predict within a few centimetres (except the pressure / wind effects). A pencil held by somebody who can divide height in twelfth and real hours into "tide hours" (that is the difference in time between HW and LW divided by 6) is even good enough.

 
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JohnMB

Super Anarchist
2,749
534
Evanston
- as best as I know, prevailing wind and barometric pressure are not factors in tide prediction.
Tide level certainly can be affected  by prevailing wind and barometric pressure.
There is no question that wind and pressure affect the tides. However they should not affect the tide predictions in the tide tables, which are usually prepared well in advance of any knowledge of the prevailing climatic conditions.

 
There is no question that wind and pressure affect the tides. However they should not affect the tide predictions in the tide tables,
One of my very best friends is hydrographer from the Spanish Navy. One day I asked him about this things and he told me he ence measured a two hour deviation from the tables due to a river flow and a very high pressure system.

It's not an exact science.

 

Panoramix

Super Anarchist
One of my very best friends is hydrographer from the Spanish Navy. One day I asked him about this things and he told me he ence measured a two hour deviation from the tables due to a river flow and a very high pressure system.

It's not an exact science.
The tidal heights (ie variations due to the moon and the sun) are well understood in most places. What's harder to predict is wind and pressure effects as it becomes weather forecasting with al the associated complexities on top of fluid dynamics!

 

Pipe Dream

Anarchist
597
86
Aust.
There is no question that wind and pressure affect the tides. However they should not affect the tide predictions in the tide tables, which are usually prepared well in advance of any knowledge of the prevailing climatic conditions.


Looking into the details I wonder if there is an adjustment for average climatic factors. In the details for the Hobart tide gauge, it mentions "Doodson’s method Sa and Ssa from a long period analysis (39.6 years) of Hobart (1960-2010) for the 2013 tide predictions".

Maybe the initial predictions are based on the astronomical data about the moon and then fine-tuned to better fit the "long period analysis" as mentioned??

 

JohnMB

Super Anarchist
2,749
534
Evanston
Looking into the details I wonder if there is an adjustment for average climatic factors. In the details for the Hobart tide gauge, it mentions "Doodson’s method Sa and Ssa from a long period analysis (39.6 years) of Hobart (1960-2010) for the 2013 tide predictions".

Maybe the initial predictions are based on the astronomical data about the moon and then fine-tuned to better fit the "long period analysis" as mentioned??
Cool ref... I learnt something new  (again) :)

Doodson came up with a method to decompose the tides in to various harmonic components based on astronomic factors.

In this case I believe long period analysis refers to the calculation for the long period factors in Doodson's decomposition  Ssa and Sa  are the Solar seminanual and solar annual parameters.

I don't believe that there is a climatic adjustment in there.

 

J_Grove

Member
102
47
Biscayne Bay
I would say that there are tides and "tides" as commonly understood by sailors, surfers, etc.

By definition, the tides are deformations due to gravitational forces, and so are not a function of wind, atmos. pressure, and river flow. Doesn't even have to be water - there are solid earth tides. Sea level tides in open ocean, continental slope, and some part of continental shelves are known to incredible accuracy now, thanks to satellite altimeters like Topex/Poseidon (launched in 1992) and follow on missions combined with tidal modeling.  Tides in coastal areas with complex bathymetry are not known nearly so well, unless you are close to a tide gauge.

"Tides" as most people experience them, i.e. I want to know when it's coming in or going out, and by how much at a particular spot on the coast, are very much affected by wind, pressure, river flow, and even wave conditions. But oceanographically speaking these are separate effects on sea level.

The really long period tides (decades+) are also really small and wouldn't have a noticeable effect on the local tide to the average observer. But there is growing evidence that the "king tides" which are increasingly common are due to the "regular" tidal harmonics combined with sea level rise due to steric effects (warmer water has very slightly more volume) and melting polar ice.

 
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