On Wednesday, September 23, 2020 at 8:00:26 PM UTC+1, wrote:
On Wednesday, September 23, 2020 at 3:42:49 PM UTC+1, N_Cook wrote:
On 23/09/2020 15:03,
On Tuesday, September 22, 2020 at 11:46:24 AM UTC+1, N_Cook wrote:
On 22/09/2020 11:28, Graham Easterling wrote:
On Tuesday, September 22, 2020 at 11:20:01 AM UTC+1, Norman Lynagh wrote:
Graham Easterling wrote:

The weekend saw the highest astronomical tide of the year. However,
it was even higher than predicted, and still is running above
predictions.

https://www.ntslf.org/data/realtime?port=Newlyn

This is despite all the weather and sea conditions suggesting it
should be below predictions. The local weather light winds (basically
calm yesterday), fairly high barometric pressure. Local sea
conditions, flat in Mount's Bay. Atlantic swell very small, so no
wave setup surges. In fact, all the factors which increase tidal
height were absent, all the normal factors which reduce it were
present. I'd expected it to be 0.3-0.5m below predictions.

I was down at the Battery Rocks for a swim yesterday, and got
chatting to a chap who had been a year long swimmer there most of his
life. He'd never seen the tide so high in totally benign conditions.

No I'm baffled, as was the chap I spoke to, & I respect the opinion
of people with his sort of observational experience. Mind you the sea
is a mysterious thing.

Graham
Penzance

Sea level rise? Perhaps only apparent in calm conditions.

--
Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.
https://peakdistrictweather.org
twitter: @TideswellWeathr

I don't think it's the whole story, experience has shown me that under recent conditions the actual tide is normally below predictions.

Mind you, over recent decades there has been a rise at Newlyn, which is supposed to be a stable site. (Though Scilly is sinking)

A study on sea level at Newlyn here
https://www.tandfonline.com/doi/full...ean%20w aters.

I can only assume that some weather event elsewhere in the north Atlantic has caused a 'dome' of water which has proved persistent.

Graham
Penzance


Although NTSLF uses supercomputer oceanographic processing of the first
pass MetO data, there are problems with it by the time it is output , if
not before.
Notoriously , www presentation so of no significance the likes of 07:60
instead of 08:00 for timings on their outputs.
Of more significance , if you look carefully at the archives, assuming
the same for Newlyn as for Southampton.
Coincident with change-over times of BST/GMT, over the week before and
after, there is a vertical "DC" offset in the mean sea level of about
0.15m, there until the next clocks change.
Unfortunately al loceanographic major processing uses Fortran still and
the people who developed the NTSLF tide predictor SW have retired, so mo
one to rectify its failings.
For Southampton it regularly fails to predict surges from the SSW, ie
wind that goes behind the Brest Peninsular and wind-stresses the water
betweem St Brieuc and IoW, worst case failing to predict about 0.8m surge..



--
Global sea level rise to 2100 from curve-fitted existing altimetry data
http://diverse.4mg.com/slr.htm

As I mentioned on UK Weather and Climate group, the unusual high tides, above astronomcal, timed at spring tide are probably a bit of extra swell. Maybe from the rather numerous Atantic hurricanes. The forecast models don't cope well with rogue waves nevermind rogue swells. Non linear interactions, unusual wave trains and all that. Notice the amplitudes are the same as astronomical.

Len
Wembury


I somehow thought Newlyn opened onto the Atlantic . Looking at a map I
see it opens on to the Eng Channel so the higher tides was simply the
east-west surge moving along the Channel over the weekend, as in
Plymouth. Weymouth, Southampton etc , due to the east wind over the
weekend. NTSLF "surge predictor" not accounting for it, well nothing new
there. Oddly it does very well at predicting delayed surges from strong
WNW wind passing over Southampton (causing immediate local negative
surge) and the positive surge reflecting off the Pas de Calais coast
back to the Solent.

--
Global sea level rise to 2100 from curve-fitted existing altimetry data
http://diverse.4mg.com/slr.htm

Interesting that.
I'll look out for it next time after an easterly down the channel.
Quite a small effect compared with North Sea surges but winds are not so strong and MSLP not so low.
How long does the easterly need to blow to have an effect?

Len
Wembury, SW Devon

Wembury, SW Devon

That wasn't the reason, I say so for 2 reasons
1. The north coast of Devon / Cornwall was also affected (look at Ifracombe..)

2. Even on the south coast I've never noticed an easterly of that strength have much affect in Mount's Bay even when it's occurring, let alone 2 days later when what breeze there was was an offshore NE. What swell left in Mount's Bay was a longish period low swell, clearly of fairly distant west Atlantic origin. (I mention the low long period swell om Weather & Climate).

I think the only feedback I had that comes close to explaining the anomalous tides, is your hurricane effect Len. That would explain both coasts being similarly affected, and there was a long period swell, albeit small, as I mentioned on Weather & Climate. The period suggesting a west Atlantic origin.

On 22/09/2020 11:37, Graham Easterling wrote:



No mystery to us in Southampton.

It's a mystery to me, as it's very different to what I'd expect based
on years of observation & the prevailing weather & sea conditions,
which are particularly significant here with the wave setup
conditions we experience, and the shape of the Bay making it very
wind affected as far as tidal height is concerned.

It is just after the new moon and at an equinox so the geometry is just
about perfect for seeing maximum tidal range. I am at a bit of a loss as
to why it should occur a couple of days after new moon though. Thursday
was perfect new moon and Friday 18/9 was the moon at closest approach.

I guess it is the difference between theory (uniform ocean on a perfect
sphere) and reality with all those nasty continents and coastlines
about. I have the astronomical tidal coefficients somewhere and will dig
it out to see if there was anything particularly unusual.

Do you have tidal curves for 17/9-21/9 ? I'd be interested to see them.

16/10 also has favourable geometry for a good high tide as well.

--
Regards,
Martin Brown

On 23/09/2020 20:00, wrote:
Interesting that.
I'll look out for it next time after an easterly down the channel.
Quite a small effect compared with North Sea surges but winds are not so strong and MSLP not so low.
How long does the easterly need to blow to have an effect?

Len
Wembury, SW Devon

Because of funnelling/shelving of the Eng channel, W-E surges tend to
get larger Newlyn to Dover and the reverse for E-W surges. Its a matter
of following the situation, tide gauge to tide gauge along the channel,
near enough real time , as NTSLF does not retain hi-,res plots .
IOC resource may have sufficient archives
http://www.ioc-sealevelmonitoring.org/station.php?code=newl2
(aside: I've never seen a newl output , the tsunami filter output, even
in academic papers)
You'd need paid-for admiralty tide data via the ukho site, eg
http://www.ukho.gov.uk/easytide/EasyTide/ShowPrediction.aspx?PortID=0062&PredictionLength=7
But all would be messy compared to some future set of NTSLF plots of
combined tide-gauge and admiralty tide data , so with residuals, with
an east wind situation.
More common is E-W surges from surges in the North Sea, eg
Feb 1953 "Canvey Island" catastrophic flooding from North sea storm, led
to a 5 foot surge being recorded at Southampton, after passing thru the
Dover Strait.

--
Global sea level rise to 2100 from curve-fitted existing altimetry data
http://diverse.4mg.com/slr.htm

On Wednesday, September 23, 2020 at 9:08:41 PM UTC+1, Martin Brown wrote:
On 22/09/2020 11:37, Graham Easterling wrote:



No mystery to us in Southampton.

It's a mystery to me, as it's very different to what I'd expect based
on years of observation & the prevailing weather & sea conditions,
which are particularly significant here with the wave setup
conditions we experience, and the shape of the Bay making it very
wind affected as far as tidal height is concerned.

It is just after the new moon and at an equinox so the geometry is just
about perfect for seeing maximum tidal range. I am at a bit of a loss as
to why it should occur a couple of days after new moon though. Thursday
was perfect new moon and Friday 18/9 was the moon at closest approach.

I guess it is the difference between theory (uniform ocean on a perfect
sphere) and reality with all those nasty continents and coastlines
about. I have the astronomical tidal coefficients somewhere and will dig
it out to see if there was anything particularly unusual.

Do you have tidal curves for 17/9-21/9 ? I'd be interested to see them.

16/10 also has favourable geometry for a good high tide as well.

--
Regards,
Martin Brown

It is not the astronomical height of the tide that is in question, it was forecast to be the highest tide of the year for the reasons you describe. It was the predicted /actual discrepancy which was a mystery. As I said in my original post, all the normal factors were in favour of decreasing the astronomical tide by a small amount (I'd expected around -0.3m at Newlyn). I think, in the absence of anything more convincing, the hurricanes must have caused some doming of the sea surface, despite the long period swell from them being of a fairly minimal height (0.5-1'). Though it was noticeable whilst swimming. Although the sea looked glassy in flat, there was a good 1' sluggish (very long period) rise and fall.

The biggest tides are typically 36-48 hours after the full/new moon. It's a complicated area, I lead a sad life studying tide tables, they reveal some interesting detail.

Further to my last post, Mount's Bay is actually very sheltered from the E. It is SSW-SSE winds which pile the water into Mount's Bay. Even easterly gales have limited affect at Newlyn, a SSE gale is a very diffeent story, especially if accompanied by a depression moving out of Biscay at just the right speed.

One study here. http://www.turnstone-cottage.co.uk/PzStorm.PDF where I say
"The sea was so huge because the storm surge and swell moved north at roughly the same speed as the developing depression, and therefore built up over 2 or 3 days before hitting Cornwall" I recently read an account of that storm by Philip Eden where he says exactly the same thing.

I've spent my life studying sea conditions, and I still get surprised.

Graham
Penzance

On 23/09/2020 20:29, Graham Easterling wrote:
That wasn't the reason, I say so for 2 reasons
1. The north coast of Devon / Cornwall was also affected (look at Ifracombe..)

2. Even on the south coast I've never noticed an easterly of that strength have much affect in Mount's Bay even when it's occurring, let alone 2 days later when what breeze there was was an offshore NE. What swell left in Mount's Bay was a longish period low swell, clearly of fairly distant west Atlantic origin. (I mention the low long period swell om Weather & Climate).

I think the only feedback I had that comes close to explaining the anomalous tides, is your hurricane effect Len. That would explain both coasts being similarly affected, and there was a long period swell, albeit small, as I mentioned on Weather & Climate. The period suggesting a west Atlantic origin.

Even then I find it surprising considering so many factors being against it..

Norman's post about MSL rise is interesting though, especially since I've now reread the article on Newlyn observations I linked to. It does suggest that benign conditions will tend to show actual readings biased high.

Graham
Penzance

I know nothing of tides in the Severn Estuary, perhaps it could be a
separate local east wind surge situation there, hugging the coast perhaps.
Perhaps Ilfracombe would show an excess height but not Avonmouth.
You need a good fetch , even with gale force wind to induce enough
windstress to the sea surface , hence surge, to be detectable on tide
gauges beyond normal storm induced jitter.

--
Global sea level rise to 2100 from curve-fitted existing altimetry data
http://diverse.4mg.com/slr.htm

A lot of strong N wind (up to sustained mean 55mph) GFS has over
sufficient fetch of the North Sea 25 Sep, driving seawater into the
bottom of the North Sea. There may be a noticeable E-W surge along the
Rng Channel 26 Sep, to have enough left to be observable on Newlyn tide
gauge. NTSLF tends to pick up such wind and surge but not east wind and
then lack of predicted consequential surge in the Eng Channel

--
Global sea level rise to 2100 from curve-fitted existing altimetry data
http://diverse.4mg.com/slr.htm

On 24/09/2020 08:31, N_Cook wrote:
A lot of strong N wind (up to sustained mean 55mph) GFS has over
sufficient fetch of the North Sea 25 Sep, driving seawater into the
bottom of the North Sea. There may be a noticeable E-W surge along the
Rng Channel 26 Sep, to have enough left to be observable on Newlyn tide
gauge. NTSLF tends to pick up such wind and surge but not east wind and
then lack of predicted consequential surge in the Eng Channel


For logging such progress over Dover,Newhavem,
Pompey,Bournemouth,Weymouth,Plymouth, Newlyn tide gauges, E-W surges
tend to be faster than the normal tide pulse going W-E along the Eng
Channel, could be about 8 hours full transit Dover to Newlym, rather
than about 12 hours.

--
Global sea level rise to 2100 from curve-fitted existing altimetry data
http://diverse.4mg.com/slr.htm

So based on NTSLF blue run, so further processed Met0 06Z modelled
atmosphere output and 24 Sep 00Z met data, so likely to change quite a bit.
Dover peak residual of 1.1m at about 22:30 25 Sep and
projected/guestimated peak residual at Newhaven about 0.6m about 01:00
26 Sep.
Guesstimate of about 0.2m residual remnant of the E-W surge, so
observable, at the Newlyn tide gauge a bit after am low tide on 26 Sep

--
Global sea level rise to 2100 from curve-fitted existing altimetry data
http://diverse.4mg.com/slr.htm

On 24/09/2020 11:31, N_Cook wrote:
So based on NTSLF blue run, so further processed Met0 06Z modelled
atmosphere output and 24 Sep 00Z met data, so likely to change quite a bit.
Dover peak residual of 1.1m at about 22:30 25 Sep and
projected/guestimated peak residual at Newhaven about 0.6m about 01:00
26 Sep.
Guesstimate of about 0.2m residual remnant of the E-W surge, so
observable, at the Newlyn tide gauge a bit after am low tide on 26 Sep


NTSLF black run showing too little E-W surge to survive attenuation down
the Eng Channel, to see on the Newlyn gauge but going by latest GFS
increased area of strong wind over the North Sea, perhaps blue and red
runs today will go higher.
Timings approx GMT and approx surge heights
Dover 22:30 25 Sep, 0.9m
Hewhaven 00:00 26 Sep, 0.35m
Pompey 03:00, 0.22
Weymouth maybe 04:00 , vague hump somewhat lost in the general rough
weather France/England seiching.
Plymouth maybe 05:00
Newlyn maybe 06:00 GMT


--
Global sea level rise to 2100 from curve-fitted existing altimetry data
http://diverse.4mg.com/slr.htm