Pressure and Sea levels

[FlyingSpud]

Sorry if this is a daft question, but we all know that air pressure affects sea level in British waters in a reasonably predictable way , but how does it work in, say, the Mediterranean?

I ask this question as I recently came back from a windsurfing/dinghy sailing holiday in Turkey. The instructors there had noticed that the sea level often seemed to rise, while air pressure was increasing.

My guess is that the Mediterranean acts as a sealed sea, and that to be able to predict the affect of air pressure on sea level you would need to know the average air pressure over the whole sea at any one time. So if the average is increasing faster than the local air pressure sea level still goes up. Is this what is happening? Or is there another explanation?

Do ‘semi enclosed’ areas such as, say, the Clyde or the Irish Sea also have this affect?


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[jimi]

I would imagine that when the pressure rose that perhaps there was a local wind system that blew onshore increasing the level?

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[Aeolus_IV]

A thought just occured to me, based on what you're saying.

Would it be true to say that the effect of atmopheric pressure on sea levels would be proportional to the average tidal range for a given area? So, where the range is small (ie the med) the affect would be small, and where the range is larger (say the channel), the effect would be more pronounced.

How does that sound? Jeff.

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[Twister_Ken]

Dunno

but logic would seem to suggest that - as we can assume that seawater is incompressible - for a sea level to fall in one spot in a sealed system it would have to rise elsewhere in that system. Thus, if it is falls most under the area of highest pressure, it may be rising elsewhere, even though the pressure at the point is also high (but not highest). Of course, if there is also a low pressure area in that sealed system, then the level wil rise unde the low, and no rise will be seen in any high pressure spot.

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[clouty]

My five years jumping on and off boats in the Balearics taught me that there at least, high pressure meant lower water and vice-versa. the tidal range was a few inches, air pressure affected water level by range about 18 inches - or 45cm, eurostyle.

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[GeorgeP]

We get +/- 30cm due to air pressure in Poole (with high pressure = less water). We then get a further +/- 20cm due to wind strength and direction (i.e. blowing into or out of the harbour). I'm more used to seeing high pressure associated with less wind. Was that the same in Turkey? And what was the normal wind strength and direction?

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[FlyingSpud]

Re: Dunno

So if there is a given amount of water in the Mediterranean and it cannot be compressed, the average height of water has got to be the same irrespective of the average pressure. All that can happen is that the water slops around.

I think you’re right Ken, my idea was crap. So why does the water go up in High Pressure. I don’t think it was the wind as Jimi or GeorgeP suggest because in western Turkey the wind is usually quite stable in direction and strength.

Odd I’nit


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[Twister_Ken]

Re: Dunno

Levels will rise under high pressure because there is a higher pressure somewhere else in the system, and no low pressure area to allow a compensating rise elsewhere under the low (presumably!!!)

If you look at todays chart <A target="_blank" HREF=http://www.wetterzentrale.de/pics/brack0.gif>(here for the moment)</A> pressure is highest just inshore of Trieste at 1030. At the heel of Italy it is still high - 1024 - but presumably the water levels will have risen there because water is being squeezed down from the top of the Adriatic. When the low currently over S.Spain makes it into the Mediterranean basin, then the levels at the heel of Italy will drop, although the pressure will remain much the same (he guessed!!!)

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[FlyingSpud]

Re: Dunno

Hang on, wasn’t that what I said at the beginning?

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[davidhand]

If you have ever sailed out of Gibraltar you will know that the tidal flow is mostly into the Med. (I have read that it replaces water lost to evaporation) Does a high pressure system over the med affect flows through the Straights of Gib?

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