Steepness of Mediterranean seas: causes?

[macd]

Anyone who's experienced a blow in the Mediterranean will know how rapidly the seas build, and how frighteningly steep they can quickly become.

I've never encountered a convincing argument as to why this should be so, but recently noticed an observation in the pilot book for Corsica and N Sardinia. In it the author, John Marchment, writes that:
"...the sea is much saltier than the Atlantic Ocean, which increases the surface tension and the surface wind effect...this surface tension effect also helps to account for the incredibly swift build-up of seas". He goes on to include their steepness as part of the same process.

I was a little sceptical of this and consulted Cap'n Google. All I was able to come up with was that surface tension is indeed a factor in the generation of 'capillary waves', but these are typically less than 2cm in length (such as cat's paws?). The process of generating larger waves goes way beyond this, and no measure of surface tension seems to figure in it.

Any thoughts?

 

[Yngmar]

I thought increased surface tension makes the water smoother. This is why sailors of old would always carry an oil bag and as part of their storm tactics would squeeze some oil into the sea (a small amount would suffice), creating an oil film that would calm the waves around their boat.

 

[Davy_S]

I think parts of the Med can be similar to a large lake, there is mainly no noticeable tide to speak of, you have a flat calm lake, the wind gets up, it does not take long for a short choppy wave action to develop. the further the fetch and conditions rapidly worsen. The Med is similar, it is wind driven, not by big tides, the shape of the land and mountains also multiply the effect. it is noticeable in a small planning hull, anything with a shallow vee hull will take a serious pounding in even a force 5, to use something with a cathedral hull, would shake everything to pieces, and yet such a hull would be ok in the tidal waters of the uk. In other words, I don't know either!

 

[guernseyman]

The Mediterranean was discussed at length in a previous thread; I'm not sure it got anywhere:

http://www.ybw.com/forums/archive/index.php/t-412621.html

 

[mjcoon]

I thought increased surface tension makes the water smoother. This is why sailors of old would always carry an oil bag and as part of their storm tactics would squeeze some oil into the sea (a small amount would suffice), creating an oil film that would calm the waves around their boat.

But the surface tension of water is high. Putting oil on it isn't going to increase the surface tension. But it is a well-known ploy...

Salt does increase the surface tension (according to my Kaye and Laby, 12th edition 1962).

Mike.

 

[BurnitBlue]

My gut feeling about this is that the vertical wind direction in the Med is steeper than the vertical wind direction in the open ocean because of the surrounding land mass causes quite big dig difference in barometric pressure at different heights.

Wind typically blows down on the surface of the med sea whereas in the open ocean it is more parallel to the surface. Energy from the wind is quickly transferred to the surface much like cooling a cup of coffee by blowing on it.

Bora and Mistral are katabatic to illustrate the point. Rod Heikal refers to wind "bullets" which in more benign areas are simply called "gusts".

 

[vyv_cox]

Since I installed my new instruments, which show current speed and direction, it has come as quite a surprise to find just how much current there is in the Aegean. Off the SE corner of Kalymnos we were in small overfalls with a current of 1.6 knots. Even in open water there is often more than half a knot. Not much compared with the Irish Sea I realise but it presumably contributes to the waves.

 

[savageseadog]

It might be an issue of perception, if you sail round Britain you expect a pounding on a regular basis.

 

[mjcoon]

My gut feeling about this is that the vertical wind direction in the Med is steeper than the vertical wind direction in the open ocean because of the surrounding land mass causes quite big dig difference in barometric pressure at different heights.

Wind typically blows down on the surface of the med sea whereas in the open ocean it is more parallel to the surface. Energy from the wind is quickly transferred to the surface much like cooling a cup of coffee by blowing on it.

Bora and Mistral are katabatic to illustrate the point. Rod Heikal refers to wind "bullets" which in more benign areas are simply called "gusts".

I don't understand the concept of vertical wind direction. Where is this mass of air going to? It cannot be dissolving in the water. So must be moving horizontally at the surface, where waves are created...

Mike.

 

[vyv_cox]

It might be an issue of perception, if you sail round Britain you expect a pounding on a regular basis.

I sailed around British coasts for long enough! Mediterranean waves are quite different, often described as 'square'. I find it quite remarkable how quickly they develop and with what stopping power. Two days ago we were on a beat for ten miles, in reasonable sized waves and wind around the top end of force three, although there was quite a bit of leftover swell. No water landing on deck. Suddenly the wind increased to about 25 knots true and within no more than a few minutes we were taking waves over the boat and the sprayhood (and we) were being soaked.

 

[homer]

I have a theory based on my own observations and nothing to do with salt or surface tension. There are 2 elements: firstly, the forecasts in the UK are more accurate; if they forecast a F3 you will probably get a F3 whereas in the med it is more likely to be between F1 and F5 - and it is the latter you will remember. Secondly, the winds in the med increase (and decrease) more rapidly and rapid wind increases produce steeper seas as opposed to a gradual increase when the waves, whilst just as high, have more time to lengthen and so are less steep.
So, you are more likely to be caught out because of duff forecasts and when you are caught out it is likely to be because the wind has increased rapidly which gives you steeper seas - which you will remember.
That's the theory, I now wait for it to be shot full of holes!

 

[BrianH]

the winds in the med increase (and decrease) more rapidly and rapid wind increases produce steeper seas as opposed to a gradual increase when the waves, whilst just as high, have more time to lengthen and so are less steep.

That fits my northern Adriatic Sea observations precisely. A warm sea fringed by steep mountains is a crucible for winds of sudden ferocity that can be local and not forecast - the katabatic bora is a case in point. The resultant waves are immediate, short and steep, just as you describe, no doubt wind persistence would result in longer wave frequency and less effect. I subscribe unreservedly to your theory.

 

[mjcoon]

That fits my northern Adriatic Sea observations precisely. A warm sea fringed by steep mountains is a crucible for winds of sudden ferocity that can be local and not forecast - the katabatic bora is a case in point. The resultant waves are immediate, short and steep, just as you describe, no doubt wind persistence would result in longer wave frequency and less effect. I subscribe unreservedly to your theory.

The reservation I have that prevents my subscribing is that waves are assumed to require a fetch distance, and hence time, before they can build up...

Mike.

 

[Mrnotming]

The cross seas from distant storms can exist in a mix with local wind driven waves.Think this is the deal breaker.

 

[Baggywrinkle]

I think it's down to the cyclic nature of winds in the med and the speed with which they appear and disappear, also warming effects on land are much more prominent than round the U.K. coast IMHO. I have noticed around the Adriatic that the wind will often change direction during the day. As the sun comes up and starts to warm the land a good onshore wind usually develops, this can then either combine with or cancel out any underlying wind due to prevailing weather systems, but when the weather is relatively settled, the best sailing can be had in the afternoon in my experience. This then reverses at night as the land cools and creates winds in the opposite direction. I often wake up with the boat facing the opposite direction from the night before. The wind also moves the water and creates currents that don't necessarily match what the state of the tide should be doing. I've noticed my GPS speed over the ground is often different to my mechanically logged speed by a knot or so, I've given up constantly re-calibrating the two because they never match, especially around the peninsula south of Pula where I sail. So .... my conclusion is that as the wind changes direction alot and there is an underlying wind generated current if it blows for any time in one direction, it is therefore often the case that a wind over current situation results, or the wind turns and goes in the opposite direction ... which results in steeper waves even though there is minimal tide.

 

[Seadawg33]

This is based upon "seat of the pants" experience not science. When a wave has fetch to build it becomes a swell and has more time to form and smooth out. When the wind has just started it tends to rile up the water and become choppy. Thus if there is no time and/or fetch you don't end up with a swell but chop. In the States the buoy reports say something like" 5' to 7' seas every 30 seconds with a 2' chop." In the Med we end up with a short wave period and this causes the waves to be choppy.

 

[chrishelen]

It's rough enough to stop the gozo ferries often in winter,grrrr.

 

[vyv_cox]

This is based upon "seat of the pants" experience not science. When a wave has fetch to build it becomes a swell and has more time to form and smooth out. When the wind has just started it tends to rile up the water and become choppy. Thus if there is no time and/or fetch you don't end up with a swell but chop. In the States the buoy reports say something like" 5' to 7' seas every 30 seconds with a 2' chop." In the Med we end up with a short wave period and this causes the waves to be choppy.

But that happens on every sea in the world. It seems to be only in the Mediterranean that we get these aggressive, steep little waves. We do also have many complex swells, in the Dodecanese largely from the SW but with reflections and diversions caused by islands they can finish up coming from a variety of directions.

 

[CAPTAIN FANTASTIC]

We spend summers in a villa by the sea facing the island of Evia. I often make a cup of coffee and sit down to admire the nice calm sea between Attica and Evia. In many cases by the time I finish the coffee the wind has picked up and the sea is rough with 2m steep waves. The wind comes down from the steep mountains and creates a hell of a chop. Late afternoons, when the land is hot, the wind creates force 5/6 in matter of 45 minutes; its fun swimming in it.

I watch the sailing yacht reducing sail and straggling to make progress, however, the kaikis (greek fishing boats) with their canoe stern and high bows manage very well. At this location there is no much reach, consequently, the sea gets calm equally as quickly. During the meltemis, we can get force 7 for days, great for windsurfers.

 

[lw395]

Deep water with very little tide?