Currents and tidal streams - what's the difference?

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Currents and tidal streams - what\'s the difference?

I didn't realise there was any until recently reading about the Straits of Gibraltar. Apparently, there is normally an east going current even when the tide is running east to west.

How's that work then ? /forums/images/graemlins/confused.gif
 
Re: Currents and tidal streams - what\'s the difference?

presumably, as a fairly dry region, there is insufficient run off from land, to create excess water, so evaporation exceeds supply, and water flows in
 
Re: Currents and tidal streams - what\'s the difference?

There are two main basins in the Med, the Eastern and Western, separated by a sill of 400 m depth extending from Sicilly to north Africa. The western basin is connected to the Atlantic ocean by the narrow Strait of Gibraltar, with a sill depth of 600 m.

There are three main water masses:

1 surface water, originating in the Atlantic inflow through the Strait of Gibraltar, and extending to about 150 m depth,

2 intermediate water, formed by cooling and evaporation in winter south of Turkey, extending from 150 to 600 m depth, and flowing out through the Strait of Gibraltar at this depth;

3 deep water, found below 600 m depth, formed by strong winter cooling and wind mixing south of France and in the Adriatic.

It takes about 70 years for circulation involving the surface and intermediate waters 70 years to flush the upper layers of the Mediterranean Sea; because the inflow occurs at the surface, the Mediterranean is sometimes called a 'reverse estuary'.

Surface evaporation causes the inflow, which generally is stronger than the tidal flow at the surface. Much depends on the direction of the wind, though.
 
Re: Currents and tidal streams - what\'s the difference?

thanks sarabande - that is very informative.

The way I understand it then, is that there is a continuous inbound current for the reasons you describe, on top of which there is are lunar tides.

So if the current is say, 2knots and the tides are +/- 1knot, the net effect is that the water is moving between 1 knot and 3 knots in an easterly direction, over a 12hour period.
 
Re: Currents and tidal streams - what\'s the difference?

I think sarabande means that the easterly current is on the surface so it might be more or less continuous. What we need is a pilot book, or you might spend more time in the Med than intended?

Mr Heikell might oblige you at the Moody AGM?
 
Re: Currents and tidal streams - what\'s the difference?

yes, but the influence of wind and baro pressure also has a big effect whenever I've been sailing there. There's a local sailing guide (name forgotten at this time of night) which will give chapter and verse on the variations.

Someone will post the proper title shortly !
 
Re: Currents and tidal streams - what\'s the difference?

[ QUOTE ]
I think sarabande means that the easterly current is on the surface so it might be more or less continuous. What we need is a pilot book, or you might spend more time in the Med than intended?

Mr Heikell might oblige you at the Moody AGM?

[/ QUOTE ]
it is the Pilot book which is confusing me, hence the question ! Will be playing with new toy in Spain at the time of the AGM - apols for absence. /forums/images/graemlins/wink.gif
 
Re: Currents and tidal streams - what\'s the difference?

Isn’t it the same here? We are in the Gulf Stream, which essentially comes up form the southwest, but the tide still goes in and out
 
Re: Currents and tidal streams - what\'s the difference?

I think you have the specific answer for Gibraltar, but in generic terms a current is horizontal motion of water; tide is vertical motion of water. As water is fluid, sloping topography will create a horizontal motion from the vertical motion of the tide - this is tidal stream. There are many drivers of current - gravity, wind, convective heating, coriolis force for instance. Where you have a river outflow (gravity) meeting the tidal stream of the flood tide, you can get a tidal bore, which would be an extreme example of the phenomenon you mentioned.
 
Re: Currents and tidal streams - what\'s the difference?

A useful addendum, expressed more clearly than I would have done !

The idea of a tidal bore at Gibralter is visually fascinating.

I would be very interested to travel back in time to when the Atlantic Ocean broke through the Straights, and started to fill up the low-lying Med. A 13km wide waterfall !

Someone has very kindly mailed me a link to the Wikipedia, which gives a picture of some astonishing waves generated by the juxtaposition of incoming and outgoing streams.

huge waves
 
Re: Currents and tidal streams - what\'s the difference?

In the pre-gps days, when you completely relied on a calculated CTS, I was often surprised by the strength of the 'surface drift' that became established in the Med after a wind had been blowing for a few days.

Post GPS laziness means this is much less obvious now, and such effects are presumably swamped by tidal streams in areas affected by tides.

But there are lots of really big offshore currents that will transport you with amazing speed (or stop you dead). The one up the E coast of Africa, when reinforced by the S Monsoon goes like a train in a narrow band.
 
Re: Currents and tidal streams - what\'s the difference?

[ QUOTE ]
So if the current is say, 2knots and the tides are +/- 1knot, the net effect is that the water is moving between 1 knot and 3 knots in an easterly direction, over a 12hour period.

[/ QUOTE ]

Your quite correct, been a few years though since i have sailed in the gib straits. you can get tidal books showing both tide and current.
 
Re: Currents and tidal streams - what\'s the difference?

[ QUOTE ]
A friend of mine died eating muesli

[/ QUOTE ]

could be a cereal killer /forums/images/graemlins/grin.gif
 
Re: Currents and tidal streams - what\'s the difference?

i have seen the phenomemna occur on a small scale when I had a mooring in a River mouth. After heavy rain the surface water would continue flowing downstream despite the considerable tide pushing up on the flood.
 
Re: Currents and tidal streams - what\'s the difference?

That would also support the business of fresh water being less dense than salt, though I wonder how much silt has to be in fresh water before it becomes heavier than sea water ?

The Amzon chucks out millions of cumecs, heavily laden with silt, but that may be a matter of scale.

amazon.jpg
 
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