Are there re­ally no tides in the Med?

Yachting Monthly - - THE KNOWLEDGE -

QThere are large tides along the At­lantic coast of Por­tu­gal and Spain and there are cur­rents in the Mediter­ranean but are these tidal or not? There are cer­tainly cur­rents and no­tice­able depth changes in var­i­ous places in the Med but the con­ven­tional wis­dom seems to be ‘the Mediter­ranean is not tidal’. Are these tidal phe­nom­ena or purely the ef­fects of weather and cli­mate? Tim Hughes

ANor­man Kean replies: The Mediter­ranean does have tides but quite small ones. The ex­pla­na­tion is sim­ply that it’s a much smaller body of wa­ter than the oceans. It’s es­sen­tially land­locked, and it takes only about two and a half hours for all of it to pass un­der the sun or the moon as the Earth ro­tates. So there’s not much chance to build up some mo­men­tum, and the short pe­riod tends to stop any pow­er­ful har­mon­ics from hap­pen­ing. Hav­ing said that, as I write, HW at Va­len­cia is at 1230 UT (range 0.1m), at Naples 1455 UT (range 0.2m) and at Akrotiri in Cyprus 1432 UT (range 0.3m), so it’s ac­tu­ally far from sim­ple. Spring range at Akrotiri is 0.5m, which isn’t zero by any means. North and south of the Strait of Messina (between Si­cily and Italy) the tides are six hours out of phase, which gen­er­ates a six-knot re­vers­ing cur­rent – hence the an­cient leg­end of Scylla and Charyb­dis (above). We are un­usual in this part of the world in hav­ing tides of any­thing up to 14m.

The east coast of North Amer­ica from Cape Cod to New­found­land also has big tides (peak­ing in the Bay of Fundy), but most places in the world have a me­tre or less. This is one rea­son why tsunamis have such dis­as­trous ef­fects – many peo­ple around the Pa­cific live right on the beach.

In the Gulf of Mex­ico there’s only one high tide a day. Fig­ure that one out! A one-time col­league of mine from New Or­leans, whose fa­ther was a tug­boat skip­per, thought I was nuts when I talked about two tides a day.

Tidal pre­dic­tions are based on an at­mo­spheric pres­sure of 1013 HPA, and each HPA (or mb as they used to be known) makes a dif­fer­ence of a cen­time­tre. So if there’s a nice big an­ti­cy­clone of 1033 HPA, the tides will be 0.2m lower than pre­dicted. Con­versely, in a win­ter de­pres­sion of 963 HPA they’ll be half a me­tre higher – that’s in calm weather. In a low pres­sure there’s likely to be a greater ef­fect from storm surges, which can add an­other half me­tre.

Tidal dif­fer­ences are the root of an an­cient leg­end: be­ing caught between Charyb­dis (a whirlpool) and the mon­ster Scylla(a rock shoal) is the equiv­a­lent of be­ing ‘between a rock and a hard place’.

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