Are there really no tides in the Med?
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’.