by Jörn Selling
Los Hileros! This exclamation from one of our former captains, rolling his eyes and looking meaningful, was meant to indicate that he would rather not sail because the Strait of Gibraltar would be too rough.
Hileros are the name given here to the wave bands that occur during the change of tide after the high tide - and during the running out of the water.
It starts when the water begins to "run off" after high tide, accelerating towards the Mediterranean Sea. A bunch of internal soliton waves will be released from the Camarinal sill. At the surface of the water near Tangier, a first band of rough seas appears, spanning north-south across the Strait of Gibraltar, which is a few hundred metres wide and is followed by up to five other bands with increasingly weaker waves. These rough bands alternate with equally broad ones where the water surface is smooth, all of them travelling towards the Mediterranean. This is the effect that the peaks and valleys of the soliton waves at a depth of one hundred metres have on the water surface.
As is always the case with waves that pass through an opening in an obstacle, they are also bent (diffracted) by the Strait. When they arrive in the Mediterranean, two boundaries running along the Strait of Gibraltar remain between the coastal waters and the body of water in the middle, one to the north and one to the south. The water in the centre flows like a river towards the Mediterranean Sea, creating "shear waves" at the frictional boundaries, which propagate departing perpendicularly from each of the two towards the north and south.
The Pilot Whales and Bottlenose Dolphins, which are mainly found in the southern half of the Strait of Gibraltar, and sometimes the Striped and Common Dolphins, surf the shear waves of the southern friction boundary towards the northwest. Although the sea is rough then and there is a risk of getting seasick, if you surf the same waves with the boat, the animals can be wonderfully observed in them.
Dolphins surf and jump in the shear waves
When the low tide is approaching, the surface current slows down towards the Mediterranean until, towards the following high tide, it even changes direction with the rising water and flows out into the Atlantic. The "spook" is over and only repeats itself after the next high tide. Until then, swimmers take advantage of the calmer sea to get across the Strait of Gibraltar.