Over my Christmas break I have been reading a history of the Mediterranean Sea. The book, recommended by the Economist magazine in their best-of-2011 list, is The Great Sea by David Abulafia. It's a pleasant enough holiday diversion although I suspect that I'll have only the vaguest recollection of what I've read after a week or so. All the stories of the Phoenician, Greek, Carthaginian, Etruscan, and Roman sailors will soon wash out of my mind like sea foam driven by the hot winds of the African scirocco. This is an admission of a weak memory rather than an indictment of Mr. Abulafia's prose.
One bit of knowledge about sea currents, however, will stick with me. In his introduction, Mr. Abulafia explained that the Atlantic Ocean supplies the lion's share of the Mediterranean's water loss from evaporation. The steady inflow of cold Atlantic Ocean water through the Straits of Gibraltar greatly exceeds the outflow of warmer, saltier Mediterranean water. What, a question? That perplexed look on your face shows that you are wondering how there could be both an inflow and an outflow of water simultaneously. The answer is that saltier water is heavier water. The saltier Mediterranean outflow hugs the sea bottom while the opposing Atlantic inflow rides above it on the surface.
The average velocity of the surface current through the Straits of Gibraltar is about 3.5 miles per hour or slightly greater than the average current velocity of the Mississippi River at New Orleans. In olden times it was quite a chore to buck such a strong current from the west in rowing or sailing out from the Mediterranean into the Atlantic. Contrariwise, the Atlantic inflow was a boon for the Vikings rowing down from Norway. They could drift through the Straits of Gibraltar, leaning on their oars and enjoying the scenery, and then arrive at Italy rested, refreshed, and ready for a traditional Norse-style rampage.