Go with the flow: ocean currents in the Mediterranean Sea

The Mediterranean is one of the most popular destinations for sailors to take their boat out and battle the elements, or to compete with other sailors in one of the many regattas. Whether you are a leisure or competitive sailor, you should have a basic understanding of everything that affects your sailing. This means being able to read the clouds and predict the weather from them, knowing what type of winds are in the region at different times, and understanding the ocean currents and how they affect sailing. These currents can be a hindrance as well as a welcome boost. Read on to find out how to use them to your advantage.

An ocean current is essentially a large body of seawater continually moving due to a variety of forces acting on it. It is a constant cycle of vertical and horizontal movement by water masses in the oceans and seas. Winds on the sea's surface, differing pressures in the water at different depths, and tidal forces from the moon and sun all contribute to the formation of ocean currents. Their direction is then determined by the rotation of the earth and the periodic winds.

The Mediterranean Sea could be considered a very large lake, as it is surrounded on all sides by land and connected to the Atlantic Ocean only by the narrow Strait of Gibraltar. If we list all the countries alphabetically, the Mediterranean Sea bathes the shores of Albania, Algeria, Bosnia and Herzegovina, Croatia, Egypt, France, Greece, Italy, Israel, Lebanon, Libya, Malta (island state), Monaco, Montenegro, Morocco, Slovenia, Spain, Syria and Turkey. It's no surprise that the Mediterranean Sea is also known as the bridge between the three continents and, with an area of approximately 2,500 km² , it spans Africa, Asia and Europe.

In the Mediterranean, you don't usually encounter powerful currents and many are weak to negligible. This is mainly due to the continental location and climate. Since the Mediterranean has hot or mild weather for much of the year, the water tends to evaporate. The lowering of the water level then leads to the inflow of water in the upper layers from the Atlantic Ocean and the Black Sea. Conversely, at greater depths, where the water is saltier, the process works in reverse — salt water flows into the Atlantic Ocean and Black Sea. Currents can therefore flow from the Atlantic and Black Sea to the Mediterranean or vice versa. 

You can easily tell by the temperature of the sea. In general, the sea is colder and fresher in the west and warms towards the east. When it is hotter and the water evaporates, you can expect a surface current towards the Mediterranean and vice versa. The currents in the Mediterranean thus follow this inflow and outflow, are directed by the coasts of the mainland and islands, and are most influenced by the winds typical of the Mediterranean. The straits between islands or between islands and the mainland can also, as in the case of winds, act as a nozzle and strengthen the current considerably.

The tides are not very strong in the Mediterranean, although of course, it all depends on the time of year and the specific wind that is blowing. Under usual conditions, sea level changes by centimetres at low tide and by a maximum of about one metre at high tide. There are, of course, exceptions, where the tide, together with the swell driven inshore from the sea, can raise the level by as much as 4 metres (typically in northern Corsica or the Strait of Genoa). As with currents, straits and narrow channels can amplify the tide. Finally, the time of year also plays a role, with the Mediterranean being more turbulent in the winter months, with bigger waves and stronger currents.

When you first think about it, it may seem that determining the temperature of water is not exactly rocket science. Either the weather is hot and sunny and the water is warm, or it is cloudy and cool and the water is cold. In reality, it's not that simple with the sea temperature affected by a variety of factors. For starters, the temperature is influenced by elements that do not fluctuate significantly over time, such as seafloor relief or depth. Then there are the variables that vary all the time. These mostly include the amount of solar radiation, currents, and pressure fluctuations at various water depths (temperature gradient). Changes in sea temperature can then tell us a lot about the currents.

Consider the following scenario — you're sailing in beautiful sunny weather, and the top half metre or so of water under the surface is being warmed by the sun's rays. However, they do not reach much further, so the water is substantially colder 10 to 20 metres deeper (five degrees Celsius or more). Such temperature differences lead to convection, a vertical upward motion that attempts to even out or reduce temperature differences. At that point, a vertical current is created.

In general, in the southern Mediterranean, currents typically flow from west to east, in the north they flow from east to west, and in the middle of the Mediterranean, currents move in eddies that can be hundreds of kilometres long. Therefore a sailor has three variables (solar radiation, wind and currents) that determine sea temperature and from which to estimate currents.

Although the currents in the Mediterranean are not very strong, preparations for sailing in the current usually lies on the shoulders of the navigator or captain. In a given location, the strength and direction of the current is constantly changing, although they often recur periodically and behave predictably. This is good news for the navigator or skipper because they can predict currents based on detailed historical data, tables, charts or maps.

Before setting sail, it is advisable to make a list of useful data, which should include information about your boat's draft, expected tide times, height, range, speed and direction. Check how the current generally behaves where your route takes you — especially in the shallows, marina entrances, headlands, estuaries, sandbanks, etc. Don't forget to include a weather forecast in your prep and have a backup plan in case of bad weather. With all of this information, you'll be able to precisely plan your departure and arrival dates to the marina, take advantage of tidal currents to reduce your travel time, and avoid the potential dangers that currents and tides can cause.

Sailing in a current has unique characteristics that depend on where you are sailing, the strength of the wind is and what direction it is in relation to the current. Although the currents in the Mediterranean are usually not very powerful, it is still worth studying nautical charts before departure, which show the usual currents on your route, and check a detailed weather forecast. A basic rule of thumb is that the lighter the wind and smaller the waves, the more the current, tide or swell will affect your boat. On the water, be on the lookout for buoys, anchored boats, breaking waves or other obstacles that the current has to go around to get an idea of where it's flowing, if it's breaking, and how strong it is. The current can be both your friend and your enemy.

True wind, boat speed wind and apparent wind

The strength of the sea current relative to the wind and their direction will determine the setup and trim of the boat. The whole principle is based on the concept — true wind, riding wind and apparent wind. The true wind is the wind we measure in a stationary position on an anemometer. The induced wind (boat speed wind) is the resistance we have to the air as we move (for example, if you move 30 km/h in no wind, the induced wind will be 30 km/h). Apparent wind is a combination of both of these forces. 

Apparent wind always veers more away from the bow of the boat, and the boat changes speed according to the course it is sailing. If you add in waves, where going from the top of a wave to the slope between the waves literally kicks the boat, the headwind changes and so does the apparent wind. This is important information especially for skippers who use the autopilot. It does not detect the actual wind, but only the apparent wind, and tries to change course so that the apparent wind is still blowing from the same direction. This can lead to unpleasant surprise manoeuvres, especially when you are sailing on a tailwind and the boat gybing.

Which sailboat will you take to meet the wind, waves and currents?