Continuing our preparation for sailing in the Adriatic, let's look at the tides and currents there.
In my sailing experience, the tides can be summarised as either huge amplitude in the Atlantic and the English Channel, particularly around the Bay of Mont Saint-Michel where the tidal amplitude can reach 12 metres, or no tide at all in the Baltic due to the very shallow and narrow opening to the northern sea!
What about the Adriatic?
Below is the tide chart for Rovinj (near Pula) for the next 17 days.
Anything unfamiliar? Let's have a look!
First, a little background on tides... In most open seas, the tides follow a semi-diurnal pattern, with two high and two low tides every day (every 24 hours and 50 minutes on average, to be precise).
The main tidal force in the open sea is caused by the gravitational forces of the Sun and Moon and the centrifugal force generated by the Earth's rotation. This is the main driving force in the oceans, with semi-diurnal variations of up to 10 metres or more in certain areas. These forces result in cycles of about 12 hours and 25 minutes that repeat themselves indefinitely.
However, in a sea that is not completely open, such as the Adriatic, this pattern is not regular, as can be seen in the image above, with clear semi-diurnal cycles in the first and last days of the period shown, and clear diurnal cycles (one per day) in the days in the middle of the period. At the same time, you can see that the amplitude of the tides is much smaller than on the open sea, ranging from 30 to 80 cm.
The reason for this is that there are other forces that influence the tides, at a lower level in the open sea, but which become significant in partially enclosed seas such as the Adriatic. These forces are "forced oscillations", "free oscillations" and "seasonal oscillations" driven by factors such as sea temperature or salinity.
Forced oscillations are driven by air pressure - the higher the pressure, the lower the water level, and vice versa - and by winds, which can be very important in coastal areas. For example, in the Italian port of Trieste, north-east of the Adriatic Sea, the gravitational and centrifugal forces cause oscillations of 26 cm on average, while the oscillations associated with the southerly winds (sirocco) can sometimes cause oscillations of 1 metre.
Free oscillations (called seiches) result from the shape of the Adriatic Sea, where any oscillation that reaches the coast will bounce back and forth. This is similar to what happens when you disturb the water in a bathtub.
Finally, seasonal variations in sea temperature (warmer sea water inflating) and salinity (saltier sea water deflating) play a role. Sea temperature is related to season and air temperature, but also to the influx of cold freshwater in spring (melting of mountain snow and ice) and autumn (increased precipitation), and salinity is directly related to the increased influx of freshwater in spring and autumn.
In short, the drivers of sea level and cyclical tidal patterns in the Adriatic are influenced by several factors that combine their effects. As a result, the sea level is not constant throughout the year (see figure below) and the variations are driven by factors that cannot be forecasted in advance as in the open sea since they are related to changing meteorological patterns. Interesting, but given the limited amplitude, not so significant for sailors in the Adriatic.
As for the current in the Baltic Sea, the pattern is influenced by the uptake of cold fresh water in the northern part of the Adriatic Sea, which flows southwards along the Italian coast, deep in the sea, creating a counter current of warm water coming from the Mediterranean Sea and flowing north-westwards along the coast of Albania, Montenegro and Croatia.
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