Adriatic Sea

The top end of the Adriatic Sea is shallow.

Farther south, the sea floor drops down into a deep hole called a pit. It is about 1,200 meters deep.

Scientists split the Adriatic into three parts. The northern part is shallow, less than 50 meters deep. The middle part gets deeper toward the Jabuka Pit, a dip in the sea floor about 270 to 280 meters deep (studies give slightly different figures).

The southern part holds the South Adriatic Pit, about 1,200 meters deep. An underwater ridge called the Otranto Sill, 780 meters deep, separates it from the Ionian Sea.

Researchers divide the Adriatic into three sub-basins. The northern Adriatic shelf is shallow (less than 50 m), deepening gradually towards the Jabuka Pit of the middle Adriatic (given as about 270 to 280 m). The Palagruža Sill separates the middle Adriatic from the roughly 1,200 m deep South Adriatic Pit (SAP), which is itself separated from the Ionian Sea by the Otranto Sill at about 780 m.

The circulation is driven partly by rivers: the river Po in the north feeds the outward Western Adriatic Current along the western coast, while the inward Eastern Adriatic Current flows along the eastern side, bringing Mediterranean water and, near the coast, water from large rivers in northern Albania and southern Croatia.

In winter, a very strong, cold wind called the bora blows down from the mountains.

It cools the sea so much that the water gets heavy. The heavy water sinks and flows away along the sea floor.

In winter, a cold, dry wind called the bora rushes down from the mountains onto the northern Adriatic. Its strongest gusts can reach hurricane strength, up to 50 meters per second. A bora usually lasts about two days, and sometimes up to a week.

The bora takes heat out of the sea. The cold water becomes dense, or heavy, forming the North Adriatic Dense Water, the densest water in the whole Mediterranean. This heavy water flows south along the sea floor, beside the Italian coast, for months.

North Adriatic Dense Water (NAddW), the densest water mass in the whole Mediterranean Sea, forms in winter during severe bora events with hurricane-strength gusts of up to 50 m/s. Bora events typically last about 2 days and up to a week, and are strongest along the northern Velebit range, where mountain gaps shape jets such as the Trieste and Senj jets.

Along the jets, large upward heat fluxes cool the sea surface, and with the mixing of coastal waters in late autumn and winter this forms dense water over the northern Adriatic and Kvarner Bay. Days to weeks later, bottom density currents start a strong circulation that generally lasts for months: the dense water travels along the Italian coast, partly filling the Jabuka Pit and partly reaching the South Adriatic Pit or leaving for the Ionian Sea.

When the heavy water slides down into the deep pit, it brings oxygen with it. Sea life in the deep needs oxygen.

Up on the rocky shore, marbled crabs climb over the stones.

The deepest water in the South Adriatic Pit is mostly renewed by heavy water from the north, which slides down the slopes as a gravity current. It carries oxygen from the surface down into the deep. This does not happen every year: one study found that between 2012 and 2022, these currents reached the deep pit only in 2012, 2017, 2018 and 2022.

On the shore, the marbled crab lives on the rocky strip between the tides. Scientists studying it in the eastern Mediterranean found that the males were larger than the females.

Le Meur and colleagues (2025) analyzed mooring records from 2012 to 2022 in the South Adriatic Pit and on the shelf slope. The renewal of the pit's bottom water is mainly set by very dense northern water arriving as gravity currents, which ventilate the deep layers, carrying dissolved oxygen from the surface into the interior; local deep convection did so only rarely. Gravity currents reached the deep pit only in 2012, 2017, 2018 and 2022, the 'bottom ventilation years'.

On the coast, Pachygrapsus marmoratus is a shore crab of the Mediterranean's rocky intertidal zone. A study of it and its relative P. transversus on the coast of Lebanon found that males were larger than females in both species, and that P. marmoratus had higher fecundity and larger eggs.

The bora wind can be very strong. Grown-ups check the weather before going out on the water.

At the beach, swim where it is safe, with a grown-up close by.

Because bora gusts can reach hurricane strength, it is worth checking the local marine weather forecast before any boat trip.

If a rip current, a powerful, narrow channel of fast-moving water, ever catches you while swimming, NOAA's advice is: don't fight it. Swim parallel to the shore, then swim back to land at an angle. Swimming straight back against it wears swimmers out.

The bora's hurricane-strength gusts make local marine forecasts the first thing to check before boating; this is PlayOwl's tip, built on the wind's known strength.

At beaches, rip currents (powerful, narrow channels of fast-moving water) can move faster than an Olympic swimmer, at up to about 2.4 m/s, according to NOAA's National Ocean Service. Swimmers who try to swim straight back against one tire quickly; the advice is not to fight it, but to swim parallel to the shore and return to land at an angle.