Adriatic Sea
Map of the Adriatic Sea
Location	Europe
Coordinates	43°N 15°E / 43°N 15°E / 43; 15Coordinates: 43°N 15°E / 43°N 15°E / 43; 15
Primary sources	Adige, Bojana, Drin, Krka, Neretva, Po, Soča
Primary outflows	Ionian Sea
Catchment area	235,000 km2 (91,000 sq mi)
Basin countries	Italy, Slovenia, Croatia, Bosnia–Herzegovina, Montenegro, and Albania (bordering); Switzerland and France (drainage basins for inflow rivers)
Max length	800 km (500 mi)
Max width	200 km (120 mi)
Surface area	138,600 km2 (53,500 sq mi)
Average depth	252.5 m (828 ft)
Max depth	1,233 m (4,045 ft)
Water volume	35,000 km3 (8,400 cu mi)
Residence time (of Ocean water)	3.4±0.4 years
Salinity	38–39 PSU
Shore length1	3,739.1 km (2,323.4 mi)
Max temperature	24 °C (75 °F)
Min temperature	9 °C (48 °F)
Islands	Over 1300
Settlements	Bari, Venice, Trieste, Split, Ravenna, Rimini, Rijeka, Pescara, Durres, Ancona

The Adriatic Sea ( /ˌeɪdriˈætɨk/) is a body of water separating the Apennine Peninsula from the Balkan peninsula and the Apennine Mountains from the Dinaric Alps and adjacent ranges. The Adriatic is the northernmost arm of the Mediterranean Sea, extending from the Strait of Otranto (where it connects to the Ionian Sea) to the northwest and the Po Valley. Its coasts are part of (in decreasing order of coastline length without islands) Croatia, Italy, Albania, Montenegro, Slovenia and Bosnia–Herzegovina. The Adriatic contains over 1300 islands, mostly located along its eastern coast. It is divided into three basins—the northern being the shallowest and the southern being the deepest, with a maximum depth of 1,233 metres (4,045 ft). The Otranto Sill, a decrease in depth, is located at the border between the Adriatic and Ionian Seas. The prevailing currents flow counterclockwise from the Strait of Otranto, along the eastern coast and back to the strait along the western (Italian) coast. Tidal movements in the Adriatic are slight, although larger amplitudes are known to occur occasionally. The Adriatic's salinity is lower than the Mediterranean's because the former collects a third of the fresh water flowing into the latter, acting as a dilution basin. The surface water temperatures generally range from 24 °C (75 °F) in summer to 12 °C (54 °F) in winter, significantly moderating the Adriatic Basin's climate.

The Adriatic Sea sits on the Apulian or Adriatic Microplate, which separated from the African Plate in the Mesozoic era. The plate's movement contributed to the Alpine orogeny and Apennine tectonic uplift via its collision with the likewise-moving Eurasian plate. All types of sediment are found in the Adriatic, with the bulk of the material transported by the Po and other rivers on the western coast. The western coast is alluvial or terraced, while the eastern coast is highly indented with pronounced karstification. There are dozens of marine protected areas in the Adriatic, designed to protect the sea's karst habitats and biodiversity. The sea is abundant in flora and fauna—more than 7,000 species are identified as native to the Adriatic, including endemic, rare and threatened ones.

The Adriatic's shores are populated by more than 3.5 million people; the largest cities are Bari, Venice, Trieste and Split. The earliest settlements on the Adriatic shores were Etruscan, Illyrian, and Greek. By the 2nd century BC, the shores were under the Roman Republic's control. In the Middle Ages, the Adriatic shores and the sea itself were controlled, to a varying extent, by a series of states—most notably the Byzantine Empire, the Republic of Venice, the Habsburg Monarchy and the Ottoman Empire. The Napoleonic Wars resulted in the First French Empire gaining coastal control and the British effort to counter the French in the area, ultimately securing most of the Eastern Adriatic and the Po Valley for Austria. Following Italian unification, the Kingdom of Italy started an eastward expansion that lasted until the 20th century. Following World War I and the collapse of Austria-Hungary and the Ottoman Empire, the eastern coast's control passed to Yugoslavia and Albania. The former disintegrated in the 1990s, resulting in four new states on the Adriatic coast. Italy and Yugoslavia agreed on their maritime boundaries by 1975 and this boundary is recognised by Yugoslavia's successor states, but the maritime boundaries between Slovenian, Croatian, Bosnian–Herzegovinian and Montenegrin waters are disputed. Italy and Albania agreed on their maritime boundary in 1992.

The Adriatic Sea is significant to the economies of the countries found along its coasts, especially in terms of fisheries and tourism. Adriatic Croatia's tourism industry has grown faster economically than the rest of the Adriatic Basin's. Maritime transport is also a significant branch of the area's economy—there are 19 seaports in the Adriatic that each handle more than a million tonnes of cargo per year. The largest Adriatic seaport by annual cargo turnover is the Port of Trieste, while the Port of Split is the largest Adriatic seaport by passengers served per year.

Geography[link]

The Adriatic Sea is a semi-enclosed sea,^[1] bordered in the southwest by the Apennine peninsula, in the northwest by the Italian regions of Veneto and Friuli-Venezia Giulia, and in the northeast by Slovenia, Croatia, Bosnia–Herzegovina, Montenegro, and Albania—the Balkan peninsula. In the southeast, the Adriatic Sea connects to the Ionian Sea at the 72-kilometre (45 mi) wide Strait of Otranto.^[2] The International Hydrographic Organization (IHO) defines the boundary between the Adriatic and the Ionian seas as a line running from the Butrinto River's mouth (latitude 39°44'N) in Albania to the Karagol Cape in Corfu, through this island to the Kephali Cape (these two capes are in latitude 39°45'N), and on to the Santa Maria di Leuca Cape (latitude 39°48'N).^[3] It extends 800 kilometres (500 mi) from the northwest to the southeast and is 200 kilometres (120 mi) wide. It covers 138,600 square kilometres (53,500 sq mi) and has a volume of 35,000 cubic kilometres (8,400 cu mi). The Adriatic extends northwest from 40° to 45°47' north, representing the Mediterranean's northernmost portion.^[2] The sea is geographically divided into the Northern Adriatic, Central (or Middle) Adriatic, and Southern Adriatic.^[4] The Adriatic Sea drainage basin encompasses 235,000 square kilometres (91,000 sq mi), yielding a land–sea ratio of 1.8. The drainage basin's mean elevation is 782 metres (2,566 ft) above sea level, with a mean slope of 12.1°.^[5] Major rivers discharging into the Adriatic include the Po, Soča, Krka, Neretva, Drin, Bojana, and Vjosë.^[6][7] In the late 19th century, Austria-Hungary established a geodetic network with an elevation benchmark using the average Adriatic Sea level at the Sartorio pier in Trieste, Italy. The benchmark was subsequently retained by Austria, adopted by Yugoslavia, and retained by the states that emerged after its dissolution.^[8][9]

Bay of Kotor, a ria in the Southern Adriatic

The Adriatic Sea contains over 1300 islands and islets, most along the Adriatic's eastern coast—especially in Croatia, with 1,246 counted. The number includes islands, islets, and rocks of all sizes, including ones emerging at ebb tide only.^[13] The Croatian islands include the largest—Cres and Krk, each covering about the same area of 405.78 square kilometres (156.67 sq mi)—and the tallest—Brač, whose peak reaches 780 metres (2,560 ft) above sea level. The islands of Cres and the adjacent Lošinj are separated only by a narrow navigable canal dug in the time of classical antiquity;^[14] the original single island was known to the Greeks as Apsyrtides.^[15] The Croatian islands include 48 permanently inhabited ones, the most populous among them being Krk and Korčula.^[16] The islands along the Adriatic's western (Italian) coast are smaller and less numerous than those along the opposite coast; the best-known ones are the 117 islands on which the city of Venice is built.^[17] The northern shore of the Greek island of Corfu also lies in the Adriatic Sea as defined by the IHO.^[18] The IHO boundary places a few smaller Greek islands (ones northwest of Corfu) in the Adriatic Sea.^[3][19]

Adriatic islands off of Croatia's coast

Bathymetry[link]

Depth of the Adriatic Sea

The Adriatic Sea's average depth is 252.5 metres (828 ft), and its maximum depth is 1,233 metres (4,045 ft); however, the North Adriatic basin rarely exceeds a depth of 100 metres (330 ft).^[10] The North Adriatic basin, extending between Venice and Trieste towards a line connecting Ancona and Zadar, is only 15 metres (49 ft) deep at its northwestern end; it gradually deepens towards the southeast. It is the largest Mediterranean shelf and is simultaneously a dilution basin and a site of bottom water formation.^[20] The Middle Adriatic basin is south of the Ancona–Zadar line, with the 270-metre (890 ft) deep Middle Adriatic Pit (also called the Pomo Depression or the Jabuka Pit). The 170-metre (560 ft) deep Palagruža Sill is south of the Middle Adriatic Pit, separating it from the 1,200-metre (3,900 ft) deep South Adriatic Pit and the Middle Adriatic basin from the South Adriatic Basin. Further on to the south, the sea floor rises to 780 metres (2,560 ft) to form the Otranto Sill at the boundary to the Ionian Sea. The South Adriatic Basin is similar in many respects to the Northern Ionian Sea, to which it is connected.^[7] Transversely, the Adriatic Sea is also asymmetric: the Apennine peninsular coast is relatively smooth with very few islands and the Monte Conero and Gargano promontories as the only significant protrusions into the sea; in contrast, the Balkan peninsular coast is rugged with numerous islands, especially in Croatia. The coast's ruggedness is exacerbated by the Dinaric Alps' proximity to the coast, in contrast to the opposite (Italian) coast where the Apennine Mountains are further away from the shoreline.^[21]

Hydrology[link]

Schematic layout of Adriatic Sea currents
                     surface currents                      benthic currents

The coastal water dynamics are determined by the asymmetric coasts and the Mediterranean seawater's inflow through the Straits of Otranto and further on along the eastern coast.^[22] The smooth Italian coast (with very few protrusions and no major islands) allows the Western Adriatic Current's smooth flow, which is composed of the surface's relatively freshwater mass and the bottom's cold and dense water mass.^[23] The coastal currents on the opposite shore are far more complex, due to the jagged shoreline, several large islands and the Dinaric Alps' proximity to the shore. The last produces significant temperature variations between the sea and the hinterland, which leads to the creation of local jets.^[21] The tidal movement is normally slight, usually remaining below 30 centimetres (12 in). The amphidromic point is at the mid-width east of Ancona.^[24]

The normal tide levels are known to increase significantly in a conducive environment, leading to coastal flooding; this phenomenon is most famously known in Italy—especially Venice—as acqua alta. Such tides can exceed normal levels by more than 140 centimetres (55 in),^[25] with the highest tide level of 194 centimetres (76 in) observed on 4 November 1966.^[26][27] Such flooding is caused by a combination of factors, including the alignment of the Sun and Moon, meteorological factors such as sirocco related storm surges,^[28] and the basin's geometric shape (which amplifies or reduces the astronomical component). Moreover, the Adriatic's long and narrow rectangular shape is the source of an oscillating water motion (French: seiche) along the basin's minor axis.^[29] Finally, Venice is increasingly vulnerable to flooding due to coastal area soil subsidence.^[30] Such unusually high tides resulting in flooding have also been observed elsewhere in the Adriatic Sea, and have been recorded in recent years in the towns of Koper, Zadar and Šibenik as well.^[31][32][33]

A submarine spring near Omiš, observed through sea surface rippling

It is estimated that the Adriatic's entire volume is exchanged through the Strait of Otranto in 3.4±0.4 years, a comparably short period. (For instance, approximately 500 years are necessary to exchange all the Black Sea's water.) This short period is particularly important as the rivers flowing into the Adriatic discharge up to 5,700 cubic metres per second (200,000 cu ft/s). This rate of discharge amounts to 0.5% of the total Adriatic Sea volume, or a 1.3-metre (4 ft 3 in) layer of water each year. The greatest portion of the discharge from any single river comes from the Po (28%),^[34] with an average discharge from it alone of 1,569 cubic metres per second (55,400 cu ft/s).^[35] In terms of the annual total discharge into the entire Mediterranean Sea, the Po is ranked second, followed by the Neretva and Drin, which rank as third and fourth.^[36] Another significant contributor of freshwater to the Adriatic is the submarine groundwater discharge through submarine springs (Croatian: vrulja); it is estimated to comprise 29% of the total water flux into the Adriatic.^[37] The submarine springs include thermal springs, discovered offshore near the town of Izola. The thermal springwater is rich with hydrogen sulfide, has a temperature of 22 to 29.6 °C (72 to 85 °F), and has enabled the development of specific ecosystems.^[38] The inflow of freshwater, representing a third of the freshwater volume flowing into the Mediterranean,^[39] makes the Adriatic a dilution basin for the Mediterranean Sea.^[40] The Middle and South Adriatic Gyres (SAG), are significant cyclonic circulation features, with the former being intermittent and the latter permanent. The SAG measures 150 kilometres (93 miles) in diameter. It contributes to the flow of bottom water from the Adriatic to the Levantine Basin through the Ionian Sea. Through that process, the Adriatic Sea produces most of the East Mediterranean deep water.^[41]

Temperature and salinity[link]

The Adriatic's surface temperature usually ranges from 22 to 24 °C (72 to 75 °F) in the summer, or 12 to 14 °C (54 to 57 °F) in the winter, except along the western Adriatic coast's northern part, where it drops to 9 °C (48 °F) in the winter. The distinct seasonal temperature variations, with a longitudinal gradient in the Northern and transversal gradient in the Middle and Southern Adriatic,^[42] are attributed to the continental characteristics of the Adriatic Sea: it is shallower and closer to land than are oceans.^[43] During particularly cold winters, sea ice may appear in the Adriatic's shallow coastal areas, especially in the Venetian Lagoon but also in isolated shallows as far south as Tisno (south of Zadar).^[44][45] The Southern Adriatic is about 8 to 10 °C (14 to 18 °F) warmer during the winter than the more northerly regions.^[4] The Adriatic's salinity variation over the year is likewise distinct:^[43] it ranges between 38 and 39 PSUs.^[42] The southern Adriatic is subjected to saltier water from the Levantine Basin.^[4]

Climate[link]

As in most Mediterranean Basin areas, the Adriatic Sea and its surrounding landmass enjoy the Mediterranean climate, a type of subtropical climate. Since the Adriatic Sea is located in the mid-latitudes, it is characterized by a seasonally variable climate; it is also characterized by warm to hot, dry summers and mild to cool, wet winters. The air temperature fluctuates by about 20 °C (68 °F) during a season.^[42] According to the Köppen climate classification, the southern and central Adriatic are classified as hot-summer Mediterranean climate (Csa) and the northern Adriatic as humid subtropical climate (Cfa) areas.^[46][47] The predominant winter winds are the bora and sirocco (called jugo along the eastern coast). The bora is significantly conditioned by wind gaps in the Dinaric Alps bringing cold and dry continental air; it reaches peak speeds in the areas of Trieste, Senj, and Split, with gusts of up to 180 kilometres per hour (97 kn; 110 mph). The sirocco brings humid and warm air, often carrying Saharan sand causing rain dust.^[48]

Population[link]

On the Adriatic Sea's coasts and islands, there are numerous small settlements, but few larger cities. Among the largest are Bari, Venice, Trieste, Ravenna, and Rimini in Italy, Split, Rijeka and Zadar in Croatia, Durrës and Vlorë in Albania and Koper in Slovenia. In total, more than 3.5 million people live on the Adriatic coasts.^[53]

Coastal management[link]

MOSE Project north of Lido di Venezia

Venice, which was originally built on islands off the coast, is most at risk due to subsidence, but the threat is present in the Po delta as well. The causes are a decrease in sedimentation rate due to loss of sediment behind dams, the deliberate excavation of sand for industrial purposes, agricultural use of water, and removal of ground water.^[54][55]

The sinking of Venice slowed after artesian wells were banned in the 1960s, but the city remains threatened by the acqua alta floods. Recent studies have suggested that the city is no longer sinking,^[56][57] but a state of alert remains in place. In May 2003, then-Prime Minister Silvio Berlusconi inaugurated the MOSE project (Italian: Modulo Sperimentale Elettromeccanico), an experimental model for evaluating the performance of inflatable gates. The project proposes laying a series of 79 inflatable pontoons across the sea bed at the three entrances to the Venetian Lagoon. When tides are predicted to rise above 110 centimetres (43 in), the pontoons will be filled with air and block the incoming water from the Adriatic Sea. This engineering work is due to be completed by 2014.^[58]

Geology[link]

Adriatic Microplate boundaries

Geophysical and geological information indicate that the Adriatic Sea and the Po Valley are associated with a tectonic microplate—identified as the Apulian or Adriatic Plate—that separated from the African Plate during the Mesozoic era. This separation began in the Middle and Late Triassic, when limestone began to be deposited in the area. Between the Norian and Late Cretaceous, the Adriatic and Apulia Carbonate Platforms formed as a thick series of carbonate sediments (dolomites and limestones), up to 8,000 metres (26,000 ft) deep.^[59] Remnants of the former are found in the Adriatic Sea, as well as in the southern Alps and the Dinaric Alps, and remnants of the latter are seen as the Gargano Promontory and the Maiella mountain. In the Eocene and early Oligocene, the plate moved north and north-east, contributing to the Alpine orogeny (along with the African and Eurasian Plates' movements) via the tectonic uplift of the Dinarides and Alps. In the Late Oligocene, the motion was reversed and the Apennine Mountains' orogeny took place.^[60] An unbroken zone of increased seismic activity borders the Adriatic Sea, with a belt of thrust faults generally oriented in the northeast–southwest direction on the east coast and the northeast–southwest normal faults in the Apennines, indicating an Adriatic counterclockwise rotation.^[61] An active 200-kilometre (120 mi) fault has been identified to the northwest of Dubrovnik, adding to the Dalmatian islands as the Eurasian Plate slides over the Adriatic microplate. Furthermore, the fault causes the Apennine peninsula's southern tip to move towards the opposite shore by about 0.4 centimetres (0.16 in) per year. If this movement continues, the seafloor will be completely consumed and the Adriatic Sea closed off in 50–70 million years.^[62] In the Northern Adriatic, the coast of the Gulf of Trieste and western Istria is gradually subsiding, having sunk about 1.5 metres (4 ft 11 in) in the past two thousand years.^[63] In the Middle Adriatic Basin, there is evidence of Permian volcanism in the area of Komiža on the island of Vis and the volcanic islands of Jabuka and Brusnik.^[64] Earthquakes have been observed in the region since the earliest historical records.^[65] A recent strong earthquake in the region was the 1979 Montenegro earthquake, measuring 7.0 on the Richter scale.^[66] Historical earthquakes in the area include the 1627 Gargano peninsula and the 1667 Dubrovnik earthquakes, both followed by strong tsunamis.^[67] In the last 600 years, fifteen tsunamis have occurred in the Adriatic Sea.^[68]