Variabilité de la température et de la salinité dans les eaux métropolitaines

• Warm, low-salinity water is less dense ("lighter") than cold, salty water ("heavier"). The former tends to be found at the surface, the latter at depth. Curiously, if these two water masses meet at the same level, due to Earth's rotation, they do not slide under one another but remain side by side, creating a current called a geostrophic current. Temperature and salinity thus allow identification of different water masses, quantification of their mixing, and inference of some of the ocean currents. Locally, salinity is modified by the balance of evaporation and precipitation, as well as by freshwater input from rivers. We distinguish concentration basins like the Red Sea and dilution basins like the Baltic Sea. The sea cools sharply under wind effects because evaporation "consumes" heat; conversely, it warms under solar radiation. The history of measurements combined with statistical tools now allows us to understand certain warming or cooling trends, which are evidence of climatic or inter-annual variations.

• The seasonal temperature cycle of the sea affects only a surface layer of several tens of meters. • In spring, solar radiation begins to dominate cooling by evaporation. The surface temperature then rises by several degrees, creating a warmer surface layer (about 20 meters deep), therefore less dense, which effectively isolates the deep water layers until autumn. The boundary between the surface layer and bottom layers is sharp and effective. It is called the seasonal thermocline and also corresponds to a biological barrier. Energy is needed to break this barrier and mix surface and deep waters. This can occur during strong winds or storms. • In the English Channel–North Sea and Celtic Seas, tidal currents, sometimes violent, prevent thermocline formation in certain places. Near the coast, depending on their orientation, wind can push the warm surface water layer offshore and "pump" deep, cold water upward. This phenomenon called "upwelling" is especially notable in the Gulf of Lions, in the Mediterranean. At greater depths, temperature varies less throughout the year.

• Salinity ranges from about 35 to 35.5 PSU (1 PSU = 1 g salt per kg water) in the English Channel–North Sea, Celtic Seas and Bay of Biscay, while it varies from 37.5 to 38.5 PSU in the western Mediterranean. Locally, at river mouths, salinity can drop sharply. Plumes from major rivers can cause freshening of surface waters far from river mouths. Thus the effects of the Gironde and Loire can be felt as far as the tip of Brittany, the Seine as far as the Pas-de-Calais, and the Rhône as far as Perpignan. Before being diluted and transported offshore, river freshwater appears as "lenses" of desalted water floating on the more saline, denser ocean water.

On the continental margins and abyssal plain, variations in temperature and salinity better reflect different water masses. For example, in the Bay of Biscay at about 800 m depth we find "pockets" of Mediterranean water that entered through the Strait of Gibraltar and are characterized by higher temperature and salinity than surrounding waters. Along the slope in the northwestern Mediterranean, the North Current flows, formed by less saline waters that entered the surface through the same strait.

• From a thermal perspective, the seas of metropolitan France's coasts can be divided vertically into 3 distinct zones: • A surface layer 20 to 30 m thick, directly influenced by the seasonal cycle. Its temperature ranges from 7–9°C in winter to 20°C in summer for the English Channel–Atlantic coast, and from 10–12°C to 25°C for the Mediterranean. This layer is not uniform and can disappear locally due to mixing induced by tidal currents (English Channel, North Sea, Celtic Seas) or strong winds bringing cold water from deeper layers to the surface (Bay of Biscay, Mediterranean); • When the continental shelf is deeper than the surface layer, water masses are isolated from spring warming. They retain more or less their winter temperature while warming slowly from spring to autumn. On the Atlantic coast, in the English Channel and North Sea, this temperature ranges from 9 to 13°C, while it fluctuates from 12 to 16°C on the Gulf of Lions shelf in the Mediterranean; • On the abyssal plains of the northwestern Mediterranean and Bay of Biscay, temperatures are nearly constant: around 12–13°C in the Mediterranean and 6 to 13°C in the Bay of Biscay. Their differences indicate the presence of different water masses.

Regarding salt content, the seasonal cycle is very weak outside highly coastal areas subject to land runoff and coastal streams, and within plumes from major rivers (Seine, Loire, Gironde, Rhône). Average salinity decreases slowly from the south of the Bay of Biscay (35.5 PSU) to the Pas-de-Calais (35 PSU). It is significantly higher in the Mediterranean (37.5–38.5 PSU). As with temperature, on the abyssal plain, it is possible to separate water masses and quantify their mixing by examining small variations in salinity.

• The English Channel–North Sea marine sub-region is characterized by a thermal front in the western Channel, river plumes, mainly from the Seine, and the arrival of slightly freshened waters toward the North Sea. • At the Channel entrance, the Ushant temperature front extends with eastward extension varying from early spring to late summer. East of this front, tidal currents are generally strong enough to prevent any water stratification. The temperature difference across the front reaches 4 to 5°C. Vertical mixing is more intense the shallower the depth and the stronger the currents, generally the case in the Eastern Channel. The eastern basin is thus distinguished from the western basin by larger thermal amplitudes between winter and summer (exceeding 10°C). • The Seine is the main freshwater contributor in the region, with an average discharge of 550 m³/s. The vertical structure of its salinity plume takes the form of a lens of heavily freshened water at the surface. The Seine plume is also generally identifiable in winter by cold surface waters. The extent of freshening is partly linked to discharge, with maximum extension in spring. Seine waters, combined with smaller rivers (Somme, Canche, Aa, Authie), generate along the Pas-de-Calais coast a band of freshened water sometimes called a "coastal river". • The average transport of water in the Channel is from west to east. Channel waters are then a succession of more or less mixed waters from the Atlantic (fairly salty) and from the Celtic Sea or even South Brittany. These are slightly less salty with less freshwater input. • At depth, the difference between coast and offshore, as at the surface, is marked during different seasons. This is explained by the average shallow depths and intense turbulence in the water column (tidal currents, winds) that prevents isolating bottom layers from summer warming or winter cooling.

• The Celtic Seas and Bay of Biscay marine sub-regions are characterized by an open gulf bounded by the Spanish coast in the south (oriented west–east) and by the French coast in the east (oriented south–north). This results in large variation in general topography, which affects temperature and salinity in the water column. The specific hydrological processes are as follows. • The Ushant thermal front, described above, is located a few kilometers offshore from the island. It meets the coast near Raz Point. The thermal gradient there is very steep and can reach 3 to 4°C in less than two kilometers. It is present from spring to autumn and is particularly marked in summer. The front is generated by the presence of strong tidal currents interfering with the establishment of the seasonal thermocline. Vertical mixing is caused by friction of these currents on the bottom, which decreases and completely homogenizes the water column. The front then forms the meeting zone between stratified offshore waters and mixed coastal waters. • When currents are weak enough and depth is sufficient, the bottom layer remains isolated from the surface and does not experience summer warming, remaining confined to the surface. This is the case of the cold bulge, a dome-shaped thermal structure located below the seasonal thermocline, distributed over two zones: the "Grande Vasière" in South Brittany and west of the Ushant front. This water mass is present year-round with temperature variations of less than 1°C, around 12°C, from spring to autumn and weak inter-annual fluctuations. • In summer and early autumn, a process called "internal waves" develops from the interaction of tidal currents with topography. It triggers the formation of thermal fronts (a zone of colder water) above the continental slope of the Bay of Biscay and Celtic Sea. • Remarkable river plumes in the region are formed by freshwater arrival from the Loire and Gironde estuaries. The vertical structure of a salinity plume takes the form of a lens of heavily freshened water. Plumes are marked in winter by cold surface waters. Their extent on the shelf is partly linked to river discharge (averaging 900 m³/s). Their extent is maximum in winter and spring, seasons when discharges can exceed 5,000 m³/s. Low-water season corresponds to late summer. Discharge can then drop to 200 m³/s. Added to this seasonal variability is inter-annual variability distinguishing dry years from rainy and/or snowy years. The natural tendency of plumes in this zone is northward displacement, due to Earth's rotation, with winds able to oppose this movement. • Upwellings are characterized by coastal water cooling due to deep water rise under wind influence. They are well marked from May to September along the Landes coast and in South Brittany with north or northwest winds. • In autumn (October–November), warm water mass accumulated during summer flows northward as a warm tongue of 14–16°C potentially extending from the Basque country to the Loire latitude. Some years it may be absent. Some winters, a slope current called Navidad presents as a warm current along the northern Iberian peninsula coast and rising in the Bay of Biscay along the continental slope.

• The northwestern Mediterranean marine sub-region is characterized by numerous hydrological processes, the most characteristic being: the North Current, the influence of a major river (the Rhône), significant eddy activity that redistributes salinities, temperatures and evaporation in the basin. • The winter situation is a consequence of this zone's hydrodynamics. Warm waters (>13°C) mark the presence at the surface of Atlantic water traceable to the Gulf of Lions slope. The 13°C line, called the Balearic front, runs from the Balearic Sea to Cape Corsica. The Gulf of Lions is the coldest region in winter, 11.4°C on average in February. It can drop below 8–10°C depending on weather events. This is due to cold freshwater arrival in winter, essentially from the Rhône. This cooling is sometimes so intense it can generate very dense water at the coast that ends up sinking from the shelf to the abyssal plain through canyons. These cold waters, occupying the western part of the Gulf of Lions, then flow along the coast southward onto the Catalan shelf. The eastern part of the Gulf of Lions, from Marseille to the Rhône, is under the influence of Atlantic water mass, consequence of frequent North Current intrusions onto the shelf. • In May, spring warming brings temperatures between 15 and 19°C. Though warmer North Current waters are still distinguishable, seasonal thermocline establishment erases differences between water masses. The western Gulf of Lions remains coldest (15°C average monthly). These remnants of winter water from the Gulf of Lions evacuate toward the Catalan shelf. Between Corsica and Sardinia and east of Bonifacio Strait, wind prevents thermocline establishment. The Balearic front is still present, more diffuse, and has shifted slightly southward. • In August, temperatures are higher (>22°C). A thermocline exists across the entire domain since temperature below the mixing layer is around 13–14°C. Mistral and Tramontane winds generate intermittent upwelling in the Gulf of Lions. This causes destratification through vertical mixing offshore of the Gulf of Lions, in the middle of the Liguria Sea. Winds east of Bonifacio Strait cause cold water rise forming an almost-permanent cold water patch in that zone. West of the Gulf of Lions, between Cape Creus at the Spanish border and Barcelona, temperature varies rapidly from 20°C to 26°C. This thermal front is very marked. It is a characteristic of late summer in this zone. It is the consequence of different wind regime north and south of the Pyrenees. • During autumn, the heat flux balance for the Mediterranean becomes negative. It triggers generalized cooling. Progressive thermocline erosion again clearly reveals the spatial distribution of water masses. The bottom of the Gulf of Lions, center of the Liguria Sea and abyssal plain south of the Gulf of Lions cool first while along the slope, the North Current is marked by warmer waters. This configuration will persist through winter. • Regarding salinity, the North Current is a water mass flowing counterclockwise at the surface from the Atlantic. This water mass passes east and west of Corsica, runs along the continental slope in the Liguria Sea, offshore of the Gulf of Lions and Catalan shelf. This water mass is characterized by "low" salinity for the Mediterranean (36.2 PSU at Gibraltar to 38.4 PSU in the Catalan Sea) and by temperature below the surface mixing layer of 14–15°C. The main freshwater input to the zone is the Rhône. Its plume takes the classic comma shape extending south of the delta. During south/southeast wind episodes, it appears as a freshened water band running along the coast to Perpignan. In autumn and winter, Mistral and Tramontane participate in cooling and salinity increase of surface waters. In winter, they can create a zone of very cold, very salty waters in the basin center capable of sinking to the bottom. This is called dense water formation. Finally, the Mediterranean reacts quickly to wind bursts. In summer, they destroy the mixing layer or bring deep waters to the surface. In winter, they generate cold, dense water formation on the shelf or abyssal plain. These processes cause eddies and filaments a few to tens of kilometers in size that also contribute to creating thermo-haline contrasts that can be locally very strong.

• The oceans store more than 90% of excess heat from climate warming. Since 1950, the temperature of the surface layer, 0–300 m, has increased overall by 0.3°C. • This progression is not uniform across all oceans. Research by IRD for the National Observatory on Climate Warming Effects (Onerc) shows, for example, that increases are weaker in the Caribbean Sea than in the Indian Ocean.

In parallel, the precipitation regime is also altered with climate change. Between evaporation at ocean surface and freshwater input, we observe a slight increase in salinity in metropolitan waters (see Onerc indicator).

The increase in surface salinity and temperature can intensify water mass stratification and limit vertical exchange. This limits nutrient and oxygen supply to deep waters and causes reduced productivity in these areas. Moreover, more marked ocean stratification can impact general ocean circulation. Several long-term projections show a slowdown in the major ocean circulation loop carrying warm, salty waters to arctic regions, which contributes to the planet's thermal balance.

• Salinity is the quantity of dissolved salts in water. It is expressed in parts per thousand, also noted ‰, or PSU (practical salinity unit). 1 PSU corresponds to 1 gram of dry salt per kilogram of water. In oceans and open seas, surface salinity ranges from 31 to 37.9 PSU. In some areas of the Baltic Sea, it can drop to 10 PSU and reach 40 PSU in the Red Sea. • Temperature is sometimes called "potential" temperature. In this case, it is corrected for pressure effects. It is the temperature the water mass would have if brought to the surface with no heat exchange with the outside. The difference between in situ temperature and potential temperature is notable only for depths exceeding a few hundred meters. • A thermal (or haline) front is the zone separating two water masses of very different temperature (or salinity). Its thickness is on the order of a few hundred meters to a few kilometers. • Author: French Institute of Research for Exploitation of the Sea (Ifremer).