Global And Mediterranean Circulation

‭GLOBAL AND MEDITERRANEAN CIRCULATION‬ ‭Great ocean conveyor belt theory: brings together the three ocean basins and their waters. The‬ ‭theory predicts that the ribbon originated with the sinking of cold, dense water at Norwegian‬ ‭Sea level; from here the waters then flow deep southwards along the Atlantic ridge. Among‬ ‭other things, they are fed by other sources including those from deep waters that come from‬ ‭the Mediterranean. In the southern hemisphere, the current represented in blue joins the‬ ‭Antarctic Circumpolar Current and then heads into the Indian Ocean towards the Pacific‬ ‭Ocean. The return route starts from the Pacific and the Indian, the one in red, warmer and more‬ ‭superficial, reaches around a maximum of 500 m‬ ‭depth, at a more superficial level it then travels along the Atlantic basin, merges with the Gulf‬ ‭Stream and returns to the starting point. It has been estimated that a mass of water that has‬ ‭sunk into the Norwegian Sea takes around 1000 years to return. According to some authors, the‬ ‭model would be even more complex: there would be two large circulation cells dependent on‬ ‭each other and connected to each other by minor circuits; the first cell connects the Atlantic to‬ ‭the Indian and Pacific Oceans through the Southern Ocean; the second concerns the return path‬ ‭from the Pacific, through the Probe Sea and through the Indian Ocean and then finally to the‬ ‭Atlantic.‬ ‭The Gibraltar threshold is visible in the image: at this level the depth is very small, reaching 320 m,‬ ‭while the width of the threshold is extremely narrow, just 13 km. All this means that trade with the‬ ‭Atlantic is relatively unchanged. The cold waters of the Atlantic, due to the presence of the Gibraltar‬ ‭threshold, do not enter the‬ ‭The waters coming from the Mediterranean instead exit at the bottom. These outgoing waters‬ ‭are surface waters, which have sunk into the Mediterranean basin because they have been‬ ‭made very salty by summer evaporation and then made even denser by winter cooling.‬ ‭Although the quantity of water exchanged in Gibraltar is relatively limited, it is important to‬ ‭underline that this exchange is sufficient to prevent the level of Mediterranean waters from‬ ‭decreasing and this would happen precisely because of the negative hydrological balance,‬ ‭i.e. evaporation prevails over contributions due to river waters and rainfall.‬ ‭Let's see here what happens to the Atlantic waters that enter Gibraltar, represented in green: they‬ ‭invade the Balearic basin and form the Algerian current, which flows along the northern coasts of‬ ‭Africa, a branch enters the here we see the circulation of Levantine intermediate waters, due to the‬ ‭progressive increase in salinity caused by evaporation. Their position then takes to greater depths‬ ‭during the winter (they can even reach 500 m), due to the effect of the decrease in temperature‬ ‭which increases the density even further. The Levantine intermediate waters do‬ ‭They are distributed in both sub-basins, the eastern and the western one. The presence of‬ ‭these waters in‬ ‭The column can be easily detected because it always coincides with a maximum salinity, which‬ ‭can then be found at variable depths, in a depth range from 200 to 600 m. Once beyond the‬ ‭Balearic area, the Levantine waters bend towards the west and are the ones that then cross the‬ ‭Strait of Gibraltar on their way out.‬ ‭The origin of deep water masses in the Mediterranean is more complex: in the western basin the‬ ‭deep waters originate mainly from the sinking of very cold waters at the level of the Gulf of Lion. In‬ ‭the eastern basin, however, until the end of the 1980s, the deep waters were mainly made up of‬ ‭cold, dense waters that had sunk due to winter cooling in the northern Adriatic but also in the‬ ‭southern Adriatic. These waters then flow beyond the Otranto threshold and end up in the deepest‬ ‭parts of the entire eastern Mediterranean basin. But at the end of the 90s there was a profound‬ ‭change in the process of the deep waters of the eastern basin and the Aegean Sea became a‬ ‭source of waters that were even denser, because saltier, than the Adriatic ones. This phenomenon‬ ‭was called EMT, Eastern Mediterranean Transient, by oceanographers. This phenomenon has led‬ ‭to a prevalent change in the eastern Mediterranean sector. Since EMT originated, it has modified‬ ‭the deep layers of the eastern Mediterranean and altered the deep circulation and recorded a‬ ‭marked change which then extended substantially from the surface to the bottom, where the‬ ‭waters coming from the Aegean replaced the old ones deep waters of Adriatic origin. In the deep‬ ‭waters of the western basin, however, a substantial trend towards an increase in temperature and‬ ‭salinity has been observed since the 1950s. Since the first decade of the 2000s, the presence of‬ ‭warmer, saltier water has been identified in the western basin, replacing the deep water that was‬ ‭previously present there. This anomaly has been called Western Mediterranean transition WMT.‬ ‭Between EMT and EMT we talk about "transient" and "transition", two related but different‬ ‭phenomena, and are the expression of an evolutionary trend of the Mediterranean as a whole. In‬ ‭both cases there was an increase in temperature and salinity in the deeper layers and there was a‬ ‭rise in the previously resident waters which rose a few hundred meters in depth, replaced by the‬ ‭new deep waters originating after the transient and the transition. However, in the western basin,‬ ‭the area of formation of the new deep waters has always remained the same, while in the eastern‬ ‭basin there has been a change in the place of origin of the deep waters which has gone from the‬ ‭Adriatic to the Aegean. This obviously resulted in a change in the circulation of the basin unlike‬ ‭what occurred in the western basin.‬ ‭This image reconstructs the surface and deep circulation models before and after the transient‬ ‭throughout the Mediterranean basin. Movements of surface water are indicated in light blue‬ ‭and movements relating to deep circulation are indicated in dark blue. Before the 90s the water‬ ‭coming out of the Mediterranean was‬ ‭ redominantly of Western origin. Since the mid-1990s it has become hotter and saltier. Sediment analysis‬ p ‭conducted over the years have then shown that the water coming out of the Mediterranean is more‬ ‭eastern than western. This phenomenon has been called Mediterranean sea transient and seems very‬ ‭linked to EMT. This modification of the waters leaving the‬ ‭The Mediterranean has led to variations in the intermediate Atlantic water with repercussions that are‬ ‭currently not foreseeable. The changes in the waters of the Mediterranean, which occurred in a short time,‬ ‭highlight how this sea is widely vulnerable to climate change.‬ ‭Other phenomena are the Nino and the Nina, which occur in succession. El Nino has this name‬ ‭because it usually occurs around Christmas time in the subtropical South Pacific. This area of the‬ ‭Pacific is affected by the cold Humboldt Current, which rises from Antarctica, creating divergence‬ ‭phenomena and causing deep waters to surface off the South American coast. As seen in the‬ ‭image on the left, the eastern side of the Pacific has a shallower thermocline and colder waters‬ ‭than the western side. This situation is kept stable by the action of the trade winds that blow from‬ ‭east to west, bringing cold and humid air to the coasts of Asia, causing precipitation. While, on the‬ ‭American side the climate is typically dry. The warm air from the trade winds that arrived on the‬ ‭Asian coasts rises in altitude and then returns eastwards, now descending, having cooled, onto‬ ‭the South American coasts. This closes a circuit called the Walker cell. When El Nino arrives, the‬ ‭surface water temperature off the American coast increases and this is due to the intensification of‬ ‭the countercurrent and equatorial current. A collapse of the thermocline occurs and these changes‬ ‭in the intensity of the equatorial current occur: in this situation the trade winds first weaken and‬ ‭then change their direction and blow from west to east, transporting warm and humid air to the‬ ‭American coasts. The reversal of the atmospheric circulation within the Walker cell also reverses‬ ‭the pressure gradient between the two sides of the Pacific: in the eastern sector the atmospheric‬ ‭pressure decreases and in the eastern one it increases. These pressure oscillations that affect the‬ ‭tropical and subtropical Pacific are called the Southern Oscillation. In reality we cannot speak of a‬ ‭normal condition and an anomalous condition: El nino and La nina represent extreme phases of a‬ ‭continuous baric oscillation always in progress. In fact, the Nina corresponds to the sudden return‬ ‭of the trade winds to their normal position. The winds became strong again, the cooling of surface‬ ‭waters on the American coast became intense with repercussions on the climate.‬