[MUSIC] Today we're going to introduce a very important subject: the formation of dense waters. All the water that is in the bottom of the ocean, Was at some point on the surface, but if it is at the bottom is that it sank. And it sank is because it acquired a higher density To the one he had when he was on the surface. Therefore the process of formation of dense water and deep water, Is fundamental in all oceans. Let us, as it can not be otherwise, to use examples from the Mediterranean. The area that concerns us, we have Barcelona in this area here. In white does not see Barcelona, we are going to put round black, perfectly. But the area where we are going to see how dense water is produced is not so much this, But in principle we are going to look at this one here. This zone, which is deeper, called the Gulf of Leon, generally receives Winds from the north in winter, cold, sometimes very cold. That on the one hand favor the cooling of water properly, But on the other they also introduce a lot of mechanical energy. And evaporation we know that the wind is important for evaporation. Therefore, it is an area where winter cooling Leads to dense water formation. However, in this area we are talking about here, This water dense, this uniformization of the top, Reaches a layer that is at a certain distance Of water that is somewhat saltier and somewhat warmer. And this is a very important question for forming deep waters. Let us distinguish what would be therefore, Dense waters that occur in winter. And they occur in the temperate zone more or less everywhere, Where the surface water is cooled, mixed, And this mixture can reach different depths. Depending on the climate of the area, Or according to the weather we have that winter. The water that is going to form will be more or less cold, more or less salty, And therefore will reach more or less important depths. This is simply dense water formation in winter. That it might not reach much depth, But it might actually sink and go To places far from the area where this dense water has formed. But the cooling and increase of salinity, generally, Are not too compatible. And therefore, really when very dense water is formed, Which has to be on a cold side and on the other hand salad. Usually it is because they mix two waters, one cold and one salade. With similar temperatures, in general the salt water is somewhat Hotter and cold water is somewhat less salty. But the mixture produces in cold and salty water and colder and More salty than the surrounding water. This water does sink to the bottom. This important mixture here occurs in winter and the truth is that in winter We have seen that this whole area here shows temperatures around 12, 13 degrees. In the southern part we see that there are somewhat higher temperatures, We are in the southern part. But in this part of here we do not see any difference in temperature. Something wrong?. Do we have the same situation everywhere? Do not. And this we see perfectly with this other photograph, where the Colors are proportional or represent chlorophyll. And we realize that the blue zones are those that have very little chlorophyll. That is, there are virtually no microscopic algae producing a Slightly different coloration, the water we could say is transparent. And the zones with yellow or red colors would be the zones where the water It really does have a lot of chlorophyll and it's a bit greener, we might say. Notice then that in this area here is an area where there is practically no algae. Why? It is a zone precisely where this intense mixture is being produced. It is curious because the life that is produced on top of what is a situation Mobile, a physical structure that is made and unravels and moves, Is the one that often reveals this operation to us. These forms, for example. These arrows I am indicating are swirls Which the organizations themselves reveal. And in the same way this, let's say blue hole, Is also better demonstrated by organisms other than their own Temperature or salinity, and why are there no organisms here? Simply because there is an intense vertical mix, and organisms, seaweed, Require light to live. And it turns out that algae spend some time on the surface with light, But then travel to depths of more than a thousand meters and pass the greater Part of the time in the dark. By spending most of the time in the dark it can not develop A population of algae. Therefore, it can not live a population of algae that of coloration and that Form important masses, and therefore the water is absolutely transparent. In this area I have said that underneath this winter water you are Mixing there is slightly warmer and slightly saltier water. Let's now see profiles of some stations of some profiles of this area And profiles of an area somewhat further south. To see the difference that there is of the position exactly of this water Something saltier, of this zone with respect to the other. And this is important because it is what determines that in this zone There is really very deep water formation or, say, very dense water. And here, however, this dense water does not form. These profiles Are in the area. This would be in the north, this would be in the north. Two nearby stations. But notice that we have, the red color would be the density. So we have water that increases the density and from a certain depth, Practically the density varies very little, it is of 1029,1 more or less. But here is an area where there is a minimum of oxygen and a maximum of salinity. Salinity is represented by fuchsia color, and blue oxygen. The minimum of oxygen and the maximum of salinity, The maximum of salinity and the minimum of oxygen, are Sign that there is an older water coming from another place. And this water is just water coming from the eastern basin of the Mediterranean, Is what we call Agua Levantina Intermediaria. A water that moves at an intermediate depth and comes from the eastern basin, It is more salty and somewhat warmer. Here in this zone of the north we find the nucleus, say, The signal to a depth of about hundred, hundred and peak meters. While further south, the Maximum salinity and minimum oxygen, Are in 400 or 500 meters. That is, 400 or 500 meters is relatively far, 100 meters is very close to the surface. What does this mean? In the north, the Levantine water is More towards the surface, is a place where this water will be incorporated to the mixture. Very cold water on the surface, relatively salty but not necessarily very salty, Is mixed with slightly warmer but significantly more salty water. And they give rise to a water that is deep water, Which has a relatively high salinity and a maximum density for the area. Not every year produces the same amount of deep water, Not every year have the same characteristics. Although more or less repeats itself. Some years, it happens that in these polar air inlets or air of the North that produce important storms throughout this area, There are years in which it is somewhat colder. These phenomena of turbulent mixing and deep water formation are Documented for the first time in a clear enough way. Late 60's early 70's. These graphics belong to these first works and really The situation we see that the areas where the water is mixed. They have a width more or less of a few tens of kilometers, Between 10, 20, 30 kilometers, variable. And that on the sides of these well mixed areas, there is a certain stratification. That is to say that these structures were baptized like chimneys, By the fact that here too there was an important vertical mix. And it also ventilated or communicated the bottom with the top. That is to say, in these zones the deep water is communicated with the one of the surface, There is a circulation. What happens after these situations? Logically after this mixture the water recaptures. And in addition, the mix has produced something else. And it has made water, let's say deeper, Or water but deep of the whole that is off the surface, Has been mixed and has been enriched with nutrients Surface water, ie spring has arrived. What is spring at sea? The end of winter begins the spring sea. At the end of winter the vertical mix To put nutrients that were in deep and dark water on the surface Within reach of sunlight and therefore is when there is algae growth. And therefore, chlorophyll increases in the surface layers in spring. And in the spring the blue hole, this zone of vertical mixing, disappears. In years where it is very cold, as was for example the year 2005, There was an irruption of cold air that lasted a few days, And also very cold, a special phenomenon occurs. And is that, in this year for example, these profiles that we saw before, With an example, we now see particularly the winter of 2005. The uniformity is incredible but also the density of surface water In the coastal zone reached not only the density of the deep water, But not a density higher than the density of the deep water. What is happening? What happens is that the coast usually in winter. This would be a T / S diagram that you saw, that is, Temperatures, and salinities, data of the coast of Catalonia, Of the coast around Barcelona of the year 2003, 2004 for example. They show water in winter with salinities Which are close to 38 grams per kilo. And temperatures that are around 12 and 13 degrees, Which are typical winter temperatures. But what happened this year? There had been a relatively dry weather and there was a lot of cold. What happened is that there was an intense mix And what increased greatly in surface water was salinity. And also with more or less equal temperatures In the typical stations we visited at that time. And yet, in stations where there was a continental water influence, Influence of the Ebro for example. As the temperatures went down more, that is to say we find ourselves That temperatures could reach temperatures of eleven degrees. With not particularly low salinities, but relatively high salinities. That is, these waters of the coast were very dense waters, The waters very dense, fixed than the dense waters, The deep water has a density of 1029.1 or so. These waters have densities equal to or greater than the deep. If we compare the 2005 winter waters on the coast With deep waters or bottom waters documented by different authors. We realize that they have higher densities and salinities of the same order, But much lower temperatures. The key issue is that there is a typical process Which always happens but sometimes is more important and is as follows. Apart from dense water formation in this area, You have to keep in mind that the platform is A place where the thickness of the water is smaller, Reaches 50, 100 meters. And where the cooling can even become a layer of water. Then when the uniformity arrives to the bottom It cools all of it, that is, it cools and can get colder. In addition in the area that receives waters of the Rhone, Or receives rainwater from runoff. Therefore it is a relatively less salty water, But that can get very cold. The result is that it often Enough densities to Can see water drop from the platform Which descends into the depths. What happens is that on this journey to the depths, Which is usually done many times along the submarine canyons, Find water somewhat warmer, dense, and mix. The result is that a certain amount of water is often produced. We would say that every year there is a certain amount of dense water going towards The bottom or important depths that come from the coastal zone. And in particular in 2005, the whole Coast really had water Very dense that it came down and formed an especially dense water this year. In summary, in winter dense water is formed, The degree of density will indicate or mark the depth it will reach. More or less everywhere every year the same type of winter is repeated, The same type of climate, with small interannual variations. What happens at the level of the Mediterranean is that they are formed, In different places, as we shall see, or as you can see in your documents, Deep water in different places, intermediate waters. And that these circulate following more or less the density itself to the height, To the depth that corresponds to them. But there is some interannual variability. Something similar happens in the oceans, it also always occurs in them Places deep water or bottom water, with some interannual variability. Therefore, we will see in the following introduction How these Mediterranean waters are distributed and how they circulate.