In this September's "In Depth" section we talk with Emili Garcia Ladona about marine currents and the importance of knowing them in detail to solve pollutant spills, adjust climate models, or contribute to the rescue of people.
Emili Garcia Ladona landed at the Institut de Ciències del Mar in 1986. He received his PhD in Physics in 1991 at the Universitat Autònoma de Barcelona (UAB) and over the years he has specialized in the mechanisms and processes that drive the movement of water masses in the Mediterranean Sea. These are processes that generate movements and structures at medium scale (mesoscale), that is, about 20 km in the Mediterranean and about 50 km in the rest of the oceans. To observe them, Emili looks at the ocean from space thanks to the information provided by satellites, but also using drifting floats. Once the data is collected, the currents can be extracted and analysed using advanced data analysis methods and simulation models.
1. What makes water masses move?
The Earth has a rotational motion about its axis. Only by this fact is it in motion if the observation is made from a fixed point outside the Earth. When we ask ourselves why bodies of water move, we usually refer to the motion we observe relative to us, for example, when we are sitting in front of the beach. The relative motion we observe is due to the wind blowing over the surface, temperature differences between the water masses near the equator and the masses at the poles, and astronomical tides, mainly from the Moon and the Sun. These are the fundamental mechanisms that generate the relative motion of water masses. However, the sea surface also deforms due to atmospheric pressure changes or other more sporadic disturbances such as a tsunami or a submarine landslide.
2. Is it true that they go around the world?
Of course, they do. But it is necessary to clarify that the processes mentioned above generate movements that form gyres or eddies of different intensity and scale: some very fast and often small, and others very slow that form very large gyres. The time scales necessary for the masses of water to "travel" around the planet are thousands of years or more, and during this period they undergo transformations of all kinds. For example, they can crumble and move along the sea floor. This means that the transformation processes they have undergone have made them denser and then it will be difficult for them to return to the surface. This is the same thing that happens when we throw a very dense object into the water and it sinks. If a process modifies the density of the surface water, it will sink to a depth where the surrounding water has the same density. The fundamental reason is the Earth's gravitational force, which tends to arrange the denser layers of water towards the center of the Earth, while the lighter ones remain above. However, in upwelling areas, the denser water from the bottom can reach the shallower layers again.
3. Are they affected in any way by global warming?
Yes, just like any other object on the Earth's surface. What is perhaps different is how it affects them and what the consequences are. Currents in the ocean are organized on a large scale as a function of rotation, tide, density gradients between equatorial and polar water masses, prevailing winds and the presence of continents. If the latter did not exist, the general circulation would be very similar to the atmospheric circulation of the winds. One of the first consequences of warming is thermal expansion, which implies an increase in volume. Therefore, we can say that warming contributes to sea level rise. In fact, it is estimated that the thermal expansion of the ocean due to warming is one of the factors that has contributed most (about 50%) to the rise in sea level over the last 50 years.
4. What other consequences of warming have you detected?
We have detected changes in the arrival of large volumes of fresh water, especially from continental glaciers (mainly Greenland), which apart from providing an extra volume of water, and thus contributing to sea level rise, change the density of the water, which distorts the density gradients and, consequently, modifies the current patterns. In this regard, it is worth noting an aspect that is very often forgotten, ignored or confused with respect to the contribution of melting ice at the poles: Arctic melting ice does not contribute to sea level rise in any way, since it is ice that is already in equilibrium in the water. On the contrary, the most significant melting and the one that contributes most to sea level variations is that of the Greenland glaciers and the continental glaciers - located above the solid ground - of Antarctica. There the ice "flows" to the sea under equilibrium conditions that, if broken, could cause the flow to the sea to accelerate, thus contributing significantly to sea level rise. In all these cases -including the Arctic- there would also be changes in ocean circulation, since continental water has different properties (density) than seawater, which would eventually alter the movement of water masses. In fact, recent paleoceanographic work has revealed changes in the intensity of the North Atlantic circulation.
5. Could warming have something to do with the alleged increase in beach drownings reported in some media this summer?
This issue has been in the news this summer and has been related to potential changes in ocean currents or their intensity. However, I am not aware of any alteration of these factors in the Mediterranean, and when the news has provided more details it has been seen that the accidents have probably been caused by a combined effect of a greater influx of bathers on the beaches and an increase in recklessness. In any case, this does not mean that traffic in the Mediterranean is always the same. Lately we tend very easily to think that any sudden change may be a consequence of global warming and this, in the case of the sea, is difficult to corroborate because we do not have sufficiently long data series to be able to correctly separate natural variability from the anthropogenic effect.
6. Is the Mediterranean a dangerous sea?
In the Mediterranean the waves are smaller and shorter than in other oceans. One might think that for this reason it should be safer for sailing, or at least for large ships. However, the meteorology of the Mediterranean, although we find relatively long periods of "good weather", changes very quickly and very strong winds can develop suddenly. I would say that it is no less dangerous than any other sea or ocean, the danger is simply related to different factors.
7. How are currents studied?
Ocean currents are probably the most relevant ocean variable. Not only because they are essential to understand how the ocean works, but also because they have direct implications on marine ecosystems and on practically all socioeconomic activities that the human species deploys over the sea. They have been and continue to be one of the basic objectives of oceanography. The development of oceanography during the last century has been, to put it simply, to be able to describe and measure them.
V.W. Ekman invented the first mechanical current meter at the beginning of the 20th century, and since then we have had different ways of measuring currents: by direct measurements through current meters fixed in a particular place (what they call moorings), leaving floats drifting and determining their trajectory, or through indirect measurements based on the thermodynamic properties (temperature and salinity) of the water masses.
8. And then came the satellites…
Yes, in the beginning, observation focused on measurements of the color and temperature of the sea surface. It was not until the early 1980s that two missions were launched (TOPEX and POSEIDON), which used altimeters to measure the height of the sea level. The fact is that currents are highly correlated with spatial variations in sea level height. This fact was exploited, for the first time, to produce global-scale maps of currents as we had never seen them before. The first temperature and color images of the ocean showed the surface of the oceans as a soup of eddies and filaments far removed from what the diagrams in books showed us. "For the first time we can see the sea and its variability at its best," said W. Munk, one of the most relevant oceanographers of the last century, who spoke of these as the "maps of ocean weather". Current missions aim to increase the accuracy of ocean current measurements. In this sense, with the available data, at the ICM we have elaborated a monthly climatology of the surface currents of the Mediterranean basin and the waters surrounding the Iberian Peninsula.
9. What are the advantages of studying the ocean from space?
They allow us to see how ocean currents are configured at an effective resolution of approximately 50 km. I say effective because the device that measures the height of the sea level -the altimeter-, from which the currents can be derived, measures with much higher resolution, but along the satellite trajectory. To obtain a map with this resolution we need to wait until the satellite has made a sufficient number of passes to be able to construct a map. However, with the combination of several satellites the uncertainties can be greatly reduced and higher resolutions can be achieved. Typically, in the Mediterranean we need 7 days to build a current map. However, the cost of these satellites is very low compared to what it would cost to deploy and maintain current meters all over the world or even in a small area such as the Mediterranean basin. Current altimeters measure point-to-point every 6-7 km. With the recent SWOT mission - a joint initiative of NASA, France and England - we will obtain small instantaneous maps that, when combined, will allow us to generate global maps with a resolution and quality with which we will be able to obtain the currents.
10. Is it possible to predict the displacement of water masses?
Ocean currents, in theory, can be predicted from the basic and classical equations of fluid dynamics (the Navier-Stokes equations) considering some effects typically particular to geophysical fluids, essentially gravity and rotation. They are the same ones used to predict atmospheric winds and to know the future weather. However, the flows are turbulent as a result of the nonlinearity of their equations, and as long as someone does not find how to solve them - it is the only unsolved classical physics problem - what we do is to simulate the movements of water (or air) masses through the computer. The strategy is analogous to that used by meteorologists: we combine models and observations to try to increase the prediction horizon, that is, the time where the prediction is reasonably careful. What happens is that, unlike meteorologists, we have much less data at our disposal, so the predictions of oceanographic models are usually more problematic.
11. Why is it important to know the dynamics of water masses?
When I say that my research is mainly focused on the study of ocean currents, the question always arises: "What is this for? Ocean currents are largely the response to the differentiated warming between the equatorial and polar zones and seek to homogenize the temperature of the planet. These same currents transport energy and all that is found in water masses. For example, the heat they transport is essential for the regulation of the Earth's climate (remember that 70% of the Earth's surface is water). We will never be able to understand or predict the Earth's climate if we do not understand ocean currents, but what is obvious to an oceanographer has taken a long time to be reflected in climate studies and reports. For example, the characteristic climate of Europe cannot be explained without the arrival of heat transported by the Gulf Stream. But, in addition, marine ecosystems have adapted to marine currents to supply nutrients to organisms to survive, some of which take advantage of or avoid currents to migrate and spread if the conditions of the water masses allow them to do so.
12. And what good does it do us as a human species to know how water masses move?
We take advantage of it to exploit the resources it provides us. Large fisheries and fishing grounds occur in areas that have a particular configuration of water mass movements, the upwellings. In addition, currents facilitate navigation to transport materials and people. In short, much of the development of our civilization depends on currents. On the other hand, part of my research has always had an eye on marine emergencies, whether due to pollutant spills - I was involved in the response to the Prestige spill crisis - or search and rescue at sea. These are two aspects where it is necessary and essential to know what the currents are like and to predict them in order to be able to prevent and respond appropriately. In cases of spills, in particular, it is important to know where the pollutant will go, while in cases of search and rescue, knowing where the water will go helps to make the search for people or drifting objects more efficient. Both of these areas are very demanding and test the capabilities and limitations of current ocean prediction systems.
13. What are currently the main challenges in your line of research?
More than challenges, I have always moved with a spirit of curiosity to understand and know the world around me. This has led me to explore other problems and topics beyond the study of water bodies. Curiosity can sometimes be more powerful than a challenge. Challenges are generally well-defined, often long-term, motivating goals. In my case, at this point in my scientific career, I can only talk about the last two projects in which I am involved. The first is the DEMON project, which aims to substantially improve the development of current ocean models to better approximate reality. If it works, we could make more detailed predictions of ocean dynamics. The second, on the other hand, consists of the deployment and consolidation of an operational oceanography system in the Catalan-Balearic basin, the ICATMAR (with two components: an observational component and a modelling component). The latter is as exciting as it is ambitious and costly, which is not always properly understood from a political point of view. The fact that we are a fundamentally terrestrial species and that the sea is practically alien to us on a daily basis, leads us to believe or ignore many times the relevance of understanding and studying the oceans.