is a senior scientist in physical oceanography at woods hole oceanographic institution she's an expert on the complex physical chemical and biological aspects of the ocean she also leads research and expeditions in ocean dynamics as matter of fact she's getting ready to for an expedition very soon and she has also developed models that predict and explain these complex ocean processes dr of the appeared on this program in two thousand and fifteen to demonstrate mechanics of phytoplankton blooms in the oceans which are important in the absorption of the atmosphere c o two and we are so delighted and honored to welcome mahadevan back again welcome thank you and i would like to start if we could with just a little background i think that most of us think the oceans are just out there and that there are not necessarily systems dynamics that we need to go back and you're the expert in the dynamics could you give us a little background about how oceans work yes so the oceans are  you know hold an enormous amount of water ninety seven percent of the earth's water is in the ocean what we see on land is just a very minuscule amount and they are constantly in motion so the ocean is forced by the winds the winds cause the currents and the currents are caused by a combination of processes one is that the winds put mechanical energy into the ocean and the other factors that you have the continents and the shape of the basins that constrain the flow and create boundaries the rd thing is that you have differences in density of the water and those differences and density also play out as differences in pressure so when you have light water was more dense water sitting next to each other that leads to currents or jets in the oceans and for the more importantly planet and the dynamics is very much affected by the fact that the planet is rotating because we are on the rotating planet viewing everything in that rotating i see and so because we are viewing  reading frame there is the effect of rotation that we see keeps turning things you know to the right in the northern hemisphere southern hemisphere and that's what shapes the currents and the oceans are not addressed i mean if you look at a forest and you go and look at the trees you come back and you measure the same place the same tree will be there but if you go and look at the oceans you measure one part of the oceans to be come back tomorrow that borders on it's moved away yeah and  water so it's a challenging environment to work in but also very interesting because everything is in motion all the time not all of the time so so there's a physics to this yes right you mention pressure like temperature differences in the different areas of the plant does that affect this these processes to yeah so the pressure in the ocean is set by differences and sea level  might occur for example because of the wind blowing and then you have a continental boundary sample if the wind blows and it moves water away from the continental boundary it can pile up at the other continental laundry so that's one way in which you generate pressure differences and you also can generate pressure differences when you have differences in density of the water which come from temperature and salinity to factors that control the density of seawater us temperature salinity i would say in a large fraction of the ocean it's temperature that dictates the density but in places where you have a lot of fresh water runoff or when the water is very cold like at the  should you get the effect of salinity being more important on that then oh i see so it depends on where you are exactly so there are multiple facets each time but consider yes in terms of these currents we hear a lot about this we're hearing a lot about temperature salinity and the currents that we kind of depend on for as far as a lot of ecosystems in so one could you give us some idea about how these currents work these major things like the conveyor belt and what happened here so the surface ocean as i said was driven is driven by the wind and it has a circulation and intensified for example the coral shoes one of them in the pacific and in the atlantic we have the gulf stream year and those boundary currents then go up the coast and they break away from the coast and they form the go across the atlantic for intense currents most prominent ones being on the western margins but in other regions two you have these currents which form a very complex network of branching out and they form and that's also a very strong change with density so the surface is obviously lighter because the less dense water would be more buoyant and stay at the surface and the ocean is very layered and density and once you're away ask you do you know we hear a great deal about like the conveyor belt for instance of the american coast and so on these things are apparently really significant for the north atlantic the high latitudes in the north atlantic are one such region there i mean of deep water formation and the water there in the winter it's very salty that water and when it starts to cool it's also salty it becomes denser and the ocean the atmospheric of these transport heat interested in the ocean currents and eddies transport he writes so that is that is really important for stability for the planet i take it right yes so we think that during thinking is that maybe we don't see the same rate of deep water formation and we may see a low that but if you were to slow it would we change the transport of heat to the poles at the same time that the oceans are warming and the region that's warming the most is in fact the arctic ocean zone northern latitudes right it's warming the most because of the ice feedback so without any ice the ice is retreating every year we have less and less ice and less and less warming rather fosters to the and at the highlight yes and and so we don't quite know how that would change the manatee and overturning circulation and there are some measurements that are ongoing where people are trying to estimate down this process where the warmer comes up and then goes under and creates this sort of conveyor even if it is diffuse that does is this a potential destabilizing mechanism in the world's oceans i think a number of like certain fish migrations depend on this but i'm not sure yet it slows these things down ye because what we see in the general media with this it's one sort of lie and it goes under and stuff and what you're saying is we know better yeah well that this is sort of ga going to happen about this because the feedback so so many in the ocean in sort of the oceans and earth system you know ice feedbacks ally all three backs salt plays a role as to whether it might form is i just have to ask you because we deal with people who are highly specialized all the time and everybody in say oceans of water everybody is studying a slice yes something and in so many fields you need a lot of minds together and combine to get to something but in your opinion is there like a really orientation to deal with complex phenomen like this because this is extremely complex the way you're outlining it like in terms of training young people to go into these fields are they looking at things your perspective is complex you know your background is yes i think students are now become a much more multifaceted what we were you know drive to train to write for and models have improved a lot can do a lot with computational models the observations have improved a equipment that you have now at hand but before that what does it mean to destabilise what will happen if the oceans are d stabilized and i magine there are number of ways that can happen the salinity the temperature all of that but what will happen to the planet if we take yeah so you know i sudden things would always between the surface ocean and the deeper ocean increases that is in fact adding stability to the oceans so it is ok in the vertical so there's different kinds of home in higher sense but if you add increase the density effect that as you one desires or sions will our fight a plankton production every base with their right another kind of thing which you might depend on is this as you mentioned you have to form deep water somewhere o and so we don't know whether we will maintain the same rates of dense water formations for this keep up this marriage annulled returning circulation at the same rate or we would change that or eat by actually warming the  has gone into the oceans in the last you know a few decades and there's a number of new estimates that have come out and they all agree to some you know fairly well in the sense that there is a lot of heat that the oceans have acquired so the atmosphere on the other have to be higher to radiate out to space yes the same amount of heat that it's receiving ok and that's why the entire planet you know that's the greenhouse effect is going to create this temperature but most where's that heat that  right somebody dealing with this on multiple levels and stuff are you concerned about this or is are we being too overly concerned about the direction of the climate change with this oh no i mean how to get it studied more and how to get help to get the governments really to pay attention to these problems right here technically is it reversible still so when you sa i meant specifically the oceans the warming of the oceans for instance are we in a position if we put our minds to it with the world policy could they slow this down or whatever yes so you know if we ask about this wonderful the the way you study these things now the way the we see these things probably you know bobbing up and down in the oceans and stuff and people can go and check them periodically and and so on but you mentioned satellites equipment in the oceans and apparently this is very sophisticated can you give us an example yes so with satellites you can measure the sea surface height or the topography of the and those satellites by measuring the height of the sea surface of course this all kinds of effects in there and the if you remove the effect of waves and things then you get the pressure for the height of the sea surface is telling you where you have high pressure and where you have low pressure  ocean yes you can also look at sea surface temperature from space so we have different kinds of instruments to look at sea surface temperature and so we can monitor babb and look at changes from one ships can only make a few measurements so we have a lot of autonomous instruments one of the most successful programs is one called the argo program which floats so floats are things which that to check these kinds of things or to do some measurements like what yeah so the expedition we're going on is to spruce look at specifically the process sees that occur or add a front to the ocean front is where you have we have a very strong flow or a jet or a current there i see that very strong current has instabilities on it and we are looking to see how things might move up or down here along those density surfaces  and so we will be going and trying to make measurements of these at much finer scales than we can see from satellite bakshi being there making the measurements and following drifting things in the water and measuring but behind the ship as  we're going along and then coming back and putting these observations into models we try to understand the process and that's the other thing is that you developed quite of model would you tell us a little bit and how models w wired because i take it that they are able to accommodate these complexities that you have to deal with that yes so what models are doing is essentially they're solving the partial differential equations that describe a called an obvious talks equations and we solve those for the motion of the fluid they describe those equations describe their century newton's laws which tell us how something will move and if a force is applied or to it so when there's differences in pressure bats one kind of might have you might have wind driving the surface ocean you might have it warmer or colder and so you prescribe these inputs to the model which is the forcing and the model is solving those equations the way we do it is we take the equations and we say that every place in the ocean this equation should be satisfied and it tells us how that the the momentum will change essentially the velocity of the water will change with time ok so it predicts that sans writing yes or clinics we do that by taking and every location is connected to the adjoining location and you're updating the solution in time so these models require a lot of computational power with more and more better better you know better computers getting better and  modeling right the these so these you're able to accommodate in these models these changes in flow but also maybe these other conditions like the that all of the heat all that sort of thing and take a look yes the models i mean boundary conditions are the surface is the ones found yes i understand and they need initial conditions you start something and you're heap integrating entire yeah and so you know either people to the entire globe what i do is i try to concentrate on some specific career or not and study a particular process which is motivated by what we saw in the ocean yeah right  and take it from there you could you mentioned but you need longer range studies and so on what would be a good length for things that would really study the very changing dynamics of the ocean the risk kinds of factors now what would be like a good timeline like twenty years or ten years we may not have that much time but that's right yeah i mean i thought i would say a few decades you know when you have a long of the things i was trying to get at was that this stuff is really complex that when scientists talk about things like climate change and stuff we need a picture in our heads of something very dynamic very complicated and that you build these models to try to address that that is right and collect city as best as possible but these may be subject to correction or adjustment over time as you get new data that's the data that we get feeds into the model right and so observations are very crucial in making the models better better yet telling us whether our models are correct but also providing the right data for the models of what an exciting field that are today just just one more thing here into the students are training you have a you have a an ability to look at the physics the chemistry the bio everything there and it seems reasonable that one has to have that capacity and you do see this in terms of students coming up today and also the ability to make models presumably yes i mean you know a lot depends on the person they were you might one might have a student who has an interest in right physics and biology but dollar everybody not everybody does and i for example the the  expedition we're just going on again next week you know we are studying the physics of that current guests in the mediterranean but one of my students is very interested in understanding the five to plankton communities and saying structure right how is that shaped by this very strong front which separates the mediterranean water and atlantic water and what we're seeing is that these are different from the plankton communities that are so many species you know i didn't realize that  there yeah and each one has a different effect on how they take up carbon round where it goes and what they do in the whole food web trying to understand the biology and how it's related to the physics is really important do that without we making a prediction of what will happen to the fire the plank and the rhino system right and we don't really know that for example will the community change when the ocean is warm  difficult for us to tell we don't understand the biology as well against how it's going to relate to the right right and certainly the adaptability of these very basic species is right up how how fast they can adapt if they depressing sometimes perhaps at least it's very helpful for the public though for us to know that it is very complex and that you don't have all the answers yet and so i do hope you get funding and long term studies out there that that is absolutely essential right thank you so much for this and wonderful information is very interesting and i wish you the best with your expedition and it's going to be great fun because you said you're going to the mediterranean that sounds that just sounds very sorry i really appreciate your doing this because i think something that people don't realize is how connected the oceans out with our lives yeah half the oxygen we've got comes from the ocean and people don't understand why most people don't think about how dynamic the ocean is and how the physics the biology the chemistry and then how it impacts us and our climate run the world we live and the other thing is that the media could do a better job maybe of presenting this if people will listen that that's been a bit of a problem but i was cheered with this this reaction of young people this huge worldwide demonstration for climate fair urgency coming from the kids for pity's sake that maybe this will also increase the moon would say curiosity but the demand for good information that people can understand how this is put together and why we have to pay attention today yes to the yes for you to some very important work thank you you do too well we really appreciate your efforts thank you so much for joining us good luck on your expedition thank you thank you very much