Across Acoustics

Ocean Acoustics in the Changing Arctic

August 04, 2022 ASA Publications' Office
Across Acoustics
Ocean Acoustics in the Changing Arctic
Show Notes Transcript

Climate change has led to a reduction in amount of ice covering the Arctic as well as the structure of the ocean. These changes have impacted many aspects of acoustics, from the communication of marine life to human navigation systems. In this episode, we talk to the editors of the JASA Special Issue on Ocean Acoustics in the Changing Arctic, Peter Worcester, Mohsen Badiey, and Hanne Sagen, about current research in Arctic ocean acoustics. 

Read the Special Issue on Ocean Acoustics in the Changing Arctic.

Read more from The Journal of the Acoustical Society of America (JASA).

Learn more about Acoustical Society of America Publications.

Music Credit: Min 2019 by minwbu from Pixabay. 

Kat Setzer (KS)


Welcome to Across Acoustics, the official podcast of the Acoustical Society of America’s Publications office. On this podcast, we will highlight research from our four publications, The Journal of the Acoustical Society of America, also known as JASA, JASA Express Letters, Proceedings of Meetings on Acoustics, also known as POMA, and Acoustics Today. I'm your host, Kat Setzer, Editorial Associate for the ASA.


Today, we're taking a bit of a different focus from our usual episode structure. We'll be discussing an entire special issue of JASA rather than a single article: the special issue on Ocean Acoustics and the Changing Arctic. Today, joining me are the guest editors of the Special Issue Peter Worcester of Scripps Institution of Oceanography, Mohsen Badiey of University of Delaware's College of Engineering, and Hanne Sagen of Nansen Environmental and Remote Sensing Center. Thank you for taking the time to speak with me today. How are you?


Peter Worcester (PW)


I think I'm good. I'm actually just getting over COVID 




Oh, no. Yeah, it's going around now.




I lost a lottery.




That's a bummer.


Mohsen Badiey (MB)


Yeah, we are doing well. Doing well.




Good. So to start, Can each of you tell us a bit about your research backgrounds?




Okay, I'd be happy to start with that. As you indicated, I'm at the Scripps Institution of Oceanography at the University of California, San Diego. I got my PhD there many, many years ago, 1977, and stayed at Scripps ever afterwards for the rest of my career. And while there, I've actually worked on acoustical oceanography and underwater acoustics—the use of sound to study the ocean and how sound propagates in the ocean. And I've been involved in the development of the required acoustic instrumentation, the deployment of it, the conduct of experiments, really all over the globe from the Arctic, which was the focus of this special issue, to Fram Strait, the Philippine Sea, Strait of Gibraltar, north Atlantic, really in a wide variety of places over the years.




Very cool. 



My background is actually kind of very similar to what Peter described, as we are working on similar research in different ways. But I have a background in applied physics and engineering. And I have been a faculty here at the University of Delaware for several years, studying the underwater acoustic and also acoustical oceanography, that means, how the sound travels underwater from one point to the other, and then how the oceanography parameters change that sound propagation. And so, in doing that, I started my work from a theoretical background; I have emphasis in applied math and signal processing. But then, later, I started noticing that the data, as we explain in this, maybe program a little bit, is very, very important in this field of research. So I started developing and designing experiments to collect field data. And we have been working in that area, designing some arrays and equipment as well as experimental setup to capture the essence of how sound travels underwater. Like Peter, we have participated in Arctic acoustics. Also, my emphasis have been in more shallow water areas, where the sound waves have a chance to interact with boundaries on the sea surface and sea bottom many times, and that makes things a little bit more complicated. And so, this, this is what we have been doing for several years right now. And we are continuing to do so.




Very interesting. Hanne?


Hanne Sagen (HS)

I'm from Bergen in Norway, and I studied at the University of Bergen, doing my doctoral degree in applied mathematics and specialized in underwater acoustics, so I have been working with acoustics since then. And I work now, and have always been, at the Nansen Center in Bergen, and we have been working on observing systems, including acoustic networks, in the Arctic, almost all the time. And we have worked very closely together with Peter and his group at Scripps. So my responsibilities is to plan and coordinate fieldwork, to analyze this observations, use of ocean models in combination with the acoustic models. And I also do some remote sensing; I use satellite images in my work. So that's more or less what I'm doing.




Awesome. Thank you.




And I have some students, too. That's fun.




So then, I guess, to get into the issue… For a bit of background, what can be learned from the study of ocean acoustics and more specifically, ocean acoustics in the Arctic?




Well, I think I would, I would like to start answering that question, by responding that I think ocean acoustics has a very special role to play in the Arctic and in the ocean in general. Ocean acoustics is the only energy that travels any distance in the ocean. Light is quickly absorbed, while sound can travel very long distances. And in the Arctic, it has an even more special role to play because the ice cover prevents observations from the surface, while the sound can propagate under the ice, and serve a variety of roles underneath the ice in the Arctic. For example, it can be used to do large-scale remote sensing of ocean temperatures and currents. Sound travels a little faster when the ocean’s warmer, for example. So if you measure acoustic travel times, shorter travel times correspond to warmer temperatures. 


You can also learn a lot just by listening to underwater sound, to the sounds that marine life make, the anthropogenic sounds, to the sound that the ice and earthquakes make. So there's a lot to be learned just by simply listening. 


Third, there's a lot of useful purposes to which underwater sound can be put. It can be used for underwater navigation, for example. If you put in acoustic sources around a region, and a moving vehicle in the middle records those transmissions, it can figure out where it is. This can be used by drifters, autonomous underwater vehicles, submarines, anything that travels underwater. 


And finally it can also be used for communication; you can send signals that transfer information from one place to another. And I might point out here that this doesn't just apply to our use of sound, to humans’ use of sound, but marine life uses sound for all of these same purposes. And so we're both, we and marine life, or other marine life, depending on how you want to portray it, find sound underwater to be extremely, extremely valuable and useful. 


And I might also mention here, there's a website that I've been involved with for many, many years called “Discovery of Sound in the Sea,” DOSITS, and I think our listeners if they want to pursue any of the things we talk about further, they may want to Google on that site. It contains a great deal of information, and it's all carefully vetted by experts in the field, unlike the content of many websites that one encounters.




So one of the things in complete agreement with what Peter mentioned is that the thing that I have been kind of saying when I am asked that question is that the acoustic waves are mechanical waves; that means when they go from point A to point B, their, the properties of the wave, can be affected by the physical properties of the medium in which it propagates through, and that is an important characteristics that can be exploited, and therefore, acoustic waves can be used as remote sensing tools. And that is to say if you send the same signal through an environment and the properties of that environment change, you can detect that change using acoustic waves. And that provides a very useful application of sound wave travel. 




Okay, so the, the using acoustics can help you basically see what you wouldn't be able to see with electromagnetic waves is what you're saying. And study what you can’t normally study.




So the other point that we can also mention is that acoustic waves propagate through solids as well. And that is another medium in which its density is very different than air and water. And so that field of study is what lends into seeing under the solid ground. And for example, all the oil explorations and things like that are used, are using the technology or the science of acoustic wave propagation through solids. So that's the other point that we can mention.




Ah, okay. So then actually, let's that kind of brings us into our next question. What factors are affecting ocean acoustics in the Arctic currently?




As we all know, the climate of the planet is changing dramatically in response to increases in greenhouse gases, carbon dioxide, methane, other gases. Well, what's happening to the planet as a whole is happening even more in the Arctic. There's a process called “Arctic amplification,” which means that surface air temperature in the Arctic has warmed at more than twice the global rate over the past 50 years. So the Arctic is just having a climate that's changing at a really terrifying rate. And we can give some specific examples: the sea ice extent has declined dramatically. The 15 years with the lowest sea ice extent, in the summertime, is the last 15 years, it's almost unimaginable. You would think that natural variability would cause that not to be possible. If you simply extrapolate current trends, if we keep releasing carbon dioxide at the rate at which we're releasing it, the Arctic will become effectively ice free in August and September in about 20 years. Of course, a lot of uncertainty in that exact date. You know, will we keep releasing greenhouse gases at the same rate? Do we fully understand the physics? But it seems highly likely that the Arctic is going to become ice free in the summertime in the not too distant future, with all that implies for the use of the Arctic, marine life and the Arctic, and acoustics in the Arctic. 


And it's not only the extent that’s changing, the ice thickness and volume are changing. One of the more dramatic examples that I know is that at the North Pole at the end of the melt season the ice thickness used to be about 3.8 meters, 12-and-a-half feet. Well, now, the average ice thickness at the North Pole at the end of the melt season is 0.9 meters, three feet. It's like it's an almost unbelievable change. And this also means that what happened with, in the past, ice would survive from season to season. So you got what was called “multi-year ice” that was many years old. It would get thick, you know, the 12-and-a-half feet that I just mentioned. As the ice moved in response to currents and winds, it would compress. It would form ridges as the ice compressed that also had ice keels underneath the water that extended down. And so the surface of the Arctic has just been changing at a tremendous, tremendous rate. 


Not only the ice is changing, the ocean stratification is changing. More warm water’s coming in from the Pacific through the Bering Strait between Alaska and Russia; more warm water’s coming in through Fram Strait between Greenland and Svalbard, and through the Barents Sea to the east of Svalbard. All this is affecting the interior structure of the ocean. 


And all of this impacts acoustics. The vertical structure of the ocean affects how sound travels in the Arctic; because of that structure, almost all of the sound interacts with the surface and interacts with the ice. And so as the ice changes, it affects how the underwater sound behaves. It's just amazing changes going on. And then this leads to other changes: with less ice, there'll be more shipping. In most of the world's oceans, the majority of low-frequency sound comes from the noise generated by ships. That obviously hasn't been the case in the Arctic with the ice present. The ice disappearing means that more shipping, more fishing, more tourism will happen during the ice-free seasons. This will impact the underwater sound that's present all the time as kind of a background noise. So the changes going on are just almost unbelievable. And it's a prime example of a part of the planet that’s changing more rapidly than the rest.




Yeah, I agree. That sounds like a massive amount of change happening. What kind of impact do these changes have?




Well, Peter have explained all these tremendous changes in the Arctic, and it is fascinating how we can use acoustics to observe these changes. First of all, that we can just sit and listen to how the icebergs are just falling off from the glaciers. We can listen to the ice flows are melting, creating small bubbles and sizzling under the ice. You have also the open ocean waves coming into the deeper Arctic, breaking up ice flows into smaller pieces, and you get all this slushy ice between. So there is a big chance for getting higher noise levels in the Arctic. Or not “noise” but maybe more like sound, natural sound. 


And in addition, when ice breaks up, and the area or larger areas look more like marginal ice zones marine life will change; you will get more life in there, and mammals and fishes and stuff, they also produce sound. In addition, you have what Peter mentioned, that the easier access to the Arctic will also change how we humans influence the soundscape or the ambient sound in the Arctic. So, all this will add on and we will get higher sound levels. And in addition, as Peter also mentioned and also Mohsen, that we can use acoustics to observe also by sending signals and measure how long time the signal takes from A to B, and use it as a thermometer. So sound propagates faster in warm water than in cold water and if the temperature goes up, the sound will go faster. And also, these changes will also influence how we design communication systems in the future. Because even if you get something to work in a temperate ocean, you can't be sure that it works in the Arctic, because it's so different environment; you get all these stratification and also the sea ice is influencing strongly how the signal is damped out, so you might need different strengths on your transducer, for example. And also these navigation systems they need also to be designed so that we can use them even if the environment are changing more than it has done already. So I think that the, all these changes going on will be interesting to observe, but also, it can be a challenge to kind of design systems that can work there.




Yeah, I might give one specific example, during the Cold War. Back in the 60s, 70s, 80s. The ice cover prevented sound from traveling long distances. Sort of the rule of thumb was that if you wanted sound to travel more than a few 100 kilometers, that the sound had to be in frequencies less than about 30 hertz, or wavelengths greater than about 50 meters. So low frequency sound just couldn't propagate very far because it scattered so much from the ice as it propagated. Now, Hanne and Matthew Dzieciuch with Scripps, who I work with, have done an experiment with 35 hertz-sound called the coordinated Arctic acoustic thermometry experiment, in which that sound traveled across the entire Arctic. So this means that we can use higher-frequency sound for things like navigation and communication, for sensing ocean temperatures, but it also means that low frequency sounds generated by ships and other sources of sound, like ice flows, can travel longer distances, so that these changes have major, major effects on what the acoustic environment in the Arctic is going to look like, in a few years. For comparison, back in the 90s, a similar experiment was done to CAATEX, in which 20 hertz sound was required in order to transmit across the entire Arctic. Now, we did it at almost twice that frequency. It's just remarkable.




So what you say, Peter is, is is also that there can be some benefits from the change, given that if the sound propagates more easily, we can reduce perhaps the length of the signals and also the strength of the sources. Is that? Is that possible? Do you think?




Yes, Hanne, you're absolutely right. It means that it's easier to use sound in the Arctic for the various purposes. It also has a practical implication that it's much easier to build the underwater sources, the underwater loudspeakers that generate the sound, and so it becomes easier in an engineering sense to use sound in the Arctic that can travel long distances.




To kind of back up a smidge, because we got into a lot of detail here, can you tell us a bit about ambient sound and what it is and how it’s studied?




So ambient sound, or as it's sometimes referred to as the ambient noise, in general is a combination of many different sounds that are generated by various signals, actually, or sources, various sources, and it's in general a summation of acoustic energy due to various ways that these sounds are generated. So, in the arctic environment, this energy comes from both natural and man-made sources, and the natural sounds that are out there already have been there for always, is due to ice breaking or sound of various marine mammals species, and interaction of sound with boundaries, for example, that come in, and then it makes the waveguide, which is what we are dealing with here, is a reverberant environment. So that means the sound gets scattered multiple times and creates this reverberation that you can hear. Now as the sea ice is changing and as well as the stratification that was mentioned, this reverberation is going to change, and therefore the background noise that is generated either naturally or manmade due to shaping or seismic explorations and, you know, navigation and things like that, that is going to as well change. 


So, as we are looking at this underwater waveguide as a soundscape problem, that means, it is an environment in which sound can travel in all directions and in a three dimensional, four dimensional including the time, therefore, the background sound or the noise is also going to be affected, again, drastically. Background noise is a signal, because any noise is basically a signal, and as those signals travel, again, they are affected by the physical parameters, physical properties, of the waveguide environment, and those are the stratification of the sound speed in depth, and in all dimensions actually, as well as the roughness of the boundaries—sea surface roughness, and the bottom roughness due to the bottom bathymetry; these are all the parameters that will affect the noise propagation in the Arctic as well. 


Now, the other question that you mentioned was how is it studied, right? So, like any other sounds that are, you know, generated, the noise can be measured using arrays of hydrophones, and once the sound is recorded, or basically gathered, then the signal processing techniques can be used to understand the nature of that noise, if it's due to, for example, ice breaking. And then what frequencies are pertinent for that kind of sound, to generate and propagate. And as well as the direction that it is coming from and how it's propagating in what ways, and how the energy of sound attenuates after it is propagated. And so, very similar techniques that are used for the any sound generated by man can be used for the noise or background noise or background sound, as you refer to it in here. 


Nowadays, recently, there has been interest by the acoustic community to start studying the sound using the artificial intelligence and machine learning techniques. And therefore, right now there are some studies that people are starting to look at the background sound with using artificial intelligence algorithms, and try to distinguish which one of these bandwidth are coming from what source, and then relate that to the phenomenon that it follows. And so that's the other thing that I guess some of the papers in this special issue started addressing that in the Arctic.




So actually stemming from that, then, is what kind of changes to ambient sounded the research in the special issue find?




Now, the Special Issue included a number of different papers on ambient sound in the Arctic. And I think I'd like to just pick out a couple of them to talk about. I mean, some, as Mohsen indicated, use techniques like machine learning to try to classify the various sounds that were recorded, but there were a couple of cases that I found particularly interesting. One had to do with marine mammal vocalizations. Marine life, marine mammals, in particular, vocalize and these are recorded when we record sounds under the sea. And in one of the reports, there were year-round recordings; underwater microphones, hydrophones, recorded sounds year round upon the Chukchi Plateau, which is shallow water area, somewhat to the north of the Bering Strait north of Alaska and Russia. And they had year-long data sets going back to 2009 and extending up through 2020. And they analyzed for the presence of vocalizations by a variety of Arctic species, including bowhead and Beluga whales, bearded seals, walrus. And what their data showed was that the composition of the marine mammal populations is changing. There's evidence of change in the far north, where endemic bowhead whales—bowhead whales that are normally present in the Arctic—are now heard into December, which would have been almost unthinkable not that many years ago because of the ice cover. Great whales have to surface to breathe. So in December, that would have been extremely difficult. Ribbon seals, which are a common species in the Arctic, have expanded their distribution northwards. Even sub-Arctic killer whales are now being heard upon the Chukchi Plateau. And so as a response to the changing ice and the changing conditions that we've talked about, marine life is adjusting, and it's tending to move. Species are tending to move north or to stay north later in the year than has previously been the case. And this is in an area, the Chukchi Plateau is still covered with ice for nine months out of the year. So all these changes are happening in spite of that, if you will. And so all of this, sort of the big implication is the biodiversity, the species that will be present there, is going to be changing as the Arctic changes, and with all that that potentially implies. So that's one example having to do with marine mammal vocalizations and how they can be used to study changing populations. 


The second example I'd like to bring up has to do with sounds caused by people, anthropogenic sounds as they're called. And in one of the papers, did a study along a shipping route in Milne Bay. This is a bay on Baffin Island up in the Canadian Arctic. And sort of in the last seven, eight years, a very large, open pit iron ore mine is open there. And they built a road about 100 kilometers to a port facility called Milne port, to which the ore’s taken and then taken out by ships during the ice-free period. And this study just recorded the sounds of the ships going by and tried to interpret the recorded sounds in terms of potential for impact on the local marine mammal species in particular. Some of these are, you know, ones that are present in the popular imagination like Narwhals, now the unicorns of the sea as they're called. And what this study did that's becoming more and more common is, in trying to assess the impact of the sound, it took account rather carefully of the animal's hearing abilities. So for example, toothed whales, like narwhals, use high frequencies; ships primarily generate low frequencies. And what they found is when you look at the frequencies the species like narwhals hear, that even though these ships and the usage of these waters by these ore-carrying ships is increased greatly, because the sound isn't in a band that narwhals hear well, that they would be unlikely to clearly perceive the shipping noise unless the ship was within about three kilometers of the animal. And the ambient sound levels, which we've just been talking about were low. And so it's not always exactly obvious what the connection is between anthropogenic sound and its potential for impacts on marine life. Not saying that there isn’t impact, but one has to be a little careful in assessing what's likely to be important, and what isn't, is what we're going to see. And I think Hanne talked about this a little bit, as the Arctic becomes more ice free, it's going to become used for a wide variety, of course, purposes from shipping to tourism, fishing, exploration for oil and gas, and in trying to make sure that we do that in a way that isn't harmful to marine life, we have to carefully assess what the impacts are likely to be, and what can be done to mitigate those impacts. 


There are a number of other studies, but I just like to emphasize those two when using marine mammal vocalizations to understand their behaviors and how their populations are changing, and the other to try to assess what impacts the sounds we're making are having on marine life in the area.




So one thing that I wanted to mention here is that there is a tendency for the background sound, or the ambient sound, to increase over time, because of these reasons—that shipping is going to increase ,and also exploration of that environment for various reasons will increase. So as the noise floor, or what they call the noise floor is the level of the noise, increases over time, then that will change, that will have a change in an effect in that environment. Because then, if we are using manmade sound for navigation, for example, we need to use higher-intensity signals to overcome that noise. If marine mammals are there, there are going to be constantly exposed to the higher levels of noise that are not there right now or haven't been in the past. So the studies of background noise or ambient noise is going to be very important over the long term. And it's an issue of signal-to-noise ratio, and what is called a “signal,” and what's called the “noise” depends on the application. So the background sound, ambient sound studies are, I would say one of the critical areas to study in the Arctic acoustics in the future.




Yeah, if I could just give one example for marine life in particular, the term that's used is “masking”; if the background ambient sound levels increase, this will mask, or prevent, marine life from hearing the vocalizations of other animals in their species. And so whether it's used for warnings or communication or reproductive purposes, they won't be able to hear some of these sounds because they're masked. It's kind of like a cocktail party, right? There's too many people talking all at once you can't hear the person next to you, you're trying to talk to.




Many of the papers in this special issue discuss ocean acoustic propagation. What is ocean acoustic propagation and why are it and changes to it important?




When you study the sound propagation in a medium, the propagation part is the part that refers to how this wave is actually affected by that environment in which it goes through. And that's why there are a lot of papers that are studying that because understanding that is the first step to do other problems in fact, like inverse problems of how to use that sound in order to obtain information about that medium. So the propagation part is a critical part that needs to be modeled and understood, actually, in general.




Yeah, I might just give one specific example of how the, the way that sound travels in the Arctic has changed in recent years. Several of the papers in the special issue comment on this. In the western Arctic, the part of the Arctic north of Alaska, warm water coming into the Bering Strait that we mentioned has formed a warm subsurface layer in the Arctic Ocean, and that part of the Arctic Ocean extending almost up to the pole. And this warm layer has gotten stronger and stronger in recent years. And sound speed increases with increasing temperature. So as this layer has gotten warmer, and also has higher sound speed. And what this has done is its created what we call a duct, it's a region of lower sound speed; there's high sound speed assorted associated with this layer of warm water coming into the Bering Strait, then there's a layer of colder, slower sound speed water, and then there's a layer of warmer water that actually came in through Fram Strait in the Barents Sea on the opposite side of the Arctic. So this creates a sound speed minimum. And one of the characteristics of how sound travels is that it's, as it travels, it's bent back toward regions of lower sound speed. For light, we use the term “refraction”; it's bent by prisms, for example, well, the same thing happens to sound. And this minimum can then trap sound so that it doesn't interact with the ice at the surface or with the sea floor, and can let it travel much, much further than it would otherwise. This can be useful for navigation, for sources and receivers in that layer, but it also has big implications for such practical applications as submarine, anti-submarine warfare tactics. Because this layering provides opportunities for submarines to hide and detect other submarines because of the way the sound travels, the way it's bent by this minimum layer. And so this is really a first order change. It's mostly restricted to the Western Arctic. But it is a big change in how sound travels in that part of the ocean.




That's very interesting. So, to touch a bit on something Mohsen was talking about, tell us a bit about more about the modeling of acoustic propagation. How is modeling done and what is it useful for?




There are several acoustic models available and used in the acoustic community, and they are based on different ways of solving the wave equations numerically. The most widely used are based on ray theory, which most of us are used to, and or you can also use mode theory that you decompose the sound field into modes, and see how the different modes are propagating through the ocean. But also, more computer power has led to more use or more complex models like finite element models or it can be beam number models. What we have focused on is to use these more easygoing models like mode theory and ray tracing, because it is tempting to link these models to ice-ocean models from the reanalysis produced by ocean modeling communities. So, that makes us capable to perhaps get the tool for designing new experiments. But it is also a tool from for the oceanographers to test how good are their models. For example, are they able to create the Beaufort lens, for example, are they able to, to have the interleaving structure that we see on the in the eastern Arctic, for example. And by using acoustics, or comparing to acoustic measurements, you can validate your model, and it's a very strong validation when you, if you are able to really predict what you observe in acoustics. 


There is several difficulties in doing acoustic modeling, because you, you don't know the bathymetry in the Arctic, to an accurate level. And that might then make your predictions not exactly as good as they should be. You don't know the ocean either, completely; the models are not so good yet. And another thing is the sea ice, which we have said many times, now, it's a very important climate factor. And we can observe the ice from space by use of satellites. And we can get a fairly accurate description of the top of the ice. But we can't see what is inside the ice; we don't know the material properties of the ice; we don't know the roughness of the ice, at the same level as we know about the surface of the ice. And it is hard to get these data; you can use a mooring and you can have upward looking sonars and you can get some statistics at that spot. But what about another spot another place? It will not be the same. And then also, it varies with time. So even if you have spatial coverage, you will come back a few hours later, and it's totally different. So therefore, it is important to establish statistical description of different kinds of ice. And I think that will be one of the key issues to get proper acoustic modeling in the Arctic is to establish a good statistical description of different kinds of sea ice. And there that can be a lot of scientific work. So please hear it's a lot of things to do. So that is my comment on the acoustic modeling.




Yeah, I think following up on that, you know, Hanne’s comments bring us back full circle to what we were talking about at the beginning—how rapidly the ice is changing. The ice that has been present for multiple years, multi-year ice, is disappearing, largely disappeared. So that all of these characteristics that are important to the acoustic modeling are different from what they were back during the Cold War. So that although a huge amount of effort was put into trying to understand the interaction with acoustics for the ice back in the 60s and 70s and 80s, current conditions are different. What was learned then really isn't applicable; the ice environment has changed. You know, one of the most dramatic examples, some of the multi-year ice, had these ice keels that I mentioned, where the big floes have been pushed together. Some of those keels would extend down 30-40 meters, over 100 feet. And this, you know, basically blocked sound that was interacting with the ice. Now with almost all first-year ice, ice that's only survived, you know, this season, there's still keels because the ice is relatively weak and it gets pushed together and it causes ridges and keels. The keels are much smaller. They're not, they haven't hardened and they're still slushy. You know, they have very different characteristics from what it used to be, so that what we're trying to model we don't really quite know it's all different from what it used to be. 




Yeah, that is very interesting about how the changes in the Arctic change how you'd have to model everything.




There are changes to the new models that are being developed right now. And the thing that I think is happening is that scientists are going towards the advancing time-evolving, range-dependent mathematical models, as opposed to in the past, they were not as much focused on the time-evolving part, or the range dependency was not as much emphasized, but right now, as Hanne mentioned, the things are changing dramatically in time. And so, there is a tendency, and so, these classes of models, in addition to what Hanne mentioned about ray models and modal studies, there is a parabolic equation, class of models that are range-dependent, and that could be are being used in in this modeling the studies, and some people recently have advanced that, hasn't been coming out as fast. But that's what happening in advancement of their numerical modeling.




So I guess to go on a different tangent, or a different direction, how are changes to ocean acoustics in the Arctic affecting positioning and navigation?




Well, how the ocean acoustics in the Arctic will affect the positioning and navigation. In fact, there is no positioning or navigation system installed in the Arctic on a permanent basis. There's been quite a few experiments with drifting positioning systems in connection to glider missions, for example. And then you have had some, some experiments in the Fram Strait and also in the Beaufort Sea, where we have set up multi-purpose acoustic networks, that should assist both the observers using acoustic thermometry, but also glider navigation. So for these experiments, we use the 250-hertz sources, and that gives you maybe 200-300 kilometer ranges, meaning that you can't, you have to constrain your your glider inside that. If you have a float, you can't control it, so if you try to put the argo float into the Arctic and just having these smaller scale range, smaller scale ranges then it will soon drift out of the network and you can't position it. So what we did in CAATEX was to move to a lower frequencies so that we can cover a larger area, meaning that we can do basin-wide system and that would, in particular help to get the argo floats, for example, operational in the Arctic. Because if you have an argo float, typically deployed in the Fram Strait, it will go into the deep Arctic under the ice and it will not be able to come up and send either position or get the position or send data or anything. So our mission is to try to get such a system implemented so that the argo people can do their stuff, the glider people can do their stuff, and we who like acoustic thermometry, we can do thermometry, and those who want to do passive acoustics can get the excellent system for following the mammals around the whole Arctic. So I think such a system will help everyone and I think that is, that is what we hope for in the future. What do you think, Peter and Mohsen?




Yeah, I agree that the changes of the ocean acoustics and environment are again, dramatically changed how the communication is occurring underwater. And navigation is part of that, and means how to position your source with respect to a particular receiver will be affected. So I think this is going to be important to have a network so that you can, similar to the marine mammals who have adapted to this new environment, we need to also have a system that will evolve over time, as things are dramatically changing right now. And so the accuracy of how to navigate is going to need to be an ongoing topic. And so it is important to have a system in place in an area that there is nothing right now.




So finally, as the Arctic continues to change, how do you see acoustics research evolving?




Well, I think maybe I'll take an initial shot at that. And I think it's important to understand that the Arctic is not done changing, in that the planet’s continuing to change, we're continuing to emit greenhouse gases. And so the papers in this special issue really only represent a snapshot of current acoustic conditions in the Arctic. And really, this research needs to be ongoing, so that we can understand the effects on the ocean acoustics as the Arctic continues to evolve. I mean, this is not a finished process, it's still changing.




Yeah, and I agree, I think there are still many areas of both basic and applied research that can, that will help our understanding of the acoustic energy propagation, which is one of the key areas to understand in continuously changing Arctic environment. And so for example, one of the areas that I think that we need, we need to really do more is what was, you know, said earlier in this discussion is more studies on the sea ice boundaries, because they are, knowing that parameter as input to improve the modeling of acoustic energy propagation is very important. And also, the other area that I think would need to get attention, because things are changing fast, is the advanced signal processing techniques—use of artificial intelligence, like machine learning and deep learning—that could be added tools to study further this, this phenomenon of ocean acoustics in the changing Arctic environment.




I'm not maybe on the other side a little bit now, because I really find the acoustic methodologies very useful for understanding the climate change. For example, the acoustic thermometry that we really can get a measure, perhaps of the heat content of the Arctic, because that even if we have managed to perhaps put up or get some knowledge about the circulation, we don't know exactly how much heat is stored in the Arctic. So, to do acoustic thermometry in combination with ocean models, I think that will be a very good contribution to climate sciences. Also, I think that it is fascinating that we can sit and listen to how the marine mammals are vocalizing and learning more about how they move around in the Arctic, because we don't know so much about that how they migrate or they react on different changes in their environment, where do they go? And then also, it is interesting that passive acoustics can also be used to monitor the or to get better knowledge about the seismic activity in the Arctic—earthquakes, you have perhaps some tsunami-like situations—and I find that passive acoustics can be kind of a window into how the environment is changing in the Arctic. You hear everything.




Well, thank you for taking the time to speak with us today. You've given us a lot to think about with regards to ocean acoustics, climate change and the impacts of the Arctic environment and Arctic research. Have a great day. 


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