The Day After Tomorrow: Truth or fiction?

Hello. Good evening and welcome to both our online and in-person audience. I'm glad you could all join us for tonight's National Science Foundation National Center for Atmospheric Research Explorer Series lecture. My name is Elizabeth Maize and I'm part of the education engagement and early career development team that brings NSF ENCAR's world-class research to you through this series. Each lecture highlights one part of the pivotal earth system science research that happens here. And that research supports priorities like protecting lives and property, strengthening our economy, and safeguarding national security. In tonight's lecture, the day after tomorrow, truth or fiction, we will hear from two scientists from NSF ENCAR's CGD laboratory. They will introduce the Atlantic meridinal overturning circulation or AMOH, share their research based on model simulations, and answer the question we all came to find out. Could the catastrophic events depicted in the 2004 disaster movie, The Day After Tomorrow, actually happen? So, for those joining in person, you will get a chance to ask the questions after the presentation. If you are online, please scroll down to the bottom of the web page you are viewing the um this live event on. There is a Slido window. If you haven't done already, please click on the green join event button where you can then add your questions on the Q&A tab. Guan and Aishu also have a few questions for us. So for both in person and virtual audiences, these questions can be found in Slido. So um on Slido um so there if you were able to um either scan this Q QR code on the screen or navigate to slido.com and enter the hashtag explorer series. Um you will be able to answer those poll questions. Um you can also even if you're in person put in Q&A questions um as you have them and we'll get to them at the end. Um so this event is also being recorded and will be available on the explorer series website along with past events. And with that I would like to formally introduce our two speakers. Dr. Guan Donna Bashulo is a senior scientist and the section head of the oceanography section of NSF Encar's CGD laboratory. His research primarily focuses on understanding the role of oceans in the earth system and the computational modeling of the ocean. Dr. Aishu Hu is a senior scientist of the NSF ENCAR CGD laboratory. His research focuses on the variability and stab stability of the Atlantic meridinal overturning circulation and its impact on earth system variability from the past into the future. All right. So with that I am going to head going to turn it over. >> Okay. Thank you. You can hear me. Okay. Okay. Great. Well, good evening and thank you for your interest on the topic and joining us uh this evening both in person here and also uh online. So we'll try to answer that question. I hope by the end of this uh presentation you will have a better idea about what we are talking about here. So in this presentation we'll try to cover these questions. What is this major Atlantic circulation that people keep talking about? Why really do we care about it? What do the observations, available observations and reconstructions of the circulation system tell us? What do our model simulations tell us with respect to its past behavior and future expectations of the circulation system? How does the particular uh this circulation system possibly change with more heat and freshwater input into the system going into the future? Then we'll finish with a few uh take-home messages. Essentially, our goal here is to convey you what we know about this circulation system. What are unknowns? What are the knowledge gaps? and what we can really tell about its present and past state and what can happen in the future. So I hope you'll get that uh that information at the end of this presentation. So uh starting with the first question, what is this major Atlantic circulation system that was the topic of a sci-fi disaster Hollywood movie back in 2004? This circulation system is uh referred to by many names and these names are used interchangeably both in news media and also uh in scientific literature. You may have heard of thisine circulation. You may have heard of ocean conveyor belt or Atlantic meridional overturning circulation or in short am. All of these represent simplified views of the ocean and I'll show you a movie that is sort of intended to show you that ocean is much more complicated than these simplified uh views even though uh they represent different things in details. We are not going to these terminologies different things in detail. We are not going to get into those details. And in this presentation both Aishu and I primarily use the term Atlantic mironal overturning circulation or amach but that's what our choice at this uh in this presentation so this is the movie on the left side you have New York on the right side you have London and as I said the movie came out about 22 years ago right around Labor Day in May 2004. four and it is by the it's a sci-fi disaster movie. So that'll probably give you a hint as to how much of this movie is truth and how much of this movie uh is a fiction. It is these are these posters are intended to show you that after a collapse of this circulation system uh the uh there are catastrophic events happening one after another and New York and London end up with tons of snow and ice. So we had actually several questions for you. I don't know whether you had a chance to answer them. The first one was have you seen the movie? And if you have seen it uh do you think that the plot rooted in extreme weather events actually is truth representing truth or fiction? So can we switch to slideo answers to see what the uh participants say? the answer side. Actually, there should be some answers there already, right? Yeah. Okay. Coming up. Okay. Going backwards in time. Should we skip this? I don't know. Maybe it's not happening. Shall we continue or Okay. Well, I'll continue. I guess we can come back to those questions later. Okay. Sorry about that. So, one of the problems right away with this movie was they were depicting a literally a light switch. And they were uh measuring some sort of uh temperature or temperatures in the northern North Atlantic. And they were showing some decline in sharp decline in temperatures and then they said oh uh this circulation system is collapsing or is about to collapse and literally within a day or two it collapsed and then you ended up with these all of those things. So we can tell you right away that any changes in amoch based on our present day knowledge based on previous con reconstructions of amach and also our model simulations and present day observations such changes in amok do not occur on those fast time scales. They occur on really slow time scales, multi-deadal, multiple tens of years and centennial time scales. So right away it is essentially uh not true in that sense. Such changes that we don't expect those changes to occur in any case in the near future cannot happen in really short time scales as depicted in the movie. So Aish is going to talk about more details right after uh this movie here and hopefully I can show the can you click on the bottom to show the movie? It's okay. So the as I said uh all of those terminologies that I indicated they represent simplified sort of versions of the ocean circulation just to give you an idea ocean circulation is rather complicated. This is one of our in-house uh model simulations at high resolution. We use community earth system model which we'll talk about in a bit more detail later on. These are showing ocean surface currents colored by temperature. So warmer colors are indicating warmer temperatures and bluish colors dark cooler colors are indicating colder temperatures and you can see the snow cover overland as well. So it is rather complicated circulation system and in this presentation uh we'll be uh uh focusing on the Atlantic uh basin here and I is going to talk more about this thing as well. You may be most of you might be more familiar with this Gulf Stream here. It is uh well my arrow is not exactly pointing but it is off the coast of US East Coast. So with that, I'll pass the microphone over to Aishu and he'll describe some uh circulation patterns and I'll pick it up uh after that. All right, as Gokan just mentioned, the ocean circulation actually complicated. It's not as simple as a switch. You can turn it on or off. Here first I will show a movie from NASA tells you what the AACH is. This side is the Pacific Ocean, Atlantic Ocean, North America and South America. The AAC is a global scale ocean circulation. Look at these white arrows. It moves the surface water warm and saltier into South Pole and North Atlantic. This labyrin and for the north that's the Nautic Sea. When this water reach the sub north atlantic, it cools lose the heat to the overlying atmosphere. It become dense in sink to deps and afterwards we look at this blue arrows. It travels southward in the deeper ocean in from the north pantic into south Atlantic water into the southern ocean drawing the deep circumpolar current. Through this current this water will travel into Indian Ocean and Pacific Ocean. In those ocean this water will change from the deep water up well to the surface become the surface water again. Then this water travels from the Pacific through the Indian through flow into the Indian Ocean and from Indian Ocean through the Africa get back to the Atlantic Ocean again. as the water travels around the world to finish this whole circle of the a mark. To simplify this or show it more clearly, we normally give this kind of schematic figure. In this figure, the red line represent the surface flow and blue line represent the deep flow and purple line represent the bottom flow. Here we can see the surface water travels from the Pacific through the Indonesia through flow into the Atlant Indian Ocean and further turn over the South of Africa into the Atlantic basin and travel northward all the way to the Labor Sea and the Nautic Sea region where this water become dense and sink to depths and travel southward along this blue line all the way into the southern ocean. Then the water will upwell partly from in the southern ocean the other part from the Indian Ocean and the Pacific. If we further zoom in into the Atlantic basin you can see is the warm color represent northward flowing water and the blue color that's a cold deep water flowing southward. As Gokan mentioned the Gulf Stream is traveling along the US east coast northward. It transfer huge amount of water and heat. Actually the Gulf Stream including two part. One part is the water travels all the way to the subpolar North Atlantic. But the other part of water actually just traveling around this subtropical Atlantic basin. This one we call the gyros circulation. And the dischar circulation is driven by the wind surface wind. And this water only travels horizontally. It does not go deep. But this aok related water transport is goes all the way to south and then is sink to so to further simplify this water transport we do a zuno direction summation. When we sum it up because for the gy circulation the northward transporting the water is the same amount as southward. When we sum them together it will cancel out. The left over is the marana flow or we call it a which we can schematic it as this surface water traveling northward into south polantic and sink and flow southward. This is the amach has a very important characteristic. It has a multiple equivalent or stable state under warm condition. This one like the hosene to now the rich hosene is about 11,000 years before present day to to now and this warm state the dense water form in the south pole north atic it can penetrate much deeper into the deeper part of the ocean basin and then flow southward. This blue color arrow represent the dense water form around Antarctic. this uh water only fill in a small portion of the Atlantic basin. But on the other hand, under a colder condition such as last glacial period, which is about 116,000 years of present day to 11,000 years before present day. Under that condition, the dense bot form in Sapo North Atlantic only penetrated to about 2,000 meter depths, which is much shallower compared with the warm condition. And the deeper part of ocean filled with the dense water formed in Antarctic. So it filled half roughly half of the ocean depths ex for both of these warm and cold condition. A mark has another stable state in which for this state there's no dense water formation in the south polantic and the whole Atlantic basin filled by the dense water formed in the around antarctic. So we call this stable state as amach on state and this state as a off state. Theical study suggests that AMA can change or transfer from this amark on state to the off state or from off state to the on state. But this transition from one state to another can be abrupt. That's why we call it a mark keeping behavior. That means once a reach a certain state it can abruptly change from a mark on to off state. That is why in the movie the day after tomorrow he said a can collapse over a matter of days which is not really true. Although AMA has these characteristics but does not means it can change that fast. Here is a preview proxy record suggest what implied a changes in the past. The top panel is the Greenland escar record. The bottom panel is the North Atlantic South polar region Marine Corps record. In the top that's the DTO 18 record. We know for water oxygen it has two major isotope. One mostly is O6 which means the oxygen has eight protons and eight neutrons. For the O8 it has two extra neutrons. That means O8 is heavier than O 16. For water we know is they have two contain two o hydrogen atoms and one oxygen. When the water evaporate at the ocean surface the heavier water will evaporate less and the light water evaporate more. That means when this water vapor reach the Greenland and fall as snow there will be more oxygen 16 water than oxygen 18 water but on the other hand under warmer temperature you will have a more evaporation that means there's have more chance the dirt 18 water can reach rand but under colder condition there will be reduced evaporation there's a less chance for O8 water reach Greenland. That's why from here you can see if the number is more negative like here that represent a colder condition and the less negative that represent a warmer condition. In this figure the time is from 80 before present day to all the way to present day. That means the time traveling this direction. So from here we can see is during the last glacial period there's a multiple this abrupt warm event we call the dans oscar event or do event this warm event has been linked to change of amark that means there's a sudden strengthening of the amark which causing a warming in around Greenland the bottom panel is the digital carbon carbonate. This is a kind of rock abundant on land. And this increase of this abundance of these rocks in this marine car that means there's a large amount of discharge of icebergs. The iceberg can carry this rock from land into the ocean and deposit to the bottom of the ocean. We during the last gl period we have about six of this event. We call it hungry event. When there's a large amount of iceberg discharge into the North Atlantic, that also means there's a lot of fresh water. When it m it can weakens the AMA and during this hungry event, it represent a cold event such as this warm temperature, this warm warmer background and this cool temperature that's a cool background. This cooling has been linked to the abrupt collapse or significant weakening of the amark. That's why it's a hungry event and the deal event that naturally happened event in the past which has been linked to the change of the AAC. Next we'll explain what is the driving force to form this amark. A mark actually is related to the change of the seawater density. The seawater density is related to the seawater temperature and the salt contained in that seawater or we call salinity. So here I want to ask how many people have watched the movie happy. Okay. In that movie during the polar night it's very cold. What do the penguins do? Yeah. They stay together keep themselves warm. And during the summer, the penguin spread out. They're looking for food or they play with each other. The same for the water molecule. When it is cold, the water molecule packed together. That means given a unit volume, there will be more water molecules there. That's why the water become dense or heavier. When temperature is warm, the water molecules stay a given unit volume. the number of molecules is reduced that's why the density is decreased when clear see here the blue line that's the density as the temperature increase this density decrease on the other hand when we talk about the the salinity you can imagine you wear a clothes we full of small pocket you can put a coins dimes quarters into the pocket the more you put in you become come heavier, right? The same for the salt. For the same temperature, if you have more salt in the water, it becomes denser. This one is a set of observed sea surface density. We can see in March in the south Atlantic those color you can see those are a denser waters. And you can imagine what if the surface density is very high and even higher than subsurface density. Or you can imagine such as you have a glass jar, you fill the bottom half with cooking oil and the top half with water. We know the water is heavier than the cooking oil. Once you do that, you will see the water will sink to the bottom of the jar and the cooking oil will go up. The similar thing happened in the ocean. We can demonstrate this from is observed the salinity change in the distribution which is in Atlantic and Pacific Ocean. Here the color is the more brownish color represent the higher salinity is the more bluish color. It's a lower density and lower salinity. And this is the North Atlantic. South Atlantic as the North Pacific and South Pacific. First we can say is at surface in the subtropical and tropical region the salinity is higher the same for the Pacific. This is related to the more intensive evaporation in those region. But in the subsurface in the subpolar region such as the salinity still higher but not the same in the Pac North Pacific and this kind of distribution of the salinity suggests you there's a flow going from the surface like a sinking like here to the depths and flow southward just like these arrows show. Because the surface water is has higher salinity, it gave the chance to raise the density. That's why the water here can sink to depths. Once this water move down, it need the due to the water continuity, it had need the surface water travel to the subpolar region to refill the water sink to depths. And once the water sink to depths, it flow southward. But in the Pacific because the surface dense salinity is so low so the water is much uh is less dense than the subsurface of water that's why the water cannot sink. This is why we have a Atlantic merit circulation but we do not have we do not have the Pacific over circulation or you can imagine this ocean basin like a a bustop at home. You fill the bus with water afterwards you can pour like orange juice or salted water with some color. you power it at one end of the bustop, you will see that water will travel down to the bottom of the bustop and flow towards another end of the bathtub. On the other hand, if you fill the water in the bathtub, but you power the cooking oil at one end of the cook the bathtub, you will see the cooking oil will stay at the surface. This is also explains why there's a overaneous circulation in the Atlantic but there's not a circulation in the Pacific. As we know the AAC is driven by the water density change. So if we want keep make the AMA slower we need to change water density or we can make the surface temperature warmer or make the water fresher. With a warmer temperature, normally it can cause an amount of the and the ice on land and increase the runoff. Therefore, make the surface ocean fresher. It can cause awakening of the AACH. Next, we will explain why we care about the AMA or we care about the change of the AACH. We know that Earth is round. It receives energy from the sun because this is round. So in the tropical subtropical region it it receives more solar energy or the short wave radiation in the polar region it receives less. Another thing we know is uh for any object in this in the universe if the temperature is higher than the temperature or the of space it will radiate it will re release heat towards those lower temperatures place. Earth is also releases heat as a longer radiation going out in the tropics. It basically receives more sunlight or more shorter radiation than the long long radiation it released. So it has a energy surplus in the tropical region. But in the polar region it receives less heat but it is released more. Therefore there's a energy deficit. But we know that the tropical region it did not change become warmer and warmer. Polar region does not become cooler and cooler. Why is that? It's because the analyation and oceanic circulation transport this surplus heat in the tropical region into the polar region. In the ocean, this is the estimated meridian of heat transport. The blue land is global ocean. The purple land, this is the Atlantic and the red line is the Pacific. And the green line, that's the Indian Ocean. In the northern hemisphere, we can see that the heat transported by in the Atlantic Ocean actually is higher than that in the Pacific Ocean. Although the Atlantic Ocean is much narrower than the Pacific Ocean. This is because AAC exists in the AMA in the Atlantic but not in the Pacific. This also means that if this amark change it can significant change this mar transport. Therefore it can generate significant impact to regional and global and weather regimes. The heat transported by the amark it is roughly 100 400 times of the total energy used in the US. That means if we can capture all the heat carried by amach in one year we can have enough energy for you at consume for 400 years. So that's a huge amount because of this we see here is the observed sea surface temperature and this uh e line is about like 40° north. When we compare the Atlantic side and Pacific side, we can find out it's at the same latitude the temperature is higher in the P Atlantic side than Pacific side. This is because the the existence of a mark. Now the question is what if the AAC is absent. We use Ankar NS NSI encar community or system model to do a simulation to see what if a collapse and what kind of surface temperature change we would see. Here we can clearly show in a subpolar North Atlantic it can generate a cooling by more than 10° C or 18 degrees Fahrenheit. And this decrease can have a significant influence to the regional and global and surface temperature and the weather regime. on the other hand is the same such as the border temperature on July 15th the border temperature on average is 90° Fahrenheit but it can be as high as 100 or as low as 70 that means the same day July 15th the temperature is changes from one year to another or we call this the temperature variability and a mark is the same amark is not always has a constant strength. Sometime some years it can be a bit stronger, some years it can be a bit weaker. So this a stronger amark or weaker a mark. It can change the mar transport associated with it. The other thing we are concerning is whether a has a long-term trend either trending become weaker and weaker or it become stronger and stronger. Either way it will affect the marren part related to it. So how this change of the amark strength and associated marran part will affect regional and global weather regime will be further targeted by goka. >> Do you want to do the slidos right now or those two questions? >> Okay. So these are the answers that you gave. So 63% of you saw the movie. Twothirds roughly. I don't know whether it's a good thing or not. And then wow that is a bit surprising. 73% thought that it was real. I'm a bit surprised by that answer but anyway that's what it is. Thank you. So let's move on. Okay. So, uh, Emoch has been in the news for a long time, especially, well, I shouldn't say for a long time, especially over the last, uh, five, maybe a little bit more, uh, years. And going back to, for example, August 2021, these were the headlines at that time. Study warns of irreversible transition in ocean currents that could rapidly freeze entire world or North Atlantic. I guess Atlantic ocean currents weaken, signaling big weather changes. Part of Gulf Stream at risk as Atlantic o as Atlantic ocean currents weaken. Climate change almost completely destabilizes AOK. How worried should you be? That's the title. And you can see more uh why the Atlantic Ocean current system is slowing down and its implications. Destabilizing currents in the Atlantic could have serious impact on your weather. And I find this Cape Cod one is actually more balanced because Cape Cod is also where Woodsol Oceanographic Institution is. It says major Atlantic currents showing signs in it may shut down, but not all scientists agree, which I agree with that one. But you can see that in order to gain your attention and probably they did they are using really sensational titles otherwise nobody would read. If I write something saying that Amok is not collapsing would you read that? Probably not really. And fast forward to today in fact I could have put something from Monday here. It was again in the news. uncertainty about uh weakening Atlantic currents isn't a reason to wait but we need to act now and bunch of others related to a co presence of cold blob in the North Atlantic. The outlook for climate regulating ocean current is not good and a critical Atlantic ocean current shows two decade decades of slowdown and I'll show you that that's not necessarily true and uh there are a bunch of others collapse could change Europe's climate 10 times faster than expected actually the that particular one the article that's based on is not that alarmist it's actually quite modest but titles are more on the sensational side to attract audienc's attention I guess readers attention so uh another set of slider questions I don't know whether you have the answers yet do you follow these news or this these pieces of news and do you think that the amach will collapse within the next we gave some options do we have answers to that or should we wait okay we do so about half you follow the news and next one. Wow, that's another interesting result. Okay, [clears throat] we can come back to these at the end and we can have a better or more extensive discussion. Okay, continuing on. So, as Aisha sort of indicated, changes in amactur temperatures. And as we are trying to tell you here that those changes occur on relatively slow time scales. So one of the uh ways that we sort of uh assess Amox impacts through something called Atlantic multi-deadal variability. What it is is essentially it is an index of sea surface temperatures in the North Atlantic. And this is it is observed time series going back to 1900 all the way to present. You can pay attention to the colored lines there. You can see that in the early 1900s it was actually in it negative phase meaning that sea surface temperatures were colder than their normal state. Then it became in its positive state from mid 1920 well 1925 through 1960 roughly and then again cold state. And now we are supposed to be in a positive uh phase. So you can see that there's some kind of variability in that or oscillation on a 60 to 70 year time scale. And if you look at the corresponding sea surface temperature anomalies, departures from the mean state, this is what you get from observations. You can see that in the Atlantic, you see large temperature differences from the normal state. They are of the order of u four to five or 3 to four degrees Celsius. That's about five 5 to six degrees Fahrenheit. So they are large amplitude relatively large magnitude uh changes in the sea surface temperature and it can impact the earth system. In general, there is actually a warming in the Pacific, but it's much weaker than the Atlantic. And we tend to have colder or cooler temperatures in the southern hemisphere when this AMV in it is positive phase. Next, I'm going to show you a movie from the UK Met Office and it's sort of summarizing the impacts of a am that's these changes are especially during the pre pre-industrial period is directly linked to changes in amok strength. So there is a direct link there. Okay. So we can see the movie now. It's uh in the past maybe up to 10 15 years ago people also call this thing Atlantic multi-deadal oscillation and it's still am or used interchangeably in its positive phase north atlantic has a warm uh temperature state and you'll see that in the it's going to be negative uh cooler temperatures in the south Atlantic in particular. So why do we care? Warmer temperatures in the North Atlantic are directly linked to increased hurricane activity because hurricanes take their energy from warm sea surface temperatures and moisture. It is associated with drought conditions in Brazil, Amazon and extreme rainfall in Sahal and Europe. There's also reductions in sea ice and in fact it can impact all the way to Indian. It has actually higher levels of monsoon or more precipitation in monsoon. Interestingly for this region 1930s dust bowl I don't think that anybody remembers that here in person. Uh the that one was essentially associated with the positive phase of MA amo or am that I indicated earlier. In its negative phase, it's pretty much the opposite impacts. And in the interest of time, I will skip that. And uh just to uh summarize the impacts of the uh AOCH related warming in the North Atlantic. Warm North Atlantic temperatures is linked to increased hurricane activity. Drought conditions like dust bowl over the United States. Increased rainfall over Sahal and also uh drought conditions over Amazon region over Brazil higher precipitation rates over Europe and reduced sea ice. I just wanted to get back to the movie again day after tomorrow. If you recall, there's a huge whatever hurricane, tropical cyclone that's coming from the northern latitudes. As I mentioned, hurricanes or those storms take their their energy. They need warm surface temperatures and more moisture. In colder conditions, they don't exist. So, they cannot really grow. So, that was another uh fiction uh in the uh movie. And that's why warm temperatures in the Atlantic is directly linked to higher or increased hurricane activity in present day conditions. So what do the present day observations and past reconstructions tell about uh tell us about amarrop uh we usually measure amok or when we talk about amok we mention swear drops. It's named after a famous oceanographer who studies who studied this kind of circulation. So one square drop in metric unit is 1 million meter cube per second. In imperial units that's we use in US that's 264 million gallons of water being transported per second. It's a lot of water. Not only heat it's a lot of water transport. So with that much water in one second you can fill 400 Olympic size swimming pools and just to compare it to the Amazon outflow. It Amazon is the largest river in the world by volume. Its transport is only 2/10 of a swear drop. So please keep these numbers in mind when I'm going to show you some numbers that we think what this transport is actually transporting in terms of volume. So it's going to be a big amount. Okay. We have unfortunately very limited observations, continuous observations. We cannot really measure or get observations of amark across the entire Atlantic basin. Instead, we have observations only at certain latitudes and they are shown here schematically and there are very few and they are not long. The longest one goes to like early 2000s 2004. And I'm going to show you a time series from that. And in the interest of time, I'm just going to show focus on the rapid line here. So rapid is not an acronym. It's the name of the program. It stands for monitoring the Atlantic miral overturning circulation to understand climate change. And this is exactly what the transct is across 26 north. They they have uh moings that they measure temperature, salinity and sometimes velocity. Then they have uh they use near surface currents are transport is measured by uh wind stress and there's gulfream contribution as well. Do you know how gulfream is measured? that transport. Uh there is an old telephone cable owned by AT&T and it was in there since 1980s or so. They use that cable because in the presence of earth magnetic field water or seawater has ions. Those ions in the presence of the magnetic field create a voltage between two ends of that cable. One is on Florida side, the other one is on the Bahamas side and that wtage is detectable enough and it is directly correlated with the amount of flow through that straight. So that's how that thing is measured. So this is interesting uh science actually uh how people scientists actually measure these things. So this is what we get from our most direct observational estimates. This is time starting around well 2005 all the way to near present. And you can see now look at the uh magnitude here. That's what I was seeing. We are talking about 17 20 rubs. So you can with that amount of water you can actually fill in about 10,000 swimming pools in one second. So it's a lot of water being transported. The second thing that I wanted to stress here is that as you can see there is quite a bit of variability in this transport on intrannual or year-to-year uh time scales. So people publish results saying that amok is declining which is a true statement if you essentially look at the blue line. Blue line shows the trend passing through all of those points and it is indicating that it is actually amok showing an amok decline of about 0.9 nearly one sw per decade. However, some scientists including myself think that amach started at a when these measurements occurring am was at a high state. If you ignore the first two years and actually do another trend calculation, you get the orange line that's the dash line and the trend diminishes quite significantly to only 3/10en of a swear drop. So with this much much variability and really short observational record, it is really difficult to tell what am prevent some of the publications. So this is a paper nature in 2005. It got a lot of publicity and the title says it all. And in this publication this is showing again emoach strength at 25 north as a function of time from mid 19 well 1955 through 2005. They use those five triangle points only and they were obtained at different times in year in a year. Of course you can get you can say that there's actually a mock is declining but then later on many people actually published other papers saying that there is so much variability oops uh in the system you can't really tell much about what amok is doing just to make the point a little bit uh clearer rapid observations actually provide am transport every 10day intervals this is now in this plot I'm showing you the entire rapid data set from 2004 4 to roughly 2018 10day records. So you can imagine now look at the transport change from 0 to 30 square drop. If you take a measurement here when this thing was high here and if you take another one right here or next time right there you would have concluded that amok was declining or collapsing. But if you take one measurement here another one here you would have said am is not changing. So these things are extremely difficult to tell. one way or another with this much variability in the natural system and also uh with short observational uh record. So we also look at other observations instead of these transbasin full observations we also transport estimates elsewhere and I'm going to show you two time series from the Nordic sea region Norwegian sea and also overflows. The top is northward flow in the Norwegian Sea region uh up up here and then the bottom is showing southward flow coming out of the overflows Denmark straight and ferro bank uh channel again quite a lot of variability these records go back to 1994 if anything these are actually showing some slight increase in AOC strength during the recent uh period so there's quite a bit of uncertainty in these estimates And we I mean we can say that no discernable slowdown in observations of the North Atlantic Ocean circulation. There are I mean of course this is not a clear-cut issue. There are other studies and I'm going to show you uh one here paleo reconstructions. This is going back to year 400 and all the way to present. In this study, they use relationships between AMO strength, sea surface temperatures that I showed with the AMV Atlantic multi-deadal variability and certain proxy records. And one of them is for example this kind of thing. These are tree rings. Tree rings are further apart from each other if they are growth favorable conditions. maybe more moisture, more precipitation and they cluster together more when there is the conditions are not favorable for tree growth and you can actually come up with this kind of proxy estimate as shown here. So these are bunch of constructions using also corals or saltable silt data to measure velocities. So what this study says that amok was pretty stable up to year 800 but then it started significantly weaken after 1800 to present day conditions. They can't really tell you what the amach strength is. They can only tell you that amok was weaker or stronger than a given sort of state. Another study contradicts that and this is essentially using inverse techniques using temperature and salinity data sets from the ocean for the 1900 to 2020 uh period and their conclusion is that amoch is not changing at all during that period and this is directly contradicting uh what these constructions are. The thing is that there are so many challenges in this. The data sets are rather sparse. They may not be necessarily reliable and when people make these uh plots there are a bunch of assumptions behind them and they may not be necessarily fully valid and small differences can actually result in uh big uh differences in the results. Okay. What do our model simulations show? And I said that while there are issues maybe in the constructions, there are challenges with the observational record. So models actually serve uh to fill our knowledge gap in many things. Before doing that, I was going to I'm going to just uh tell you what these models are. We call them global earth system models. We use them as the primary tools to understand Earth's past and present weather and longer time scale events. And we also use them to make uh weather and earth system variability uh predictions. We take physical and mathematical equations of flute dynamics. We actually then discretize them. That's what we call discretization. Meaning that we take the earth, divide it into tiny little cubes or cube like rectangular prisms and we solve these equations for each of those. And that's an example there on the upper right. Just to give you an idea because of the computational limits and we need the we need long simulations one grid box one cell is roughly 100 kilometers by 100 km 60 miles by 60 miles roughly. So that is for long simulations and for some of the present computers that's what we do but we need to include if you go outside you'll see a bunch of things land surface clouds and all that stuff all of those things need to be somehow included in our models. What we do mostly we actually represent if you go out there you'll see clouds really small scale much smaller than 60 miles one uh dimension. So we end we end up essentially what we call parameterizing their impacts in the earth system. So that is one of the uh most difficult challenges of earth system modeling. How to best represent our state-of-the-art knowledge of what's going on with the clouds and everything or other physics and incorporate them into uh global uh models. ENCAR has one of these and it is called community earth system model. We actually represent atmosphere, sea ice, land ice, ocean, surface waves, river runoff, land and all the biochemistry in the whole system through a coupler. It's a computer code. It has more than several million lines of it. These are big big uh codes and CSM has been uh in development uh has been developed at NSF ENAR in strong collaboration with both the national and international research and university communities. It provides uh worldleading state-of-the-art computer simulations of the earth's past, present, and future states, including, as I said, uh short-term uh predictions all the way probably up to decal uh time scales. That's a movie. I'm not going to get into detail. It's one of our in-house simulations again showing some details of uh clouds and sea surface temperature and atmospheric uh rivers. This is the schematic that sort of uh equivalent of that schematic Aishu showed earlier. This is what amok looks like in the model. This is a depth here from surface zero to 5,500 m 30 south to 60° north. Warmer colors represent clockwise circulation. So it's essentially near the surface the flow is going northward. There is sinking happening in the northern north atlantic and the dense water flows southward. And there's this area down here that's the region that I already mentioned associated with the u Antarctic bottom water. So we can use models to see what's going to happen or what happened in the past or uh what's happening uh what's going to happen in the future but we need to essentially make sure that our model is actually doing the right thing. We can compare it to available observations to evaluate and validate our models. And I'll show you one example for am as I said earlier am is measured at 26° north. So how is our model doing when we do the simulations and this is the result. The solid line black line is the observations and the other two are showing a low res and high resm simulation. So I think we are doing quite well. This is an emerging property of the system. So we gain confidence in our model uh by doing this kind of comparison. And I can well I'm biased but encarc models in the world. So you may want to ask how other models are doing. Well that's the answer. So this now includes 20 models or so. Observations again shown rapid is the purple line. Our models are shown in the green somewhere nearly on top of the observations and all the other models are all over the place. You may ask why do we need so many models? Well, these models are contributed by many nations. So, it's a national pride to essentially to have an earth system model. But these differences among models actually help un help us to understand what our challenges are in the models. We can actually look at what why some models have low transport or high transport. which processes are actually responsible with that and we can try to improve our models even uh even better better ways. So though that diversity has really uh uses uh for us for scientists. This is now showing uh the same am from 18 different models. Our model is right here an older version CCSM4. So there is quite a bit of uh spread in the structure and strength. These are now showing the time series uh of uh those maximum transports from 1850 to 2010. Even though they look quite different, if you look at it compared to the uh existing observations that I showed, most of these models are actually more or less in agreement with observations. They are within the observational uncertainty. They all show multi-deadal to decal variability. Ankar model is the uh yellow one here. One thing that's clear from these models, we think it's robust. You do not see any weakening or strengthening trend. They are pretty flat during this period. This is a summary of all of those models. This is now showing the AOC strength from 1850 all the way to 2100. And I'll come back to the future uh period in a second. So up to 2010 or so these models use are best estimates of forcing observed forcing. So they are the historical period. Earlier models were shown by the ones that I showed earlier are the light gray line and it was showing again as I said the entire average of all of those models are flat. The newer models tend to show an AOC increase actually those are the black lines. So again this is directly contradicting some of the paleo or previous reconstructions. I think that the newer models are in better agreement with also uh oceanon only models that are forced with our best estimates of atmospheric data sets. They also show a strengthening of AMO in the mid 1980s or 1990s future for all the models. These are the future estimates of amach strength. They all show a weakening of amok and it depends upon the emissions rate. I assume many people are familiar with what emissions are CO2 emissions. So if the they are higher we tend to see weaker AMO but with lower emission rates we can see actually less reduction uh in amach there's quite a bit of uncertainty of course we don't know what the emissions are and there is so much variability in the whole uh system but I think we can confidently say that up to at least year 2100 we do not expect am to collapse it may weaken but I don't think that personally it's going to collapse perhaps in the near future. Okay, we just almost wrapping up. How does the circulation possibly change with more heat and meltwater? As Aishu alluded to, we call these sort of idealized simulations as uh hosing experiments and that's why that hose picture is up here. So how and we simply impose additional fresh water in the North Atlantic and what happens strongly depends upon the amount of fresh water and its uh duration. So again I'll show a set of results from our director's old place UK Met Office and uh uh they did a bunch of experiments using their model. They did uh 0.1 swear drop hosing and their control case is the black line here. Then the hosing experiment is shown by the gray line here. Just to give you an idea about how much do you think that hosing amount equal to in terms of Greenland melt and the answer is here current rate of Greenland melting is about 0.04 square drop. So it's still higher than that rate. And if one uses that rate 0.1 uh you can melt the entire Greenland ice sheet in about a thousand years. It is an idealized simulation but it's still comparable to maybe present day uh rate. And what they did then is essentially when you stop the hosing at year 25 those are different colors at 50 100 and 150 amok recovers. If you do the same experiment with a higher rate of hosing five times more then you do not see the recovery. AOK actually does not recover. So the point here is that how am going to behave in the future depends upon amount of freshwater input to the system also its duration. You may ask are these results robust? Are the models showing the same results? And here's the answer. Uh these are now from same experiments with slightly higher rate of hosing 0.3 from eight different models. CSM is right here. Uh a version of CSM2 our version from five years ago. In each case the uh the black line is the control and then when fresh water is imposed that's the gray line. The thing the main message here is that if hosing stops early on the blue line all of the models are recovering. So that's a robust result. But if the freshwater uh hosing continues up to year 50 uh some models do not recover, some models do recover. So CSM does not and Headley Center low resolution does. At Enkar, we also looked at impacts of warming and model resolution. This is from our in-house simulations. We are not imposing any fresh water, but as Aishi indicated, ocean density is controlled by both freshwater and ocean temperature. In this case, we are essentially increasing ocean temperatures and looking at change in amoch and it's shown at on the left side here y-axis and as a function of simulation year. We do two sets of experiments the same with one low resolution order 100 kilometer that's the 60 mile uh grid dimension. Then we do the same experiment with our high resolution version which is about six miles or so in the ocean one grid. You can see quite contrasting differences. In low res amapsing with increased warming. Uh however at high res am weakens but does not collapse. These are relatively new experiments and we are trying to understand. So when I was telling you that we are going to convey you what we know what we don't know. This is one of the things that we really don't uh we are trying to understand why the high-res is behaving this way compared to low res version. We have some ideas and we are currently essentially trying to understand why certain things are happening differently. I'll just give you a quick hint. We think that high-res model is actually representing the processes that I mentioned better than the low res version. Maybe that's a not a counterintuitive thing but that's we think what's happening. So with that now we can wrap up. Uh just three take-home messages. I hope you got that uh AMO is indeed important for the earth system. Its changes can have really really significant impacts both in water transport and energy transport. There are many challenges, uncertainties and knowledge gaps remaining. And in particular, we need longer observations to be able to definitely definitively say what's happening to AOK at present. And we need also further modeling advancements uh to represent things better than uh what we have right now. And finally, we think that AMO is going to weaken in the future, but by how much is rather difficult to uh predict in the presence of large variability and forcing uncertainties that come with it. So, we can come back to the movie now and I don't know whether you want to change some of your answers now based on uh what we showed you. Hopefully, we did not confuse you too much. We had to get into some uh science details a little bit but we tried to keep it at a relatively sort of hopefully understandable level for you all. So with that we'll thank you all for your time and interest and we'll take questions. [applause] All right. Um so I'm going to pass the mic off to Kyle. There's already one question there, I think. >> Well, thank you. Um, how does how does um the effect that AMO has compare with the effect of what we're doing to the atmosphere and are and I assume they're somehow interconnected. >> Yes. I mean that's why uh we were talking about warmer temperatures and uh I mean that's part of the sort of reason for some of those u uh news articles uh sens what I call sensational warmer temperatures as Aishu indicated can actually uh result in uh smaller amounts of deep water formation and that can result in weaker mark and that has that can result in cooler temperatures in Europe for example but not as in the depicted in the movie maybe Aisha can I mean he has done a lot of work on this thing he can say more >> yeah usually we see is that if we have a background of warming and also waking a mark that will generate a competing effect we ache in support north landing also the west in Europe and with background warming it warm it if we add these two together that means for Europe it will experience less warm if there's a weaken a than without weaken a but for the rest of the world maybe we also experience a bit a little bit less warming but in the southern hemisphere in general we will experience more warming with a weaker amark >> it's complicated. It's a it's a it's a tug tug-of-war between what's happening with warmer temperatures and if AOK weakens at the same time that's be transporting less heat northward and that's going to try to compensate for the warmer temperatures by making the surface colder. It's not [laughter] >> there's one more question here. Okay. >> All right. We're gonna um alternate with a online question real quick. Um so if we could pull up the slideo. So this question is, is it possible for temperatures to drop several degrees per second like they did in the movies eye of the storm scene? And for those of you who have not seen it as recently, um, a little synopsis of this scene is essentially, uh, the idea that, uh, going back to the cold tropical cyclone, which you described are not really a thing. Uh, but in this movie, um, they are. And so essentially the when the eye of the storm passes over an area, the rate at which higher um cold air from the atmosphere is being pulled down is so rapid that temperatures cause people and things to essentially freeze instantly. >> My well do you want to take that question? My immediate answer >> I think this is very good question and u we can think about this if there's hurricane go through the tropical ocean right and along the hurricane track you will see the temperature near the center of the hurricane it drops the ocean time drop by a few degrees in ter of this is it could drop the temperature but not that large That's area that large the smaller region is good but for the larger region it's not. I should have actually pointed out in the one of the movies that we showed that I did not dwell on, there were SSD uh movies and in the Pacific in particular, there was actually a typhoon forming and going uh northward and it it had a very small uh what is it? The cold wake. >> Yeah, >> but it was only colder by a few degrees. It wasn't and it was a it was relatively small. Um what's typically the cycle time for the emoch from like going from you know the north to the south and back up again? >> That's a good question. I can't remember that. I mean uh I think it's several tens of decades. uh because it is the advective time scale of the water parcels from north to south essentially and I computed that one once. It depends on the model simulation because uh if it well it depends on the velocity. I I think I can't give you an exact answer but my guess is that a few uh decades if maybe longer but it's pro well I mean uh right I think it's a few decade I mean it's order maybe less than a century >> travel from the south Atlantic to the southern ocean this takes about 60 or 80 years >> on the other hand for the water is traveling all around the world come back to surface that take normally we think it takes about 1,000 years, but that's why we're not really validated using the model. >> So, it is slow. Everything is slow. >> Yeah, >> there's Yeah. Okay. >> Uh, we're going to go to another online question just to alternate. Um, so the next one question is, how does the weakening or strengthening cycles of the AOCH impact the global weather? I understand it impacts hurricanes, but how about winter weather or monsoon seasons? >> Uh well, from the movie that we showed the hurricane impacts because I mean the actual impacts of AOCH changes can be ordered decades, but if the for example uh North Atlantic temperatures stay warm for a while, then it's associated with increased hurricane activity. uh over Europe the precipitation actually is mostly summertime not necessarily winter time but the drought conditions that can happen that 1930s drought that great drought I think that's that happened that persisted for I don't know how long but it was actually not just one season it actually persisted and for the monsoons we know that for Indian monsoons actually it's increasing precipitation amount uh I don't have a good sense of uh I think Sahel precipitation increases as well. So those are all on weather time scales >> right >> actually there if there's a variation in strength of the AMA it can also affect the intertropical convergent zoom right by moving north southward and it can affect the hot circulation such as related to monsoon system and also it can affect the inter tropical basin interaction through the vo circulation. ation there's a meralo anal circulation basically along the equator >> those circulations that I are me is mentioning there are various circulation patterns in the atmosphere either longitudinal or latitudinal patterns so sea surface temperature changes can impact those atmospheric circulation patterns and they are really large circulation patterns so what happens in Vegas in this case does not stay in Vegas what happens in the [laughter] Atlantic actually impacts other places. So, >> yeah. So, we know that the oceans absorb a lot of carbon and I wonder what the uh AAC uh OMAC am. >> Yeah. >> What um you know variability is going to how it's going to affect that? Is it going to increase it or? >> Well, so as far as I'm not an expert, but as far as I know, solubility of carbon uh is higher in colder surface temperatures. So if the surface temperatures are colder, one would expect higher solubility and more carbon intake by the oceans, right? That's I believe that's what happens. >> Yeah. All right, another online question. If we can go to >> I'm sorry for my cough. I have a lingering cough for the last two, three weeks and that's why it keeps happening. Anyway, >> okay. Okay, the next question is what kind of future work are you and other scientists planning to do to better understand amo and hopefully find some sort of agreement between studies? >> Very very good and loaded question. [laughter] So uh I so based on our recent results I am uh now a sort of more advocate of going towards higher resolution models and we think that at least I think that it's actually representing the processes better and we think that uh in amark behavior or at least I think that representation of certain processes like the meoscale it is in the ocean is quite important in terms of how they transport energy that we talked about and even though our parameterizations are great in fact I'm responsible for implementing one of those parameatizations in the low resolution model 30 whatever 35 years ago I still think that I I and we continuously learn how to do things better. So I think high resolution does not necessarily is the penaceia to solve all of our challenges or fill in all the knowledge gaps. But if we can use high-res, high resolution, low resolution or nominal whatever 60-mile uh models and maybe other idealized process models in a hierarchical way, we can learn from all of those to represent things processes a lot better maybe than what we are doing right now. So I would say that essentially it is a combination of many things improving our understanding learning from other models other we collaborate by the way with many other centers so we don't work in isolation here we collaborate with university community national international we try to do our I mean it's always a learning thing and you can always I guess teach an old dog new tricks or new science so that's what we try to do essentially So I would say improve our understanding and fill of our knowledge gaps. But one thing that I was trying to stress here is that the I think even though they are expensive we need continued observations and that is key to our understanding of nature and also validating our models. We can run our models but if there's no observation to say whether we are doing it the right way or wrong way then what good is it? So I think modeling and observations go hand in hand and theory of course. So >> are changes in the AOCH thought to be causal to the onset of the younger dus or the little ice age in the Renaissance? >> That's specialty here. In general um we think yes during young dress that's a cold event and uh normally we attribute that cooling relate to the significant weakening of the aug but this is knowledge about 10 or 15 years ago and more recently there's a more available paleo proxies which make the attribution to the hungri event and also So this younger DS less um purely due to changes of emark and other processes may also contributed to this cooling event or we say not only emark pos still play a role but it is not only processes contribute to these code events. So at the end of the day, we are going to quiz you how to pronounce Amok and Mary. [clears throat] >> All right, we have another question um that is kind of a good followup to your um the last online question you answered, but what are some of the biggest constraints in terms of developing more accurate models? for example, availability of real world historical data, real world number of real world data collection points, computer computing power, etc. Conversely, what are the greatest opportunities for further refining these models? >> Well, I part partially answered that question, I guess, with the previous one, but I think another thing that we can add is that I used to advocate for more observations we have better it is it's still true but I want to modify that saying that we need also observations that tell us how things work I don't just want a number for am 24 swear drops I want to understand to improve our models as I showed models have quite diverse depictions of amach if we understand the processes that are contri contributing towards differences. Uh that's important from the modeling side. We can essentially say what process is contributing to something but how do we verify that and we need essentially observations more on the process based side. So I think that's one thing that I would uh advocate. Another thing that is that is you as I said high resolution high resolution models are expensive and uh we need to run our models faster and that's why we are trying to essentially take advantage of emerging uh technology. I don't know whether you heard GPUs with the data centers and all that stuff they are based on graphical processing units. So we are trying to essentially adapt our models to run on those because well they don't necessarily well they may run faster but [cough] we can take a take advantage of emerging technologies as we move uh forward and I mean computer power is a big thing and as just observations getting a computer with high power to perform some of these calculations is expensive as well. Yeah. Think in addition we also need the people power. >> People power >> the more um like the model resolution increase we need more people to analyze the model result and al more people to code those models. >> Uh well that reminds me also another challenge that we face in the modeling community. the number of people who are actually developing models is collapsing just like whatever the movie Amok I guess the uh [laughter] there is not much fame and fortune in model development one can write papers and get those publications out a lot quicker than developing models and there's less interest in model development unfortunately and that's true particularly for ocean modeling and we always joke we should not take the same plane going somewhere. It's [laughter] literally worldwide. It is uh I mean you can pretty much count the number of ocean model developers in the world. >> It's a bit better on the atmospheric side I think. But anyway, that's my beef. I always say [laughter] that >> my question is uh Atlantic multid >> can you be louder or yeah >> Atlantic multidicattleal variability plot you shows warm phase and the cold phases the past warm phases the magnitude is higher compared to the present day. I just want to know why it is and what is the future runs shows in the model. >> Yeah, I mean that's a good question as well. I don't we don't want to get into that much too much detail. AMV for the past climate without the anthropogenic forcing is primarily related to changes in AMO. However, the situation becomes a lot murkier with the uh warming and other stuff going on right now. So that's why I'm not sure how much of that uh change or weaker AMV towards the end of the whatever the last century and the recent period. We know that during that period there are also impacts of certain uh certain warming uh contributions. So I think that may be related to it but I'm not that familiar with that literature. As in general when we derive this AMV index we need to or at least we at first we assume the glomian temperature does not change or we in a a stable climate but in reality is if we look at glumin temperature actually it has this rising trend. >> This one is actually taking it away. >> Yeah. How to remove that trend become very tricky. At first we thought we can remove a linear trend X assuming that trend is constant over time. But when we do that we can see the am variability increase or the partial negative is value increase towards the end of the 20 century or 20 early 21st century. On the other hand, if we remove a nonlinear trend like then we will remove a larger number towards the end of the 20th century and the early 21st century. In that way somehow I think it reduces the amplitude towards the 21st century. That could be one of the reason. >> So actually I wanted to tell you two more things I think related to the movie. So we did not touch upon those things because they were not directly related. One was the um I think there was a really rogue wave of something the single wave about 40 mters or 40 ft that cannot as far as we know happen. I mean the I think the on record the largest wave that they ever observed in that region is about 20 feet or 10 feet and and it comes not just a single wave it's like uh more than that certain things need to I mean certain things need to align well I guess to get a 40 ft thing and then there was another one I think related to I mean that sort of thing can happen with a tsunami perhaps but with the any ice sheet collapse even from Antarctica I think that was another thing in the other land blocks sort of continents will not permit that uh sort of that kind of wave to reach all the way to London or not London New York I guess. So, that was another uh thing. I guess they're trying to make money, right? That's the [laughter] >> All right. Well, on that note, if you have more questions, perhaps um you'll be able to stick around and and >> Yeah, sure. I mean, I can stand >> uh directly. Um otherwise, I want to say thank you all again for attending this lecture series in our explorer series. And if you are interested in more explorer series events, please check out our website for upcoming events and to view recordings of past events. Um we have a lot of upcoming events in August on wildfire and space weather. Um understanding more about floods and models and management, air quality, smoke. Um uh so please check out um on our explore series website to learn more and if you are 18 years or older please take a moment to fill out our 3 to five minute anonymous survey to help us better understand the impact of the program and how we can improve our event and it will close next Monday. Um, and I know I learned a lot that I probably questions I didn't even know to have 20 years ago when watching this movie. Um, but a lot of questions answered today. So, let's give a round of applause um, again for our speakers. >> Thank you. [applause and cheering] Heat up here.