Heather: ENSO and the IOD are definitely related to one another. ENSO is related to the Walker circulation in the tropical Pacific. And the Indian Ocean Dipole (or IOD) in the Indian Ocean is related to a Walker-type circulation and depending on the phase of both they can either act destructively or constructively with one another.

So you can think of these Walker-type circulations is sort of gears in the atmosphere sort of gears of air-sea interactions and depending on the phase of each those gears can either work together and sort of mesh together and reinforce one another or they can work against each other and sort of break down a little bit.

So, for example, a positive Indian Ocean Dipole will have increased precipitation over the Arabian Sea and decreased precipitation and subsidence over the Maritime Continent and at the same time if you have El Niño conditions you also have more descending air over the Maritime Continent. So those gears will both be sort of working together a little bit.

Then, conversely, if you have a negative Indian Ocean Dipole you'll have an enhancement of normal conditions, so it's similar to a La Niña phase and that is sort of an enhancement of what our neutral phase is. So, for the negative Indian Ocean Dipole, you'll have increased precipitation over the Maritime Continent and decreased precipitation over the Arabian Sea.

Then for La Niña again, you'll have increased precipitation of Maritime Continent and you'll have that cold tongue and decreased precipitation over the eastern Pacific.

So in that case, you'll have those gears again meshing together. You'll have a lot more precipitation over Maritime Continent in the Bay of Bengal in that region of the Indian Ocean. All of that works together with the Indian monsoon and because it's affecting the Walker-type circulation in the tropical Indian Ocean that's affecting wind stress perturbations in the equatorial Indian Ocean. And that impacts all of the Rossby waves and Kelvin waves in the Indian Ocean which then, in turn, impacts fresh water transport through coastal Kelvin waves in the Bay of Bengal into the southeastern Arabian Sea, you can actually impact the timing and strength of monsoon onset.

So there are a lot of different ways that ENSO the IOD work together, but mainly see how that's going to impact the monsoon itself is through rainfall distribution. So, where that rainfall is whether it's over the ocean or over India or parts of Southeast Asia depending on the phase of the IOD and ENSO then how those Walker-type circulations are interacting that's going to impact where rainfall is and then also it's going to impact these planetary waves not all be a little bit later so that we'll have a delayed effect where it's going to impact freshwater transports and the western boundary current in the Bay of Bengal.

Heather: So, the IOD actually has a pretty profound impact on monsoon rainfall distribution spatially and it also has a really strong impact on the northern Indian Ocean circulation overall. In terms of rainfall, a positive Indian Ocean Dipole is going to have more precipitation over the Arabian Sea and the eastern Indian Ocean overall and that's because of the impact the SST (sea surface temperature) differences and the Walker type circulation where you have ascending air and convection over the Arabian Sea and then descending air over the maritime continent.

During a positive Indian Ocean dipole, you'll also have a suppressed freshwater flux signature coming out of the Bay of Bengal. So because of the direction of that Walker type circulation you'll have a really weak wind stress perturbation over the equatorial Indian Ocean. So that's going to cause weakening Kelvin waves in the equatorial Indian Ocean and also weakening coastal Kelvin waves in the Bay of Bengal.

There's four main coastal Kelvin waves in the Bay of Bengal, but the most important one is the second downwelling Kelvin wave which is characterized by positive sea surface and sea level anomalies so that second downwelling Kelvin wave, because of its timing, it really starts around October through December and it will arrive in the Southeastern Arabian Sea around in the winter, so January and February and that's going to be what transports all the freshwater from the Bay of Bengal into the southeastern Arabian Sea and creates the conditions necessary for monsoon onset later on in June.

So, during a positive Indian Ocean dipole because you have that suppressed coastal Kelvin wave response, you also have less freshwater leaving the Bay of Bengal and entering the southeastern Arabian Sea. In climatology, you should actually have a delayed monsoon onset. So, for example, 2019 had a really strong positive Indian Ocean Dipole. We had a really strong monsoon also, but we had really weak freshwater fluxes leaving the Bay of Bengal and entering the southeastern Arabian Sea.

Heather: Now, a negative IOD is really the reverse of this. So, a negative IOD similar to a La Niña is really an enhancement of neutral conditions to a lot of precipitation of the Maritime Continent and a lot of convection and rising air over there. And then in the Arabian Sea, which is already evaporation dominated, you have a lot of descending air and you've much drier conditions over there. So, during a negative Indian Ocean Dipole you have a much stronger wind stress perturbation over the equator. That's going to cause really strong Kelvin waves and also a lot of freshwater flux leaving the Bay of Bengal and being transported into the southeast Arabian Sea. So in climatology, you'll actually have an earlier Southwest Monsoon onset.

The IOD can also impact eddying In the Bay of Bengal, so these coastal Kelvin waves... depending on the strength and timing of these IOD events and whether it's a positive or negative IOD... again, those impact those coastal Kelvin waves. Those coastal Kelvin waves radiate Rossby waves and those Rossby waves excite eddying in the western Bay of Bengal near the East India Coastal Current region. So, during a positive Indian Ocean Dipole – immediately following it, rather – you will have a suppressed eddying signature there versus after a negative IOD, you'll have a lot more eddying there. These eddies can actually be associated with a lot of atmospheric convection. So, that will sort of feedback into the next monsoon season. If you have a really strong IOD in say, 2019... and then in sort of March through May of 2020 you'll have a suppressed eddy field and suppressed convection over that region of the Bay of Bengal, just for example. So that's just a few ways that the IOD can affect monsoon precipitation and Indian Ocean dynamics.

Heather: The Indian Ocean Dipole interacts with ENSO and there's actually some debate within the community as to which one causes which. I know people get really antsy if you start saying that the Indian Ocean Dipole causes ENSO or ENSO causes the Indian Ocean Dipole, but because it does impact the Southwest Monsoon and because impact ENSO and interact with both of these systems, it actually has really far-reaching consequences.

So in addition to if you say that it's associated with ENSO, so then it sort of gets piggybacked onto whatever ENSO is doing in terms of impacting weather globally. But in terms of just impacting the Southwest Monsoon overall, the Southwest Monsoon is the greatest contribution to diabatic heating in the atmosphere. So the effects of that are pretty far-reaching overall, so if you're modulating that, then you're impacting global heat overall. So I'd say the impact of the IOD really shouldn't be ignored. I know there's a huge focus on modeling ENSO but I would like to see more of an emphasis on the IOD because I feel like it's often overlooked, but it's also a really important dynamical system that really shouldn't be overlooked.

Heather: So I would say that it does make predicting the IOD feel a little bit more urgent, especially with global climate change and rising sea levels. I think the threats to coastal communities and the uncertainty with how the southwest Pacific is going to respond to climate change is a really strong motivator for understanding all the processes that really impact coastal communities like coastal Kelvin waves, eddying tropical systems and also the Southwest Monsoon, which impacts well over a billion people and the global economy overall. So any process that impacts all of those different things is necessarily really important.

So, because the IOD impacts all of these different things, it's really important that we model it really well. There have been... a lot of efforts to start modeling the IOD really well. So JAMSTEC has done a lot of work on that and there are a lot of studies now working on really improving studying of IOD asymmetry. So why do we have more strong events of one type... why do we have more positive IOD events or more negative IOD events and things like that. And why do we have more strong events or more weak events? There have been a lot of studies looking at modeling that. And I think there is starting to be a lot more focus on the IOD instead of ENSO which, obviously there's still a lot of work on ENSO, but I think there's been a lot of bit more attention on the IOD. I think it does feel a lot more urgent, especially with global climate change that we really nail down how we can predict these things. So while forecasting still isn't great, there's been a ton of monitoring done. So, the one I know best is the Australian Bureau of Meteorology. They have a really nice IOD monitoring website that has really nice diagrams and time series that are ongoing and I believe are updated weekly. So, there's a lot of monitoring going on and I think there's a lot of research going into forecasting, as well.

Heather: So, there have been a lot of studies that have looked into how climate change could affect the IOD. Because climate change is sort of a hot-button issue right now, there have been a lot of studies by Cai et al. that have been published in Nature that have looked at modeling the IOD and how that should change in different Intergovernmental Panel on Climate Change (IPCC) predictions in different greenhouse gas warming situations.

The general result of these that I've seen so far has been that there's predicted to be an increased frequency and positive IOD events. Like in 2019, we had this anomalously strong positive IOD event. Though we can't directly contribute that to climate change by any means, statistically what we're seeing in these models is that positive IOD events could be up to three times as likely in the future. So, we're going to see more positive IOD events in the future, moving from having one every 17 years to possibly every 6 years in the future.

And what we're also going to see because of this global climate change and the warming oceans, mean conditions are going to be shifting more towards a positive ID state. So right now, our mean neutral conditions are more similar to a negative IOD where you have more precipitation over the Maritime Continent and less precipitation over the Arabian Sea. But because of the changing climate, we're actually getting a shift where neutral conditions are appearing a little bit more like positive IOD conditions and that's projected to continue to change in the future. So that's sort of their understanding of why we would get more positive IOD events. So that's going to have a lot of far-reaching consequences for patterns of precipitation, monsoon dynamics overall, and possibly global climate, which obviously global climate change is going to impact global climate, but the changing IOD will also have far-reaching impacts, as well. So yes, it's definitely a very interesting topic and there's a lot of studies that are looking at this now.

Heather: My vision for the future of IOD prediction and satellite oceanography overall is that we'll have a lot of better satellite observations. So, with the upcoming Surface Water and Ocean Topography (SWOT) project from NASA, we'll have a lot of really high-resolution coastal observations. So, that'll help with things like coastal Kelvin wave observations and eddy observations. In terms of predicting and understanding the effects of the Indian Ocean Dipole, we will really be able to understand how that impacts the ocean, especially in places like the coastal Bay of Bengal.

In terms of predicting the Indian Ocean Dipole, where I see the future going is better assimilation of these high-resolution observations. For example, we've written a paper recently on high-frequency synoptic oscillations that are 3- to 7-day oscillations in the Indian Ocean overall. They're really interesting phenomena but the issue is that because of the time between observations in the satellites – and because of the footprint of these satellites – we really can't capture the structure and temporal resolution of these 3- to 7-day observations. So, if we have more satellites – I know this is sort of optimistic from my side of things in an ideal world... in the future – we'll have more satellites observing the Indian Ocean and oceans in general to better understand the Indian Ocean Dipole and will be higher temporal resolution as well as spatial resolution.

And then where forecasting of these events, not just monitoring them, really comes into play is then assimilating all of that data. So right now, this is sort of my plug for assimilating satellite salinity data, right now I do not know of any operational forecast models that actually assimilate satellite salinity data. So, pretty much everyone assimilates altimetry and sea surface temperature. To my knowledge, no one assimilates salinity. They do like a climatological situation with river runoff, but no one is assimilating satellite salinity data... and that's not great, especially if you're doing something like 3- to 7-day oscillations and you have a tropical storm coming through – or something like that – where the salinity response is really rapid and then the river runoff is obviously going to change... not just with climatology, it's going to be affected as well.

We've done a few studies on that but where I really see improving IOD and intraseasonal oscillation prediction, as well... where I see all of that moving in the future is the assimilation of satellite salinity data to really improve our forecasts and then also higher resolution both temporal and spatial satellite observations. And hopefully more satellites because it would be nice to have more salinity satellites and more scatterometers. That's my plug for more satellites and model assimilation.

Subra: The Argo program really started in 2000, immediately after my Ph.D. when I came to Florida State University, but we reached 3,000 floats by 2007. So, that is the where we started using the Argo float data or gridded Argo data. Everyone got an interest on the salinity because the temperature we have from the AVHRR from 1975-78, we have the temperature data, but getting the salinity data was very tough because that's the only way you used to get from Argo at the surface salinity... even though that is not exactly at the skin, it is at 5 meters depth. Also, we widely used the Argo data for monsoon studies and also El Niño and IOD studies and all those things.

After 2009, we got SMOS then followed by Aquarius and SMAP salinity. They really improved our knowledge of the salinity, and we widely use it now for the monsoon studies. In our lab, me and my students, we work a lot with the satellite-derived salinity. So right now, we are using the satellite-derived salinity and how we could use it for the 3- to 7-day synoptic oscillations, you know, for the Indian monsoon studies... that is a big change. If you're looking from the last decade, at that time we don't have that many studies using the salinity.

Heather: My name is Heather Roman-Stork. I'm originally from Simsbury, Connecticut. So, I did my undergraduate at Boston College in Environmental Geoscience where I actually did my undergraduate thesis on paleoclimate. So, I reconstructed 4,500 years of paleo rainfall in Puerto Rico, which has nothing to do with anything I'm doing now, but it got me really interested in the tropics, in climatology and meteorology.

Then after I graduated in 2016, I went to Florida State University and did my Master's in Meteorology with Dr. Mark Bourassa. I used scatterometer winds to look at the 10-20-day, 3-7-day intraseasonal oscillation in the Indian Ocean and I really loved tropical meteorology. So, I took a ton of classes at Florida State. Obviously, because it's in Florida... lots of tropical meteorology classes. We also had multiple hurricanes come through while I was there. I decided I did not want to study the things that we're trying to kill me and turn off my power.

I really liked studying the monsoon. I took a whole class that was basically on ENSO and IOD dynamics in Monsoon Dynamics and it was like the coolest class I ever took at Florida State University. So there's a plug for that class. After taking that class and after doing my Master's thesis, I really wanted to study the monsoon, which is where Subra and my Ph.D. comes into play. Subra had a really awesome opportunity to work in his lab as a Ph.D. student. So, I moved from Florida State University in 2018 after I got my Master's to University of South Carolina, which is where I am now. I'm finishing up my Ph.D. on ocean-atmosphere interactions during intraseasonal oscillations in the northern Indian Ocean. I really love my research here. So, my Ph.D. will be in Marine Science with a concentration in Physical Oceanography within the satellite oceanography lab, which is a mouthful. But, basically, it's satellite oceanography and physical oceanography and meteorology all sort of combined into one. So, that's sort of my journey to where I am now. And then once I graduate in December (2020), I will be starting a job at NOAA STAR as a contractor with GST in the altimetry division. I'm really excited.

Awards: National Association of Geoscience Teachers Outstanding Teaching Assistant Award; NASA/South Carolina Space Grant Graduate Fellowship; Breakthrough Graduate Scholar, University of South Carolina; and 2020 Outstanding Publication Award in Marine Science

Subra: So, I started my Ph.D. in 1995 at Southampton in England with Ian Robinson. The TOPEX/Poseidon was launched... so that is the beginning of getting a good altimetry data. In 1997-98 we had a strong El Niño, and also at the same time had a strong IOD. Then people who were interested, they started looking at what happened... why we have a strong IOD. Luckily, we had the altimetry data at the time, so we were looking at the connection between the El Niño and this IOD in 1997- 1998. Saji et al. published a paper, then Webster et al. had a following paper sparked the interest in the IOD. But really, that is the time we had also a scatterometer wind scan. Unfortunately, we didn't have satellite-derived salinity, but we had the altimetry data to understand the IOD dynamics and all those things. That's where I started my interest. Also, there was a very mild El Niño in 1994 and also we had a mild IOD... but, you know, not many people have noticed that until we noticed the 1997-98 El Niño and IOD.