NASA Physical Oceanographer Anastasia Romanou Leads a Group
at GISS Studying the Interactions Between Earth’s Oceans and Climate


To honor World Ocean Month and discover how NASA scientists are helping us better understand the world’s oceans in a changing climate, the NASA Center for Climate Simulation (NCCS) interviewed Anastasia Romanou, a NASA scientist who has for decades developed and used some of the most advanced models of the complex Earth system.


Anastasia Romanou is a GISS physical oceanographer. Photo by Jay Friedlander, NASA Goddard Space Flight Center.

Current Role: I work at the NASA Goddard Institute for Space Studies (GISS) in New York City. I am interested in studying the ocean circulation, how stable it is under climate change, what are the heat and carbon exchanges between the ocean and the atmosphere, and the change of climate extremes with climate change. I am a civil servant and lead a small but energetic group at GISS looking at different aspects of the interactions between Earth’s oceans and climate.

Career Path to NASA: I attended college at the University of Athens (UofA), where I completed a Bachelor of Science in Physics and a Master of Science in Oceanography. I then did a Ph.D. in Physical Oceanography and Geophysical Fluid Dynamics at Florida State University (FSU) in Tallahassee. After a postdoc at Los Alamos National Laboratory, and another one at Courant Institute of Mathematical Science at New York University, I came to GISS as a research scientist.

When I first arrived here, I knew very little about what climate models are and how to think about the Earth system as a whole. Over time, my research perspective and understanding of the Earth system has become much deeper and more holistic. I have learned that the oceans regulate Earth’s climate via large-scale currents that transport heat from the equator to the poles and that they control many extreme weather phenomena.

During my career at GISS, I have worked on diverse projects, from air-sea interaction, to model development and evaluation, to carbon cycle research and now to extreme events, tipping points, and the reversibility of climate change.

Is your work done in a lab or in the field, or both? Which do you enjoy most? My work is done entirely from my office in New York City, on computers at GISS and remotely at NASA Goddard Space Flight Center. I use complex numerical models to simulate the Earth system, that is, the Earth’s atmosphere, oceans, land, ice, marine life, and more. This is what I enjoy most: being able to use mathematical equations and computers to try to recreate aspects of the real world, which are indeed very close to how the real world actually is! Comparing our model output with actual field observations and finding them comparable is very satisfying and indeed gratifying.

Left: A view of the historic, neoclassic building designed in 1841 by Danish architect Christian Hansen, the original main building at the National and Kapodistrian University of Athens [Ethnikó ke Kapodistriakó Panepistímio Athinón (NKU)], a.k.a. the University of Athens (UofA). Photo by Alexander Savin, CC BY-SA 3.0, via Wikimedia. Right: A view of the Main entrance to Dodd Hall, built in 1925, at Florida State University in Tallahassee. Dodd Hall was the location of Florida State's library until 1956. Rendered in gold leaf is the phrase, "The half of knowledge is to know where to find knowledge." Photo by James Roberts, CC BY-SA 3.0, via Wikimedia.

I enjoy my work a lot. I like using mathematical models to simulate ocean currents and exchanges with the atmosphere, the land, and the ice. It astounds me that we know so much and at the same time so little, yet we keep learning more and making our models better and furthering our understanding of the world. I love my job at NASA GISS because it is much more than just work for me. I learn a lot about how nature works and how science is done and also about how policy utilizes scientific knowledge and how public opinion perceives the results of our research. I travel and meet interesting people in conferences and research campaigns.

What is the overall focus of your research? I am interested in how ocean circulation and physical and biogeochemical interactions with the rest of the Earth system change over time and how climate change can affect the oceans in small and big ways. Recent climate change is anthropogenic, caused mainly by human actions. Only a small part is due to natural variations, which, for the most part, are reversible. The ocean produces 50% of the oxygen on Earth and absorbs 90% of the heat and about 25% of the carbon added to the atmosphere through fossil fuel use and land use change.

This oceanic sink of heat and carbon – a major focus of my research – is maintained by large-scale ocean circulation, those vibrant currents which include the Gulf Stream, the Kuroshio, the Antarctic Circumpolar current, and others. Some of these currents flow northward, carrying heat and carbon that sink to greater depths at high latitudes and are stored in the deep ocean for centuries. If those currents and those sinks are not maintained as we go into a much warmer world, global warming will advance faster with detrimental consequences to all of Earth’s natural systems. Nobody really knows whether this complex system of oceanic circulations is prone to collapse abruptly as some paleoclimate studies suggest. We do not know how close we can be in a possible tipping point or what the broader impacts on Earth’s climate can be in that case.

The two scientific visualizations in the video above show the flows of ocean currents, first at sea level, then at 2,000 meters below sea level. Notice some of the differences in speed and direction of the flows between those at sea level and those at 2,000 meters below sea level. For example, the Gulf Stream off the coast of Eastern North America flows strongly towards the northeast, while at 2,000 meters below sea level, the flow is weaker and in the opposite direction. Visualizations by Greg Shirah, NASA Scientific Visualization Studio (SVS).

Where did your passion for oceans come from? I was born and grew up in Athens, Greece. My other hometown is, of course, New York City, which I love dearly. That’s where I have lived and worked for the last 20 years, between the Hudson River and the North Atlantic Ocean.

Growing up along the west coast of the Aegean Sea, I always appreciated all that the sea gives us, the nutritious food, marvelous travel, recreational opportunities, and tranquility. Yet, as a child, I never knew that the oceans are important in other ways for the planetary balance and the health of the ecosystems.

I became interested in studying the oceans during my undergraduate studies at the UofA in Greece, where I studied physics. Towards the end of my studies and while considering different career paths, I came across a master's program in oceanography that included physical, biological, chemical, and geological oceanography. This was a great combination of science and real-world phenomena with my love for the sea. It was rather serendipitous that I got into this type of research, but it grew on me. And since then, I have been very fortunate because climate science came to the forefront of science due to climate change and, more recently, oceans gained even more recognition because of their critical role in Earth’s climate.

Starting in college in Athens, and later, along the Gulf Coast in Tallahassee, Florida, where I completed my Ph.D., and during my many years of study, I have learned that the oceans encompass many complex phenomena. Ocean currents, waves, eddies, tides, turbulence, and exchanges with the atmosphere contain the chemical and geological records of Earth’s history. Oceans nurture thousands of species, from the microscopic phytoplankton to the ultra-big fish and sea mammals.

I enjoy the fact that the ocean has a very complex physical science. We may see calm water at the surface, yet there is a plethora of phenomena that we may not see, exchanges and interactions at all depths, along with complex biological and chemical interactions and a massive number of ecosystems and species that live in Earth’s oceans. The seas span very different scales from small lagoons, to bays and estuaries, to larger basins, to oceans. Tropical waters are very different from Arctic waters and certainly different from the vast Southern Ocean.

The map above shows the three Nordic Seas (the Greenland Sea, Norwegian Sea, and Iceland Sea) and subpolar basins with schematic circulation of surface currents (solid curves) and deep currents (dashed curves) that form a portion of the Atlantic Meridional Overturning Circulation (AMOC). Colors of curves indicate approximate temperatures. Map by R. Curry, Woods Hole Oceanographic Institution/Science/USGCRP, CC BY-SA 3.0, via Wikimedia.

Can you talk about some of your most recent research for understanding the role of oceans in climate change? My colleagues and I published a very exciting paper in May 2023 in the Journal of Climate that followed a long and painstaking investigation into a set of simulations conducted in recent years at GISS. All simulations in this set represent the same climate state, or, in more technical terms, they all evolve under the same climate forcing (the same amount of greenhouse gases and other external forcings). However, we found that the large-scale oceanic circulation and, in particular, the Atlantic Meridional Overturning Circulation (AMOC), can abruptly diverge in its behavior due to intrinsic climate variability and non-linear feedbacks. This divergence occurs after we have crossed the 2-degree warming threshold, which is known to be a threshold for potentially large and catastrophic instabilities, or tipping points, in the Earth System.

The ocean’s overturning circulation has weakened substantially, and even though we start to reduce greenhouse warming, in 2 out of 10 simulations, the ocean’s overturning circulation will collapse completely. In these two simulations, the entire North Atlantic and surrounding continents freeze for hundreds of years. These results show that we need to curb greenhouse gas emissions and global warming early, before we reach a point where there is a good chance that there will be detrimental and long-lasting effects on the planet and all the species that live on it.


This paper is a collaborative effort on a phenomenon that is very important for climate – the overturning circulation in the Atlantic Ocean, also called the Atlantic Meridional Overturning Circulation (AMOC). This circulation is responsible for taking up heat and carbon from the atmosphere and storing it in the deep ocean. The deep branch of the AMOC will resurface eventually: it takes hundreds to thousands of years for deep water to come back up to the surface, release the stored heat and carbon, and continue on the same conveyor belt-like journey.

This circulation is very important because it moderates the global climate. However, what we are seeing is that if we continue warming the climate, this overturning circulation will slow down and maybe weaken, or even collapse, and that's what we see in our simulations.

This is something all climate models show: if you keep warming the planet to a certain point, the overturning circulation will collapse. However, in our case, we are seeing that there is a good chance (~20%) for this to happen earlier than previously thought. Why is there such a degree of chance?

As we have discovered, some years we might have an El Niño developing in the Pacific Ocean, which generally warms the planet, or sometimes we have a La Niña year, which has the opposite effect, with the climate becoming a bit colder. This natural variability of the Earth’s climate can interact with greenhouse warming and lead to higher chances of unexpected tipping points, such as the collapse of the AMOC.

We know from paleoclimate records that whenever AMOC collapsed during the Earth’s geological past, there were profound impacts for the entire globe. Several studies have pointed out that AMOC collapse will lead to severe winters in the Northern Hemisphere and losses in agricultural output and in ecosystems. This is the first time that AMOC collapse has been triggered by internal variability in a numerical simulation of a plausible future climate, so we are interested to explore this phenomenon even further. We expect that this result will be of interest to the rest of the scientific community as well.





How have NASA’s advanced computing resources helped you and your team conduct and accelerate your research? My work is based entirely on NASA computer resources. All my simulations are done on supercomputers like the NASA Center for Climate Simulation (NCCS) Discover supercomputer, which is also where all the data storage is taking place. I also use graphics and analysis software that NCCS provides.

I rely heavily on the IT team at NCCS who work tirelessly to maintain a high standard of infrastructure and resources and support many scientists like myself. I have come to know many of the IT team very well through remote interactions, and despite the fact that I have never met them in person!

What inspires you? During my teenage years, I was inspired by some amazing teachers in math and science in my hometown in Athens, Greece. I learned that physics helps us explain what happens in the world around us, how things work, and how to best try to predict the future. I think the scientific approach to understanding the world is a great foundation for dealing with everything in life. As a research scientist, I find that people inspire me: my colleagues, other researchers in my field, men and women in completely different areas of science and technology or the liberal arts who have a genuine enthusiasm for learning, understanding, and exploring. I always find that passion irresistible, and it makes me want to be even more enthusiastic and productive myself.

What does World Ocean Month mean to you? I appreciate that June is celebrated as World Ocean Month each year, and that the United Nations celebrates World Oceans Day each year on June 8 as well. It is important to take this opportunity to highlight the role of the oceans in our everyday life and in the health of the planet.

Any final thoughts? I would like people to think more about studying, exploring, and protecting the oceans. What would happen if the oceans changed irreversibly due to pollution, overfishing, climate change and other human activities? Life would be very different, not just along the coasts and the margins of continents but also inland, far away from the sea.

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