A Scientist's Life: Paola Cessi

Paola Cessi is a professor of oceanography at Scripps Institution of Oceanography at UC San Diego. She received her laurea, equivalent to a bachelor’s degree, in physics at the University of Bologna in 1982 and her PhD from a joint Massachusetts Institute of Technology (MIT)/Woods Hole Oceanographic Institution (WHOI) program in Boston, Mass. in 1988. She joined Scripps Oceanography as a postdoctoral researcher that same year. 

explorations now: What do you do for a living?

Paola Cessi: I'm a physical oceanographer interested in large-scale ocean circulation and its contribution to the climate of the earth. My interests are mostly in the large-scale components of the circulation, large-scale currents, and how they transport heat, salt, carbon, and other tracers around the ocean on long timescales, longer than five to 10 years.

en: What are some of the tools you use in your research?

PC: My tools are paper and pencil equations, but also computer simulations and models. When the equations become a bit too complicated and you can't solve them with pencil and paper on the board, you have to use a computer and I do. These days, we have so much data and this information is organized using computer models of ocean circulation. That’s the best way to handle big data. So I use computers ranging from my little laptop to supercomputers.

An example is a recent computation where we released virtual parcels of water in a model that assimilates lots of observations. These billions of observations include data from Argo floats and satellite data. These observations are incorporated into a model of the general circulation of the ocean. The advantage of putting everything into a model is that you can represent velocity on a regular latitude-longitude-depth grid and you can move virtual floats around using a velocity constrained by Argo and satellite observations. Although the dataset is only 25 years long, you can repeat it over and over again and regard this as a hypothesis of the long-term motion of fluid parcels. These calculations were done using about 100,000 parcels and the computer took them on a tour of the ocean for a little over 8,000 years. This required about three or four months of computation on a big supercomputer. So you can see that supercomputers are essential for this type of work – we are very grateful to the National Science Foundation and their XSEDE supercomputer facility.

Our computations showed where the Atlantic Meridional Overturning Circulation (AMOC) expands in the global oceans and how long this grand tour of the ocean takes. The parcels started in the equatorial Atlantic. We followed them through the abyssal depths of the Pacific Ocean and all the way around the Indian ocean and then back into the Atlantic in a tour that took an average of 4,000 years. Some particles skip going into the deep abyssal Pacific and stay in the upper Pacific, where the currents are fast. These quick guys take about 300 years to complete their tour. On the other hand, there are a bunch of slowpokes that take a very long time. These slowpokes get stuck in the abyssal Pacific, where velocities are very small.

When these particles move, they carry ocean properties such as temperature, salinity, carbon. About half of the 100,000 parcels are slowpokes that go into the abyssal Pacific. So only half of the parcels are able to hide carbon from the atmosphere for a long time. Another thing that we found is that this circulation brings salt into the Atlantic. That implies that there's what we call a positive feedback on the AMOC. That’s a good thing because it maintains the circulation, but it's also a fragile situation. If you were to freshen the surface waters slightly, that might have a large effect in slowing or even halting the AMOC.

en: What are some of the main questions in your field?

PC: Our big question is how global warming will affect the AMOC. That's very important, especially for the countries on the eastern side of the Atlantic, such as Scandinavia, the United Kingdom, and France, that enjoy a much milder climate than they would if the Atlantic circulation were not there. The answer is not clear because, on one hand, ice will melt, no doubt about it, and that will make the Northern Atlantic fresher. That will weaken the overturning, but on the other hand, because it's also getting warmer, there'll be more evaporation and that might make it more salty, which would strengthen it. Which one will win? 

en: What do you tell people worried that that circulation might suddenly stop, like in the movies?
PC: It’s not going to be like that. Even if you dump a huge amount of fresh water in the right spot, the AMOC doesn’t stop immediately. It takes hundreds of years to slow down. It won't be like “The Day After Tomorrow.”

en: Although it's reassuring that it wouldn't happen overnight, it seems like it would be more alarming because what takes decades to start would probably also take decades to undo.
PC: This is why we have to stop putting greenhouse gases in the atmosphere as quickly as we can. The system has a big inertia. It keeps going in the same direction for a while, even after you change things. It’s very important that we are aware of this. So, fly less, let your house be a bit warmer in the summer, a bit colder in the winter, and walk more or bicycle more. I think that everybody has to do their bit. 

en: How did you first get excited about physics?
PC: The truth is my father wanted to be a physicist, but instead he was a doctor. He just infused in me his love for physics and math, and I was pretty good at math in school, where I benefited from great math and physics teachers. I think that's where it came about. 

en: Why did you want to come to Scripps?
PC: I mean it's the best place in the country, maybe the world, for oceanography, right? So who wouldn't want to come here? Plus the winters are so much better than in Boston.