A Scientist's Life: Sarah Purkey

Sarah Purkey is an oceanographer at Scripps Institution of Oceanography at the University of California San Diego. She received her bachelor’s degree in mathematics-physics from Whitman College in Washington in 2005 and her PhD in oceanography from the University of Washington in 2014.

explorations now: What do you do for a living?

Sarah Purkey: I look at the ocean’s role in climate and how ocean circulation and thermodynamics affect the amount of anthropogenic (human-caused) heat the ocean takes up.

I was always interested in math and physics. Oceanography is a wonderful interdisciplinary subject because if you like the ocean, there’s a place for you. If you like biology or if you like chemistry or if you're like me, if you like physics, there's still a home for you studying the ocean. Through school I always loved math and physics. And then as I got into it, I realized that I liked more of the applied side of things. I really wanted to do something where you're applying those concepts to real life problems. And I think climate change is one of the biggest problems that we're going to face as a society moving into the future.

So I find this as the best place to be. I get to do something that I love, which is working with data and thinking about statistics and thinking about physics. And at the end of the day when I do all that work, I feel like it's still going to be something that is societally relevant.

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

SP: One of the biggest questions in our field right now is really understanding the ocean’s role in climate, particularly how the ocean is mitigating the effects of climate change. A lot of people don’t realize how important the ocean is. The ocean is really big and water has a really high heat capacity. Those two things together mean if you have any change of temperature in the atmosphere, it’s going to equilibrate with the ocean. Most of that energy is going to go into the ocean, so if we didn’t have the ocean, we’d be experiencing just a tremendous amount of climate change. Our atmosphere would have warmed by astonishing numbers over the last few decades, but instead the ocean is taking up over 90 percent of that heat.

What I do is understand the mechanisms that allow the ocean to take up that heat and I measure it. The work I do is to measure how ocean temperature has changed year-to-year and calculate how much extra heat is in the ocean and also where it is. It’s not going uniformly throughout the ocean so we want to understand where it’s going and why.

en: What are you finding?

SP: The ocean is steadily warming. It affects ecosystems and very much contributes to sea-level rise. In addition to ice coming off of land and going into the ocean, about half the increase in sea level over the past three decades is  because the water is warming, and when it warms, it expands. The basic idea is that the volume water is taking up is a function of its temperature and so as you warm it up, it’s going to take up more volume and then we’re going to see sea-level rise. Sea-level rise is another of the biggest issues that we’re facing as a society. In that respect, the amount of heat that's going into the ocean can be somewhat alarming and we are seeing those effects of sea-level rise.

On the other hand, when the ocean takes up all that heat, it means it's leaving less heat in the atmosphere and therefore we're seeing less temperature effect in the places that we live. In some regards it is actually a positive thing in the sense that it's mitigating some of the effects of climate change. While it is disturbing, it might be more disturbing if the heat content stopped going up because it would mean atmospheric temperature would rise quite dramatically.

en: What are some of the tools of your trade?

SP: There are a few global-scale international observational programs that provide data that we rely on to monitor the ocean. There are two at Scripps that I'm heavily involved with. The first one is the Argo program. Argo is a fleet of autonomous floats that are all over the globe and they are profiling the upper 2,000 meters (6,560 feet) of the ocean. It’s a robot that’s about as tall as me and it’s a very thin cylinder. It has a very simple engine that inflates an external balloon that allows the float to go up and down in the water column by changing its buoyancy, much like a hot air balloon but in the ocean. As it does that, it measures temperature and salinity. When it comes to the surface every 10 days, it calls home – it has an antenna and through satellite, it transmits its data back. We have about 4,000 of these floats all around the globe. This network has produced millions of temperature profiles over the last decade or so and it gives us a really good picture of the temperature and salinity throughout the ocean.

The other program that I'm heavily involved with is called GO-SHIP and this is again an international program. It involves us going out on ships. Basically there are a bunch of lines that grid the ocean along longitude or latitude. We go out and we occupy these lines once a decade. We stop every 50 kilometers. We drop a CTD [conductivity, temperature, depth] rosette into the water. It goes all the way down to the bottom. The rosette has a bunch of electronics on it that measure a whole suite of things so it can tell us about not only temperature and salinity but also about the velocity of the ocean, mixing, oxygen levels, and optical properties that tell us something about biology.  From shipboard analysis, we get a whole bunch of information about ocean carbon chemistry. We can learn about all the different nutrients in the ocean. We can learn about tracers – inert gases that can tell us something about ventilation time scales and circulation, which is really exciting.

One of the main things that I look at is heat content and in order to know what the content is, we need to know what the temperature is. The instrument that I rely most heavily on is a thermistor, a type of thermometer. We probe the ocean in different ways to measure the temperature and then we can look at how that's changing in time. It's a simple thermometer, but we have really, really, really exact thermometers that can tell us what the temperature is down to .001 degrees C, which is the accuracy we need to know how temperature is changing.

The other thing we care about in physical oceanography is salinity. The reason we care about temperature and salinity together is because those two things along with pressure tell us what the density of the water is and the density tells us something about circulation.

en: Why did you want to come to Scripps?

SP: Scripps is really the mecca for observational oceanography, especially physical oceanography, the kind of stuff that I do. I just talked about two of the biggest international parts of physical oceanography observations and Scripps is a leader in both programs. We have one of four Argo labs located here in the U.S., so we produce about one-fourth of the U.S. Argo array and it’s exciting to work with the engineers and scientists who build the Argo floats. But most of all, there’s a really deep breadth of exciting oceanography and climate science going on at Scripps, so it’s a great place to be and work.