Physical oceanography seems like magic, but it is science

Forecasting is like magic

My way into science started with my interest in weather forecasting. I must have been in junior high school already. I remember watching the weather forecast on TV and feeling amazed by how we could predict something in the near future based on what we knew about the current state. It felt like magic, and I fell in love with climate and weather systems.

As I studied further, I learned how basic physical equations can tell us about how states change over time. Since I had this vague interest in physics as well, I wanted to choose a university known for its excellence in physics. So, I joined the undergraduate Department of Earth and Planetary Science. As I studied even further, I realized that the weather system and physics were interconnected.

I feel like many undergraduates majoring in basic science go on to complete a master’s degree as well. So, I did not hesitate to enter graduate school. I estimate that about 50% percent of students come from a physics background, and about 50% come from an environmental science background in the Graduate Department of Earth and Planetary Science. I find this interdisciplinarity fascinating.

My main interest is physical oceanography, with a focus on ocean dynamics. In my opinion, the ocean, for example, El Nino and El Nina, is the key driver of longer-time-scale climate phenomena. Oceanic phenomena influence the atmosphere, and ultimately, we can get a hint of those influences in our weather forecast.

There is a jump from my earlier interest in weather forecasting, which makes predictions only at a shorter, about a week-long time scale. What caught my attention about longer time scale phenomena was precisely jumping over this hurdle: I was surprised that we could predict what the ocean would be like a year from now, even though we are barely able to predict what it would be like a week from now.

Mesoscale eddies around strong currents

Knowing the physics behind ocean currents and other phenomena, we use model simulations to make predictions. We plug observational data into the model. Then, the computer runs the necessary calculations of the basic equations. If the model is set to the right conditions, the various changes that might occur over time in the sea can be projected into the future.

For my master’s degree, I worked specifically on mesoscale eddies in the region around Japan called the Kuroshio (Black Tide) Extension. Many eddies are observed in this region because the Kuroshio is an extremely strong current. So, if we want to think about the state of the ocean around Japan, eddies are a great target. Mesoscale eddies are about a hundred kilometers wide and can go as deep as a kilometer. Still, they are considerably smaller and a bit more difficult to observe. Thanks to recent advances in satellite imaging, however, we can make more observations of and learn more about these eddies.

My topic started with the question of how the atmosphere affected the ocean surface. For example, it was already known that some currents in the upper ocean (the top layer of the ocean) were associated with the winds blowing above the surface. We had a very basic theory describing how the surface winds drove the ocean currents, but little was known about how those winds drove circulation in the presence of eddies.

The wind and the ocean collide

The basic theory is called the Ekman theory. It states that if the wind is blowing in one direction, the surface current will not be exactly along the surface wind but about 45 ° to the right in the Northern Hemisphere due to the rotation of Earth. This results in a spiral structure in the surface layer of the water, which is called the Ekman response. The strength of this theory is that if we know the direction and strength of the wind, we can understand the direction and strength of the wind-driven currents in the upper ocean. However, since several assumptions are made to create this theory, I think it might not be precise enough for some structures, for example, when mesoscale eddies are present. Mesoscale eddies are characterized by their rotational motion and a temperature anomaly, a difference in temperature between the center of the eddy and its surroundings.

My findings in my master’s thesis were that this wind-driven upper ocean Ekman response is not solely determined by surface winds, but also it is determined by the rotational velocity temperature, the way temperature and rotation interact in mesoscale eddies, which had not been stated explicitly before.

I have just started my doctoral program and am searching for a new problem to work on. Lately, I have been interested in how relatively small-scale phenomena, eddies, for example, feed back to large-scale phenomena in the ocean. At this point, I am not sure which specific large-scale phenomenon, such as El Nino or La Nina, I want to target. But I intend to follow where my curiosity takes me. My ultimate goal is to define the connections of how small-scale phenomena combine to drive a phenomenon on the scale of the North Pacific Ocean. I believe the eddies, which I specifically worked on in my master course, transport heat in the ocean and play an important role in causing climate variations. If I succeed in my next goal, then we will know the precise mechanisms of how these small-scale phenomena drive changes and be able to make more precise predictions about the North Pacific on a year-long timescale.

Models, theories, and equations

Running simulations of climate systems means we let a computer solve a system of equations. The Navier-Stokes equations are often used for this purpose and make up the backbone of our models. These equations have three components because we are making calculations for three dimensions in the ocean. We also have the advection-diffusion equations to calculate how “things” such as temperature or salinity “move around” in the ocean. We also have an equation for calculating density, determined by salinity or temperature. For example, if the temperature is high, water expands, and thus density decreases. These equations are then combined in the model as we try to understand physical phenomena in the ocean.

To make our models better, be it climate models or models of ocean currents, we make detailed comparisons of our model outputs and our observations of the actual ocean. A mismatch between the two signals that there might be something wrong with how the model represents the target phenomenon. We then adjust the model and repeat this cycle until we get it right. I am not that involved in the adjustments of the model, but I can describe in broad terms what “adjustments” might entail. For example, there is a method called “parameterization,” which is used to represent processes that are too small to represent in the model using simple statistically determined parameters or physics. What scientists can do is to think deeply about the physics of the phenomenon and find new ways to approach its mathematical representation.

Much of what I do for my research starts with running a model, which can take a day or two. During that time, I read relevant papers and analyze the output of a previous run of the model or the output of another model. This is my favorite part of research. I am using Python to write code that analyzes these outputs. I learned it in my undergraduate classes. I picked it up quickly and still enjoy it. I like coding in general, which is very lucky because oceanography requires a lot of coding. After the analysis, we plot the resulting data on a map to make it easier to visualize and understand what is happening in the target phenomenon. We, of course, compare our model output with observational data. I have to say that writing code to solve the differential equations for the analysis was the most difficult part of my master’s thesis.

A "gap year" in Chicago

I was in the same class with the same people for six years during junior high school and high school, which is relatively rare in Japan. So, when the opportunity arose to experience another environment, I took it without hesitation. I participated in a high school student exchange program and went to a local school in Chicago for a year.

I started learning English in junior high school, so I only had about four years of learning behind me, and my English skills were average. Going abroad provided great motivation to improve and I studied a lot in preparation.

It might seem like going overseas for a whole year took a great deal of confidence and courage. But honestly, I had fun and learned a lot. So, if anybody else has a chance to do something similar, do not hesitate. It is not as scary as it first seems.

I am involved in International Graduate Program for Excellence in Earth-Space Science (IGPEES) since 2023. In this program, students are required to study abroad during the doctoral course. There is one researcher at the National Oceanic and Atmospheric Administration (NOAA) whom I have been working with. I met her when she was here as a guest professor around two years ago. She has been giving me advice and comments on my study despite her busy schedule since then. I hope I get to go back to the US and do research there one day.

I feel so lucky to live in a day and age where it is easier to pursue one’s interests than ever before. Watching weather forecasts piqued my curiosity, which led me to major in a related field. Having a chance to go overseas in high school helped me prepare to collaborate internationally in graduate school. So, I would suggest students to just start following their curiosity. If they find something interesting in their everyday life, start to get to know it more, look it up online, or read a book about it. That is probably be the best advice that I can give.

※Year of Interview:2025
Photography:KAIZUKA Junichi
Text:Belta Emese
The interview was edited for brevity and clarity.

USUI Kento

Usui’s interest in science started early when he was captivated by weather forecasts and their ability to predict the future. This, combined with his later interest in physics, led him to the Department of Earth and Planetary Science to focus on physical oceanography. He worked on mesoscale eddies for his master’s thesis and has just started his doctoral studies in the same department.