Every Japanese child has learned about the Kuroshio and Oyashio currents. They know where the good stuff comes from. By good stuff I mean delicious seafood that forms the foundation of Japanese cuisine.

I must admit I haven’t spent much time wondering about water currents and their influence on my everyday life. But I guess that is the difference between the people of my country and the people of Japan, a country that is surrounded by and depends on the sea. 

The sea does not only nourish the Japanese nation. For hundreds of years it has also protected the country from foreign enemies. 

Yet every once in a while the ocean itself turns into an enemy, as though trying to remind us that the sea is not merely water spilled on the borders of the earth. On the contrary, the land is just a lucky piece of soil dropped in the midst of a never-ending field of ocean.

Professor Tanaka doesn’t spend much time in his office. Since the disaster in March 2011, you would more likely find him doing his research in Otsuchi town. 

This is not his first time to witness a natural disaster. He was a student at Kyoto University when his parents’ house in Kobe was damaged by the Great Hanshin Earthquake of 1995. 

He never dreamed he would witness something like this again. At the time his studies had little to do with the disaster, but now he is putting his knowledge and experience to good use.  

Slowly getting an idea of how important the complex system of water movement in the oceans is and grateful for the opportunity to ask questions from an expert like Professor Tanaka, I carefully choose the words for the first question:

“The northwestern part of the Pacific Ocean is extremely abundant in fish and it makes the Pacific coast of Tohoku a great area for fishing. I know that has to do with the currents but could you explain why exactly?” 

Professor Tanaka springs up from his chair and walks to the whiteboard. 

“It’s a mixture of various reasons,” he says drafting a map of Japan on the board.  

He explains that the Oyashio is a cold water current coming from the northeast Pacific. It has an important role in bringing nutritious water and oxygen to the area. The Oyashio influences the growth of kombu kelp and wakame seaweed.

The warm Kuroshio Current flows past northeastern Japan. It is the meeting of these two currents - the Oyashio and the Kuroshio, that most contributes to the abundance of fish in the Tohoku area. 

The third important current, called Tsushima, splits from Kuroshio in the East China Sea and passes through the Sea of Japan. It then eventually flows into the Pacific Ocean via the Tsugaru Strait south of Hokkaido and forms a warm southward current along the coast of Tohoku.

The river water inflow also plays an important role in the Tohoku coastal area. River water is known to be very nutritious because it flows through forests and mountains, which brings nutrition to the seawater. This altogether makes the area an abundant source of delicious cuisine.

When I ask Professor Tanaka about which aspect he loves about physical oceanography, his area of expertise, he replies: “All of it.” 

“Ocean dynamics in general is very interesting,” he continues, “such as why and how it moves   the mechanics of it.”

How do water currents flow? What is the speed of the water? How does the temperature change? What is the water salinity? This kind of information tells us a lot about the ocean, because these factors have a great influence on the organisms inhabiting the sea. 

“What are the elements and forces that influence water movement?” I ask. 

“Let’s take the top layer of the sea as an example,” he says and continues explaining that usually the top layer is influenced by the wind, and hence it flows differently from the lower layers of water. Additionally, it is affected by the friction between the different water layers, speed and direction of the currents and, last but not least, the river water inflow. 

“Our work is technically quite similar to weather forecasting,” Professor Tanaka explains. “It’s all about numerical models and approximate solutions.”

Numerical models are well suited for measuring the dynamic behavior of physical systems, such as water or the atmosphere. 

“Just like the weather forecast, the information we collect enables us to make future estimations, as well as track past movements in the ocean.” 

“Is it also possible to learn about the distant past?” I ask, “for example, what was the situation a hundred years ago?”

“Well, it depends on what exactly you want to know,” Professor Tanaka replies. “There are things you can say quite surely about the past, but there are also things that one can only assume.” 

“We have positioned several measuring devices inside and outside Otsuchi bay,” Professor Tanaka continues.  

“Measuring devices?”

He jumps up again and draws a sketch on the whiteboard. 

  

From above the water it looks like a buoy. Its job is to collect ocean data. Measuring takes place inside the water, depending on which layer of water one wants to collect data from.  

The bay is divided into 10 square meter cubes. What Professor Tanaka and his team do, based on the data taken simultaneously in all the cubes, is they calculate the speed and direction of the water by solving the equations of fluid dynamics. 

As there are many forces that influence the water, the formula becomes very complicated and therefore computers are needed to do the calculations. 

“Even then it is impossible to add every force that influences the water to the formula,” Professor Tanaka admits. Wind and water friction are two of the examples. 

Also, no matter how good the model is, there is a limit in terms of how small the area of measurement, the ‘cube’ can be.   

“Even the best model has limits and sometimes reality may differ from calculations,” says Professor Tanaka.

“The computer can make errors and after all it gives us only approximate solutions. That’s why we always compare the model with observations and make corrections if needed. Good models always go together with good observations.” 

After the earthquake and the tsunami hit the region, Professor Tanaka joined the researchers who work in the Tohoku area. 

“It seems to me that up until now pure science had a relatively small part in this field,” he says, “but now a perspective of pure science is crucial to approaching the problems we are facing now and in the future. This is a new challenge for the scientists and I do think pure science plays an important role in the recovery of the Tohoku area.” 

“It is difficult to find the balance between doing research just for the sake of research and doing it for the community,” Professor Tanaka admits. “Now, since the disaster the times have changed. We have to find the right way to help.”

This is the first article in the series of interviews with the researchers who are helping to reconstruct Otsuchi town and study its surrounding waters, after the disaster in March 2011.

October 19, 2013