Associate Professor OKAJIMA Satoru, Institute of Life and Environmental Sciences

In the mid-latitudes of the Northern Hemisphere—where Japan is situated—weather patterns are primarily governed by the movement of low- and high-pressure systems. Professor OKAJIMA's research focuses on the seasonal variability of these systems, with particular emphasis on Kuroshio cyclones--a prominent weather phenomenon that frequently occurs in early spring and carries significant socioeconomic consequences.

These Kuroshio cyclones travel eastward along the southern coast of Honshu, Japan's main island, often bringing heavy snowfall or rainfall to the densely populated Pacific side of the country. In February 2014, two such systems struck in rapid succession. The second event, in particular, caused record-breaking snowfall—114 cm in Kofu City and 73 cm in Maebashi City—highlighting the severe impact of these cyclones.

Accurately forecasting whether these systems will produce snow remains a major challenge. Instead of focusing solely on prediction, Professor OKAJIMA and his team posed a more fundamental question: Why do these cyclones form so frequently in early spring? To investigate, they analyzed several decades of global atmospheric data—including temperature, wind direction, and wind speed—and successfully identified the key conditions that lead to cyclone formation.

Here is how it works (Fig. 1). In spring, the atmosphere over Eurasia warms compared to winter, and the downstream region (eastern side)—along with the overlying westerly winds—also becomes warmer. This thermal change promotes cyclonic circulation (updrafts) near the surface and strengthens the westerly jet stream over southern China and the East China Sea. As a result, low-pressure systems are more likely to form in this region. These systems draw in moisture from the south, intensify, and track eastward toward Japan's southern coast.

To uncover the mechanisms behind this phenomenon, Professor OKAJIMA employed a technique he developed during his doctoral studies at the University of Tokyo: Curvature-Based Eulerian Decomposition (CUBED). This method, along with other analytical tools, enables researchers to more precisely isolate and analyze mobile low- and high-pressure systems.

Previously, it was difficult to distinguish between the broad westerly jet stream and the localized swirling winds around individual pressure systems. However, the innovative CUBED method has made this differentiation much more precise. Using this approach, their research uncovered a clear link between the recent rise in rapidly intensifying bomb cyclones in the Northwest Pacific and the presence of warmer, more humid air over the East China Sea.

Professor OKAJIMA believes that this research can enhance seasonal forecasting and improve predictions of how global warming will affect different regions. He emphasizes that understanding today's climate through the lens of the past is essential for anticipating future changes. Looking ahead, he plans to investigate how variations in sea surface temperatures influence climate variability.

Professor OKAJIMA's fascination with weather began early—he passed Japan's national weather forecaster certification exam at just 14 years old. "I don't remember exactly why I became so interested in weather maps," he recalls, "but I used to read meteorology books at the library all the time as a kid." Despite his early passion, his path to becoming a climate scientist was not straightforward. He was also interested in the life sciences and deliberately explored a wide range of disciplines during his first two years of college. "That's when I realized how exciting it is to take a bird's-eye view and think about how different systems interact," he explains. The mechanisms behind Kuroshio cyclones involve a complex interplay of factors—from atmospheric heating over Eurasia to jet stream dynamics. It is this kind of large-scale, long-term scientific inquiry that captivates Professor OKAJIMA: research that goes beyond daily weather forecasts to uncover the deeper workings of our planet.