Scientists find source of cooler summers

The Okhotsk high-pressure system phenomenon, which sometimes causes unusually cold summers and prolonged rainy seasons in Japan, originates from atmospheric movements over northern Europe and reaches Japan via Siberia, researchers have found.

The discovery represents a breakthrough in the researchers’ longtime study of the mechanism of the formation of the so-called Okhotsk High, and they say it could lead to more accurate weather forecasts.

The massive Okhotsk high-pressure system occurs annually above the Sea of Okhotsk from late April to August.

From June to July, a rain front is formed between the Okhotsk High and the Pacific high-pressure system in the south, causing the rainy season in Japan.

If the Okhotsk high-pressure system becomes stronger in the summer, a cold wet wind known here as “yamase” moves from the northeast and stays over northern Japan, causing crops to suffer cold weather damage.

Hisashi Nakamura, a University of Tokyo assistant professor on climate dynamics, and Tomohiro Fukamachi, who works at the Japan Meteorological Agency’s maritime weather information office, compiled the study based on their analyses of meteorological data collected by the U.S. National Oceanic and Atmospheric Administration over the past 30 years.

According to the study, a high-pressure system gains strength above northern Europe about a week before the Okhotsk high-pressure system develops in July.

When the high pressure above northern Europe weakens, an atmospheric low-pressure system is formed along the Siberian coast of the Arctic Ocean.

The Okhotsk high-pressure system is formed when the low pressure system is weakening.

The moves are caused by waves of atmospheric energy — known as Rossby waves — when they travel east on a jet stream, according to the study.

The jet stream moves at about 36 kph, some 10 km above the Arctic coast.

According to the study, high pressure breaks out at an altitude of about 10 km in the Okhotsk Sea’s north, which serves as the exit of the jet stream, and produces a cold, easterly wind above the sea.

The air that cooled off at sea builds up in the western side of the sea and develops into the Okhotsk high-pressure system, according to the researchers.

Movement of Rossby waves was particularly visible during Japan’s cool summers of 1993 and last year, the researchers said.

“For better forecasting accuracy, further research on the jet stream and monitoring of the atmospheric pressure of northern Europe is required,” Nakamura remarked.