High moisture confluence in Japan Sea polar air mass convergence zone captured by hourly radiosonde launches from a ship.

Some of the heaviest snowfalls in urban areas in the world occur in Japan, particularly in regions that face the Japan Sea. Many heavy snowfalls are produced by a Japan Sea polar air mass convergence zone (JPCZ), which is an atmospheric river-like cloud zone that forms when Siberian cold air flows over the warm Japan Sea. Quantifying how the air-sea interaction strengthens the JPCZ is key to snowfall prediction. However, until our observations with hourly meteorological balloon launches from a training vessel in 2022, no simultaneous air-sea observations targeting the JPCZ had been conducted. Our observations showed that wind direction shifted drastically by about 90 degrees from the surface to an altitude of about 3.5 km within a narrow horizontal range of about 15 km, indicating airflow convergence from the surroundings. Maximum temperature difference between surface air (3 °C) and water was 11 °C near the JPCZ centre with 17 m s-1 wind speed. Large amounts of heat, 718 W m-2, was thus gained from the warm sea. Water vapour was also concentrated by the horizontal convergence, which caused heavy snow, equivalent to 100 cm of snowfall in 7 h. The surrounding sea greatly affects moisture formation within the JPCZ.

Pacific western boundary currents and their roles in climate.

Pacific Ocean western boundary currents and the interlinked equatorial Pacific circulation system were among the first currents of these types to be explored by pioneering oceanographers. The widely accepted but poorly quantified importance of these currents-in processes such as the El Niño/Southern Oscillation, the Pacific Decadal Oscillation and the Indonesian Throughflow-has triggered renewed interest. Ongoing efforts are seeking to understand the heat and mass balances of the equatorial Pacific, and possible changes associated with greenhouse-gas-induced climate change. Only a concerted international effort will close the observational, theoretical and technical gaps currently limiting a robust answer to these elusive questions.