Tropical cyclones give rise to unique type of heat wave in Japan
Tokyo, Japan – Researchers from Tokyo Metropolitan University have categorized a unique, previously unclassified type of heat wave in Japan, so-called “moist heat waves” which are accompanied by an approaching tropical cyclone.
These heat waves are accompanied by high humidity and/or heavy precipitation, and their frequency has been rising over the last thirty years, accounting for approximately a quarter of the heat wave days surveyed. They may potentially trigger multiple natural hazards at once.
Heat waves and droughts are dangerous weather events which are expected to become more frequent and dangerous with global warming. Understanding how they occur is crucial to accurate forecasting and risk mitigation. On large continental masses such as Europe or the United States, there is a well-known positive feedback between hot weather and dry land; as land becomes drier, there is less cooling due to evaporation, leading to the prolonged spells of high temperatures and low humidity. This contrasts with what happens near the sea, where this feedback is much weaker. While continental heat waves have been extensively studied, heat waves in places like Japan, surrounded by sea, are yet to be fully understood and categorized.
A team led by Associate Professor Hiroshi G. Takahashi from Tokyo Metropolitan University has been looking at typical air circulation patterns around the Japanese archipelago when heat waves hit. They used an algorithm to extract dominant patterns in air flow over 108 heat wave days in the period 1992-2021. Their analysis firstly confirmed the most well-known pattern, a high-pressure system jutting out westwards from the Pacific Ocean. This type is said to be due to a connection between air flow waves in the mid-latitudes and Japan, a so-called “teleconnection pattern,” where air circulations can affect weather over great distances. The analysis showed these to be hot, dry, and short, in agreement with what was previously known.
But this was not the only pattern they found. They also classified a heat wave type associated with the approach of a tropical cyclone. These do not involve dry weather; in fact, the low-pressure system of the cyclone sends in large amounts of moisture, causing a “moist heat wave.” Not only do these entail hot days but extreme humidity as well, contrasting strongly with continental heat waves. According to the team’s findings, “moist heat waves” accounted for around a quarter of the days they surveyed. Worryingly, their frequency has been systematically increasing over the last thirty years.
The team’s successful categorization of this new pattern is a first step to understanding an underappreciated mechanism behind heat waves in a maritime environment. The properties of “moist heat waves” make them particularly hazardous, as they can potentially trigger both extreme temperatures and torrential rain at the same time. Continued monitoring and further study will prove crucial to predicting and mitigating their impact in the future.