Cyclone “Seed” survival affects hurricane season intensity

Predicting how many tropical hurricanes will form during a hurricane season is important for mitigation and disaster preparedness, but can be tricky to get right. Previous investigations have shed light on two factors that affect the number of tropical cyclones: the number of tropical cyclone “seeds,” which are weak vortices in the lower troposphere, and their survival rate. However, a better understanding of the conditions that lead seed storms to evolve into full-blown hurricanes and typhoons is needed to help scientists predict the intensity of storm seasons in future climate change scenarios.

Using climate conditions to estimate the survival rate of potential tropical cyclone seeds is a useful approach to predicting future hurricanes, according to new research. Ikehata and Satoh show that although plenty of would-be hurricanes and typhoons form over the oceans, it’s not just a numbers game in which more seed storms mean more hurricanes. The seeds need the right conditions to survive.

“The change in [the number of] seeds is not so important in the future—we need to study more survival rates,” said Masaki Satoh, a climate scientist at the University of Tokyo’s Atmosphere and Ocean Research Institute and one of the study’s authors. “This is something contrary to the previous notions,” which suggested that the number of tropical cyclone seeds affected primarily the intensity of hurricane seasons.

To come to their conclusion, the authors examined a database of tropical cyclones and tropical cyclone seeds during summers in the Northern Hemisphere from 2000 to 2018. They also collected atmospheric variables and sea surface temperatures during those storms to determine the best conditions needed for tropical storm seeds to grow.

They found that the number of seeds that appeared in a given month was related to the average vorticity, or the spin speed, of the weather systems in an area, as well as the average vertical wind speed. During the study period, only about 10%–20% of seeds converted into tropical cyclones. Climate conditions, such as sea surface temperature, humidity, and winds, determined whether seeds grew into hurricanes.

Although the researchers didn’t consider climate change in this paper, a number of studies have shown that climate change will likely affect future formations of tropical cyclones across the world. Satoh suggests that their study’s findings about tropical cyclone seed survival can be applied to future studies examining how hurricanes may change in frequency or severity in the future with our ever changing climate.