Earlier arriving tropical cyclones reason for serious concern say climate scientists

Researchers say they’ve identified changing seasonal cycles which are bringing forward the date on which intense tropical cyclones are more likely to form up each year.

In what they describe as “a significant seasonal advance of intense Tropical Cyclones (TCs) since the 1980s in most tropical oceans,” the researchers from Beijing and Hawaii, found  earlier-shifting rates of 3.7 and 3.2 days per decade for the Northern and Southern Hemispheres respectively.

The research was co-authored by a team from China and Hawaii, led by Kaiyue Shan from State Key Laboratory of Hydroscience and Engineering at Tsinghua University in Beijing and Pao-Shin Chu from the Department of Atmospheric Studies at the School of Ocean and Earth Science at the University of Hawaii in Honolulu.

The paper was published in Nature.

The occurrence time of an intense TC event is defined as the date on which it first achieves its lifetime maximum intensity. The “active season” is defined as June to November in the northern hemisphere and December to April in the southern hemisphere.

The report says that since the 1980s, Category 4 and 5 hurricanes (maximum wind speed greater than 211km/h or 131mph) have been arriving three to four days earlier with each passing decade of climate change.

“When intense tropical cyclones occur earlier than usual, they cause unexpected problems for communities,” said Pao-Shin Chu, atmospheric sciences professor and Hawai‘i State Climatologist.

“Moreover, the earlier advance of these storms will overlap with other weather systems, for example local thunderstorms or seasonal monsoon rainfall, and can produce compounding extreme events and strain the emergency response.”

The seasonal shift was particularly observed in the eastern North Pacific off the coast of Mexico, where most hurricanes near Hawai‘i come from; the western North Pacific; the South Pacific; the Gulf of Mexico; and the Atlantic coast of Florida and the Caribbean.  

“It was surprising to consistently see earlier arrivals when we independently assessed satellite data and conventional ground-based observations of intense tropical cyclones,” said Chu.

Using simulations from multiple global climate models (e.g., high-resolution CMIP6 models), the team detected warmer oceanic conditions developed earlier, which favored the earlier onset of intense tropical cyclones. Further, they found that the warming was primarily driven by greenhouse gas forcing. 

“In a future with high carbon dioxide emissions, the earlier shifting trend is projected to be amplified,” said Chu. 

In South China and the Gulf of Mexico, the earlier onset of intense tropical cyclones contributes significantly to an earlier onset of extreme rainfall. 

“Given the seasonal advance of intense tropical cyclones, as shown in this study, the potential for simultaneous occurrence with other high-impact weather events should be a serious concern for the society,” said Chu. “Understanding potential changes in hurricane activity in response to global warming is important for disaster prevention, resource management and community preparedness.”

Another study released last year shows that climate change is reducing the number of tropical cyclones.