Beyond El Niño: Winter weather may help predict East Asian summer typhoons

Typhoon season has been particularly brutal this year, with 26 named storms battering East Asia. The storms are difficult to predict, especially months in advance when mitigation planning could be most useful, but large-scale weather patterns may offer clues. This year, many are pointing at the strong El Niño, a cyclical phenomenon of warming ocean surface temperatures, as the primary driver of the increased number and severity of the storms — but it might not be entirely El Niño’s fault and there may be better predictors, according to researchers at Zhejiang University.

In a study published on Oct. 12 in Ocean-Land-Atmosphere Research, the team assessed more than 40 years of observation and identified a significant connection between the activity of a specific large-scale atmospheric circulation called the Pacific–North American (PNA) pattern in February and tropical cyclone activity over East Asia during the following summer. After removing the potential influence of the El Niño-Southern Oscillation (ENSO) — the climate system that includes El Niño’s higher ocean surface temperatures — from their analysis, the researchers found that nearly 80% of the East Asian tropical cyclone variance could be explained by the PNA activity.

“Our findings suggest that late-winter atmospheric variability may provide valuable information about East Asian summer tropical cyclone activity beyond the influence of ENSO," said Zeming Wu, first author on the study and doctoral student in the State Key Laboratory of Ocean Sensing and Ocean College at Zhejiang University. "This highlights the potential importance of atmospheric circulation outside the tropics as an additional source of seasonal predictability."

The PNA pattern is large scale, distributing air and heat produced by solar energy across the North Pacific and North America. It operates in phases, with its “positive phase” characterized by warmer temperatures across the western United States and Canada and cooler temperatures across central and southeastern United States. The negative phase is the inverse.

The researchers found that PNA’s positive phase in late winter is associated with fewer tropical cyclones forming over the western North Pacific and fewer storms reach East Asia several months later. But how did an atmospheric pattern in February remain connected to tropical cyclone activity months later, when the original atmospheric anomalies may have weakened or disappeared? Ocean temperatures kept the events linked, according to paper co-author Chundi Hu, researcher in the State Key Laboratory of Ocean Sensing and Ocean College at Zhejiang University.

“Through interactions between the atmosphere and ocean, sea surface temperature anomalies can preserve and transmit climate signals across seasons,” Hu said.

Wu explained that they identified two complementary ocean-mediated pathways linking late-winter PNA variability to summer tropical cyclone activity.

"The ocean appears to act as a bridge between winter atmospheric variability and summer tropical cyclone activity," Wu said. "More importantly, our results suggest that this connection involves not only the familiar ENSO-related pathway but also another pathway associated with atmospheric variability beyond preceding-winter ENSO."

The first pathway was the one associated with ENSO, where El Niño’s warmer sea temperatures accompany a positive late-winter PNA pattern in the tropical North Atlantic. The tropical Pacific cools relatively rapidly toward neutral conditions. According to the researchers, these temperature changes are consistent with the changes in large-scale atmospheric circulation conditions that favored the development and persistence of an anomalous anticyclone over the western North Pacific into summer.

The second pathway operates beyond the linear influence of preceding-winter ENSO, Hu said, pointing to a mechanism called seasonal footprinting that allows anomalies in atmospheric circulation to leave a lasting and developing imprint on ocean temperatures. In this case, the anomalies were wind changes over the subtropical North Pacific that were associated with cooling in the central tropical Pacific.  

“Despite their different origins, both pathways converge on a similar atmospheric response: an anomalous anticyclone, or unusual system of high pressure, over the western North Pacific,” Wu said. “This circulation pattern suppresses conditions favorable for tropical cyclone formation and modifies the winds that steer storms, reducing the number of tropical cyclones reaching East Asia during summer.”

Next, the researchers said they plan to conduct targeted climate model experiments to further investigate these pathways.

“Ultimately, our goal is to better understand the sources of tropical cyclone predictability and contribute to more reliable seasonal risk assessments for East Asian coastal communities,” Hu said.

The Key R&D Program of Zhejiang Province, National Natural Science Foundation of China, Natural Science Foundation of Guangxi, Guangdong Major Project of Basic and Applied Basic Research, Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhejiang University, the Ocean College Seed Fund, Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, and the Innovation Group Project of Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) helped fund this research.

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