The Western Pacific Subtropical High (WPSH) functions like Earth's atmospheric traffic controller, directing summer monsoon flows that regulate rainfall and temperatures across East Asia. When this high-pressure system misbehaves, the consequences can be dramatic - from the devastating Yangtze River floods of 1931 and 1998, to 2020's endless rainy season, and the record-shattering 2022 heatwaves that baked the Yangtze basin. These extremes underscore the urgent need to predict the WPSH’s variations. Yet our understanding remains incomplete. While researchers have made significant progress in studying the WPSH’s seasonal behavior over recent decades, a critical question persists: How has its predictability evolved over longer timescales?

A team of climate scientists reveal how our ability to predict the WPSH has transformed over the past century. Published in the Journal of Geophysical Research: Atmospheres, the research shows the prediction skill of summer WPSH has significantly improved since the 1960s compared to earlier decades. What's behind this prediction breakthrough? The answer lies in increasingly robust oceanic signals from the Indo-Pacific region.

“We identified three persistent oceanic predictors of WPSH variability that serve as crucial sources of predictable signals”, said first author Ye Wanheng from the Institute of Atmospheric Physics at the Chinese Academy of Sciences. These predictors include sea surface temperature anomalies in the tropical central‐eastern Pacific, tropical North Atlantic, and Indo‐Pacific.

Notably, the Indo‐Pacific has emerged as the most significant contributor to WPSH prediction skill, accounting for 59% after 1960, compared to 40% before 1950. This shift is mainly attributed to two factors: the systematic improvement in the quality and coverage of observational data, and an enhancement in the quasi-biennial variability of El Niño-Southern Oscillation since the 1960s.

“This centennial analysis gives us insight into how oceanic drivers have systematically improved WPSH forecasting,” said corresponding author Hu Shuai. "Before 1950, limited sea surface temperature predictability constrained our ability to forecast the WPSH. The post-1960 era brought a remarkable transformation. The strengthened predictable signals emerging from three-ocean interactions began dominating over atmospheric noise from distant regions like the North Atlantic."

By quantifying the evolving contributions of three oceanic regions across a 100-year period, this study establishes a new framework for understanding WPSH predictability. The findings demonstrate how shifting oceanic influences have gradually transformed our ability to make accurate seasonal forecasts.