What Is the Subtropical High? The "Hidden Hand" Steering Typhoon Tracks
The subtropical high is the primary guide for typhoon paths: typhoons glide along its periphery. A strong subtropical high drives typhoons westward, while a weak or broken one allows them to turn northward.
What Is the Subtropical High?
The subtropical high (abbreviated as "Sub-High") is a warm-core high-pressure system hovering over latitudes 20°–40°N. The Western North Pacific Subtropical High acts as the "master switch" for weather in China: it dominates summer heatwaves, the position of rain belts, and the direction of typhoons.
On weather charts, the Sub-High resembles a massive elliptical "fortress of air," rotating clockwise. Its position and intensity change daily.
How the Sub-High "Directs" Typhoons
Typhoons lack their own propulsion; like spinning tops drifting in atmospheric rivers, they are "steered" by large-scale circulation. The airflow along the periphery of the Sub-High serves as the primary steering current:
- Strong Sub-High extending westward: Typhoons travel westward along the easterly flow on the southern side of the Sub-High—heading straight for South China (e.g., the paths of RAMMASUN and YAGI).
- Sub-High positioned northward with a lifted ridge line: Typhoons turn along the western edge of the Sub-High, moving northwestward to make landfall in Fujian and Zhejiang or recurving into the East China Sea.
- Sub-High retreating eastward or breaking apart: The steering flow collapses, allowing typhoons to move northward through gaps, or causing them to spin in place with erratic paths (a typical example is IN-FA lingering off the coast in 2021).
Forecasters often say, "Forecasting typhoons is forecasting the Sub-High." Errors in predicting the Sub-High directly translate into errors in typhoon track forecasts.
Two Classic Scenarios
- Variable Paths of Autumn Typhoons: In autumn, the Sub-High begins its seasonal retreat eastward and southward. Its unstable position makes typhoon track forecasting the most difficult of the year, leading to the greatest divergence among global models.
- Binary Typhoons + Sub-High: The Fujiwhara effect combined with changes in the Sub-High makes track forecasting notoriously difficult, akin to "hell mode."
How to Apply This Knowledge on TyphoonSays
When comparing multi-agency forecasts: if the predicted tracks from different countries fan out in a "fan shape," it usually indicates that models disagree on the evolution of the Sub-High. In such cases, do not bet on any single path; instead, broaden your preparedness range. As time progresses and the Sub-High pattern becomes clearer, forecasts from various agencies will gradually converge.