Why Do Typhoon Tracks Suddenly Change Direction? What Is the Subtropical High?
An analysis of the subtropical high, steering flows, and the causes of anomalous typhoon tracks.
Typhoons Don't "Move" on Their Own
A typhoon is essentially a vortex within the atmospheric river. Its movement is primarily "steered" by large-scale surrounding circulation patterns, with the most critical factor being the Subtropical High (Sub-High).
How the Sub-High Determines Typhoon Tracks
The Sub-High is a massive warm-core high-pressure system hovering over the Western Pacific. Typhoons typically move along its outer periphery:
- When the Sub-High is strong and positioned further west → The typhoon is steered westward into the South China Sea, affecting South China.
- When the Sub-High retreats eastward, creating a gap → The typhoon turns northward through the gap, affecting East China, Japan, and Korea.
- When the Sub-High splits into two rings → The typhoon moves slowly or spins in place within the saddle field.
Common Anomalous Tracks
- Looping/Stalling: Weak steering currents cause the typhoon to get "lost." For example, Typhoon Wayne in 1986 had a 23-day lifespan and looped three times.
- Sudden Westward Turn: A sudden strengthening and westward extension of the Sub-High.
- Binary Interaction (Fujiwhara Effect): Two typhoons within 1,000–1,200 km of each other exert mutual influence on their paths.
Why Forecasting Turns Is Difficult
The advance and retreat of the Sub-High are influenced by multiple systems, such as mid-latitude westerly troughs and the South Asia High. Numerical models often have significant errors in predicting the Sub-High's structure beyond three days. Therefore, track forecasts often show the greatest divergence around the turning point, so it is advisable to compare forecasts from multiple agencies.