What Is a Typhoon's "Eyewall Replacement" (Eye Replacement)? Why Do Super Typhoons Suddenly Weaken and Then Strengthen Again?

What Is an Eyewall Replacement Cycle (EWRC)?

When a typhoon intensifies to a severe typhoon or super typhoon (typically with maximum sustained winds of ≥50 m/s), its inner eyewall contracts into an extremely compact ring (with the eye diameter sometimes shrinking to just 10–20 km). At this stage, outer spiral rainbands organize into a new ring of convection roughly 50–80 km outside the inner eyewall. This is the "outer eyewall," which creates a striking "concentric dual-eyewall (double-eye) structure" on satellite imagery and microwave scans.

The Three Stages of Eyewall Replacement: How Does the Outer Eyewall "Take Over" from the Inner One?

• Stage 1 (Two Eyewalls Coexist, Reaching the Peak): The outer eyewall forms and closes in, beginning to act like a dam that blocks the flow of moisture and angular momentum from the outer environment toward the center.
• Stage 2 (The Inner Eyewall Suffocates, and Intensity Temporarily Drops): Cut off from its moisture supply, the inner eyewall gradually dries out, collapses, and dissipates. The typhoon's central pressure rises at this point, and its maximum sustained wind speed may fall by 10–15 m/s. However, the radii of its force-8 and force-10 wind circles actually expand significantly!
• Stage 3 (The Outer Eyewall Contracts, and the Typhoon Returns to Its Peak): The formerly broad outer eyewall takes over as the new primary eyewall and contracts inward into a tighter ring. The typhoon then intensifies explosively once more, potentially surpassing its previous peak intensity.

Why Can the "Timing of Eyewall Replacement" Determine a City's Fate?

A complete eyewall replacement cycle usually takes 12–36 hours. When a super typhoon approaches the coast, the stage of the eyewall replacement cycle can directly determine the scale of the disaster:

• The lucky scenario (landfall occurs during the temporary lull of the replacement cycle): The inner eyewall has collapsed, but the outer eyewall has not yet contracted. The typhoon makes landfall with a loose, broad wind eye, substantially reducing the severity of its extreme winds. For example, some typhoons that made landfall in Taiwan, Fujian, or Zhejiang weakened markedly after their eyewall replacement attempts failed over the nearby waters.
• The worst-case scenario (the eyewall replacement is completed just before landfall): The new eyewall has only just contracted over the nearby waters. Not only has the maximum sustained wind speed returned to force 17 or above, but the larger wind circle following the replacement means that the area swept by force-12 hurricane winds is twice as large as it was before the replacement (for example, "MANGKHUT" in 2018 and "YAGI" in 2024).