Two Climate Drivers Steer When Pacific Storms Suddenly Explode in Strength

Some of the most dangerous moments in a tropical cyclone’s life arrive with almost no warning. A storm that has been grinding along as a modest tropical storm can, within a single day, blossom into a ferocious typhoon, catching forecasters and coastal communities off guard. Meteorologists call this explosive strengthening rapid intensification, and it remains one of the hardest phenomena in tropical meteorology to anticipate. A new study published in Climate Dynamics by Zi-Xuan Chen and Yi-Peng Guo of Nanjing University and Xu Chen of the University of Tokyo now offers a clearer picture of why some years see far more of these events than others in the western North Pacific, the most active tropical cyclone basin on Earth.

The research focuses specifically on offshore rapid intensification, meaning the dramatic strengthening that occurs close to land rather than in the open ocean. This distinction matters enormously for disaster preparedness. A storm that intensifies rapidly a thousand kilometers from any coastline is a problem for shipping; a storm that does the same thing a few hundred kilometers offshore, as it bears down on the Philippines, Vietnam, or southern China, is a potential catastrophe. Recent work by other research groups has documented a global increase in rapid intensification events in offshore regions, and a separate migration of tropical cyclones toward coastlines, which makes understanding the year-to-year variability of near-shore intensification an urgent scientific and practical priority.

Using the International Best Track Archive for Climate Stewardship, the standard global database of tropical cyclone positions and intensities, together with atmospheric reanalysis products from the ERA5 dataset and ocean reanalysis data from SODA version 3, the team compiled a record of offshore rapid intensification events across the western North Pacific. They then examined how the annual counts of these events fluctuate from one year to the next and searched for the climate fingerprints behind those fluctuations. A crucial early finding emerged from this analysis: offshore rapid intensification does not simply track the overall level of tropical cyclone activity in the basin. A year can be busy with storms yet quiet with respect to near-shore explosive intensification, or the reverse, which means the two phenomena are governed by partly independent sets of climate drivers.

Through a systematic screening of candidate climate indices, the researchers identified two dominant players. The first is the El Niño–Southern Oscillation, the famous Pacific climate pendulum, represented here by the La Niña index. The second is a less familiar but increasingly influential pattern: the boreal spring Hadley circulation over the Indo-Pacific warm pool, abbreviated IPWP_HC. This circulation describes the large-scale vertical and meridional overturning of air over the warmest ocean waters on the planet during the Northern Hemisphere spring, months before the peak typhoon season arrives. When the team fed both indices into a multiple linear regression model, the combined picture explained a substantial fraction of the year-to-year variability in offshore rapid intensification, and, remarkably, each driver stamped its influence on a different part of the basin.

The La Niña signal concentrates its effects near the Philippines. During La Niña-like conditions, the tropical Pacific reorganizes in ways that raise the heat content of the upper ocean in the Philippine Sea region. Upper-ocean heat content, often quantified as the integrated thermal energy stored in the layer above the thermocline, is a critical fuel gauge for intensifying hurricanes. A deep, warm layer resists the cooling that a storm’s own winds would otherwise stir up through mixing, allowing the cyclone to keep drawing energy from the sea surface without throttling itself. The study shows that when the La Niña index points to these conditions, offshore waters near the Philippines carry more of this thermal reserve, tilting the odds toward rapid intensification for storms passing through the region.

The spring Indo-Pacific warm pool Hadley circulation, by contrast, exerts its strongest influence over the South China Sea, and it does so mainly through dynamical rather than thermodynamic channels. A stronger or differently configured spring Hadley circulation leaves behind an enhanced low-level cyclonic circulation over the South China Sea during the following typhoon season. This background rotation does two things at once. It helps storms spin up by providing ambient vorticity that the developing vortex can exploit, and it reshapes the vertical structure of the atmosphere in ways that favor intensification. The researchers found that both the La Niña index and the spring Hadley circulation index are associated with increased mid-level relative humidity and reduced vertical wind shear in their respective regions, two of the most consistently identified preconditions for rapid intensification in the global literature.

Vertical wind shear, the change in wind speed and direction with height, is the great destroyer of developing tropical cyclones. Strong shear tilts the storm’s core, vents its heat and moisture away from the center, and disrupts the organized eyewall convection needed for explosive growth. Mid-level humidity matters because dry air entrained into a storm’s circulation evaporates, cools, and suppresses the deep convective bursts that drive pressure falls at the center. By independently promoting a moist, low-shear environment, each climate driver creates a window in which a passing storm can cross the threshold into rapid intensification. The regional separation of these windows, one anchored near the Philippines by ocean heat content and the other over the South China Sea by circulation dynamics, is the study’s central and most actionable insight.

The implications for seasonal forecasting are significant. Because the spring Indo-Pacific warm pool Hadley circulation is established months before the typhoon season peaks, it functions as a potential precursor signal, giving forecasters a head start of one to two seasons in anticipating whether the South China Sea will be unusually prone to offshore intensification events. Similarly, the state of the ENSO system, which is monitored continuously by climate centers worldwide, provides guidance on the Philippine Sea sector. A two-parameter statistical model of the kind the authors constructed cannot predict individual storms, but it can sketch the seasonal landscape: which sub-basins are likely to be loaded with the ingredients that turn ordinary typhoons into monsters just before landfall.

The study also fits into a broader and rapidly evolving research landscape. Other recent work has linked ENSO to rapid intensification globally, documented shifts in where and when western North Pacific storms intensify under different ENSO flavors, and connected decadal patterns such as the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation to intensification behavior in the basin. By demonstrating that offshore events obey their own interannual rules, distinct from basin-wide storm counts, the new analysis adds an important layer of nuance. It suggests that operational agencies and risk modelers who estimate coastal typhoon threat from storm frequency alone may be systematically missing a key dimension of the hazard.

For the tens of millions of people living along the rim of the South China Sea and the Philippine Sea, the difference between a gradually strengthening storm and one that detonates offshore can be measured in lives. Rapid intensification compresses the time available for evacuation, and history is full of storms whose final-day transformation turned manageable events into disasters. This research does not eliminate that uncertainty, but it narrows it. By identifying two independently verifiable climate signals, one oceanic and thermal, one atmospheric and dynamical, and mapping exactly where each one loads the dice, the study converts a chaotic-seeming year-to-year pattern into a structure that seasonal forecasts can begin to exploit. As offshore intensification events continue to rise worldwide, that kind of foresight will only grow more valuable.

Subject of Research: Interannual variability of offshore tropical cyclone rapid intensification in the western North Pacific

Article Title: Interannual variability of tropical cyclone rapid intensification in offshore regions of the western North Pacific

Article References: Chen, Z.-X., Guo, Y.-P., & Chen, X. (2026). Interannual variability of tropical cyclone rapid intensification in offshore regions of the western North Pacific. Climate Dynamics, 64(11), Article 444. https://doi.org/10.1007/s00382-026-08405-8

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08405-8

Keywords: tropical cyclones, rapid intensification, western North Pacific, ENSO, La Niña, Hadley circulation, South China Sea, Philippine Sea, vertical wind shear, upper-ocean heat content, seasonal prediction, Climate Dynamics