Stratosphere-troposphere exchange, or STE, is far more than a technical curiosity. It governs the global atmospheric budget of chemical constituents, including the water vapor and aerosols that shape the planet’s radiative balance. When air rises from the surface into the stratosphere through the so-called troposphere-to-stratosphere transport pathway, it can carry anthropogenic pollutants along with it, altering stratospheric chemistry and feeding back on climate. In the reverse direction, ozone-rich stratospheric air can sink into the troposphere, where it threatens human health and ecosystems. Because these exchanges span scales from planetary circulations down to small-scale turbulence, scientists have struggled to fully qualify, let alone quantify, the contributions of individual mechanisms. Tropical cyclones, with their towering convective thunderstorms, have recently emerged as one important piece of the puzzle.

Earlier work had established that roughly 10 to 15 percent of the global lower stratospheric water vapor budget can be traced to overshooting convection in the tropics, and that around 15 percent of that overshooting convection occurs inside tropical cyclones. Satellite observations have revealed pronounced moistening of the lower stratosphere above storm centers and even a downward flux of ozone through the eye. Yet the exact dynamics of how air crosses the tropical tropopause, a transition that in the tropics takes place across an extended layer called the tropical tropopause layer, or TTL, remain incompletely understood. A team led by Massimo Martina and Petr Šácha of Charles University in Prague suspected a missing ingredient: the mountains beneath the storm.

The researchers’ hypothesis rested on a well-known but underexplored coupling. Typhoons, the most intense manifestation of tropical cyclones, pack maximum winds of at least 33 meters per second and drive deep convection that spans the entire troposphere. When such winds encounter mountains rising more than 2,000 meters, as they do across the Philippine archipelago, they generate orographic gravity waves, atmospheric ripples that propagate upward from the terrain. These waves are known to initiate precipitation and modulate convection, and in the extratropics their role in stratosphere-troposphere exchange is well documented. In the tropics, however, the possibility that mountain waves could boost transport during a typhoon landfall had received virtually no attention.

To test the idea, the team ran a high-resolution simulation of Molave’s landfall using the Weather Research and Forecasting model, configured with 180 vertical levels and a horizontal grid spacing of 3 kilometers, fine enough to resolve convection explicitly rather than approximating it with a parameterization. The simulation, driven by ERA5 reanalysis data and initialized with high-resolution terrain information, accurately reproduced the typhoon’s observed track across the Philippines, along with its minimum central pressure of 965 hectopascals and mean winds of roughly 38 meters per second, according to Japanese Meteorological Agency records. Some expected intensity biases appeared, but the simulation captured the essential spatiotemporal behavior of the storm, making it a trustworthy laboratory for studying terrain-storm interactions.

Into this simulated storm the researchers released four million virtual air parcels from an idealized surface source near the typhoon’s path, using the Lagrangian particle dispersion model FLEXPART-WRF. Each parcel’s trajectory was tracked, and a newly developed algorithm called AirIntrusions flagged the precise moment and location where any parcel crossed the lapse rate tropopause, the cold point tropopause, or the planetary boundary layer. The algorithm even recorded transition times and residence times, sorting intrusions into classes ranging from brief excursions of under three hours to prolonged stays exceeding nine hours. The results were striking. As Molave made landfall on 25 October 2020, intrusions spiked abruptly, and in total about 36 percent of all parcels reached the TTL, while roughly 13 percent penetrated into the stratosphere itself. The fastest parcels made the journey from the boundary layer to the stratosphere in just three to six hours.

Mapping where these crossings clustered revealed a telling pattern. The distribution of intrusions did not spread uniformly along the typhoon’s track; instead, it broke into several distinct hotspots, many of them aligned with subtle features of the underlying topography. In regions directly beneath the storm’s strongest updrafts, parcels made deep, one-way penetrations across both tropopause boundaries, driven by the ferocious overshooting convection at the cyclone’s core. But in hotspots over mountain ridges away from the center, a different and previously unrecognized mechanism dominated: parcels executed multiple shallow, back-and-forth crossings, oscillating around the tropopause boundaries. Approximately 73 percent of all parcels entering the TTL or stratosphere engaged in such repeated crossings, some more than ten times, suggesting a persistent two-way mixing process rather than a single convective punch.

Analysis of the Richardson number and turbulent kinetic energy along the parcel trajectories pointed the finger squarely at orographic gravity waves. In cross-sections over the mountains, the team identified vertically propagating waves with vertical wind amplitudes of about 3 meters per second that modulated both tropopause definitions and extended into the stratosphere. Where these waves broke, they generated pockets of dynamical instability and enhanced turbulence, precisely the conditions needed to mix parcels already hovering near the tropopause across the boundary. Over the typhoon center, by contrast, the cross-sections showed convective instability at the crowns of overshooting updrafts, where first-time deep intrusions occurred. The two mechanisms acted in concert: the typhoon’s core lifted boundary layer air rapidly toward the tropopause, and the mountains, through wave breaking and orographic lifting, provided additional gateways for entry and continued exchange.

The implications extend well beyond a single storm. Parcels that reached the TTL tended to settle into that layer for the remainder of the simulation, hinting at a possible role for orography-typhoon interactions in feeding phenomena such as the Asian Tropopause Aerosol Layer, a persistent veil of aerosols near the tropical tropopause with climate relevance. Parcels that crossed into the stratosphere proper can subsequently be swept around the globe by the Brewer-Dobson circulation, spreading tropical surface emissions far from their source in a region of the atmosphere where the Philippines sits squarely among documented pollution hotspots. Even modest errors in representing these pathways could translate into significant biases in modeled stratospheric composition and radiative forcing.

The study also carries a pointed message for climate model developers. Global chemistry-climate and earth system models cannot resolve individual mountains or gravity waves, relying instead on parameterizations of subgrid-scale orography. The new results demonstrate that even fine variations in terrain leave discernible fingerprints on the geography of tropopause crossings, meaning that inadequate representation of these effects could bias modeled transport in the upper troposphere and lower stratosphere, a region critical to the global radiation budget. The authors note that their conclusions rest on passive tracers without deposition processes and on a turbulence scheme not originally designed for the strongly stable stratification near the tropopause, and they call for follow-up experiments at even finer resolution. Still, the core message stands: typhoons do not just devastate the ground beneath them. When they collide with mountains, they can punch surface air, and everything it carries, into the stratosphere, at speeds and by routes the atmosphere’s accountants had never tallied.

Subject of Research: How orography and orographic gravity waves enhance troposphere-to-stratosphere transport during a tropical typhoon landfall

Article Title: The impact of orography on the troposphere-to-stratosphere transport during a typhoon event in the tropics

Article References: Martina, M., Villalba-Pradas, A., Bartoň, Š., & Šácha, P. (2026). The impact of orography on the troposphere-to-stratosphere transport during a typhoon event in the tropics. Atmospheric Chemistry and Physics, 26(19), 13845-13860. https://doi.org/10.5194/acp-26-13845-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-13845-2026

Keywords: typhoon, stratosphere-troposphere exchange, orographic gravity waves, tropical tropopause layer, Philippines, Typhoon Molave, WRF model, FLEXPART-WRF, convection, pollution transport, turbulence, climate modeling

The research, led by Anjela A. Ilagan of the Philippine Atmospheric, Geophysical and Astronomical Services Administration (DOST-PAGASA) and the University of the Philippines Diliman, together with Olivia C. Cabrera and Marcelino Q. Villafuerte II, tackles a persistent blind spot in Philippine hazard science. Most previous investigations of the country’s storm surges have concentrated either on individual catastrophic events or on numerical simulations of specific tropical cyclones. What was missing was a systematic, observation-based reconstruction spanning the whole archipelago and many decades. Because the Philippines is made up of more than seven thousand islands with an extraordinarily long and complex coastline, local bathymetry, shelf width, and coastal orientation can dramatically alter how high the sea rises when a typhoon makes landfall. A national picture assembled from actual measurements, rather than models alone, is therefore an essential foundation for hazard assessment.

The team’s approach hinged on a deceptively simple but technically demanding manipulation of tide gauge data. Tide gauges record the total water level at the coast, which is a mixture of the predictable astronomical tide, longer-term sea level variations, and the meteorological contribution from winds and pressure changes associated with storms. To isolate the storm signal, the researchers first removed the tidal component using harmonic analysis with the UTide software package, which fits the known tidal constituents to the observed record and allows the tide to be subtracted out. What remains is the residual sea level, the non-tidal part of the water level that carries the fingerprint of storm-driven water pile-up. The raw residuals were then smoothed with a low-pass filter to suppress high-frequency noise, ensuring that short-lived fluctuations unrelated to genuine surge events would not contaminate the detection.

Identifying storm surge events from residuals required a detection threshold, and here the researchers made a methodologically important choice. Rather than applying a single fixed cutoff across the entire country and record, they used an annually varying threshold, which accounts for the fact that baseline sea levels and seasonal conditions differ from year to year and from station to station. This approach, consistent with skew surge methods used by established sea level monitoring facilities, allows the detection algorithm to remain sensitive to genuine surge events even in years or locations where the background water level is naturally higher. Applying this threshold to the de-tided records across multiple coastal stations, the team identified 133 distinct storm surge events over the 77-year study period, the most comprehensive instrument-based storm surge inventory ever assembled for the Philippines.

The statistics that emerged from this inventory carry a message with direct implications for coastal safety. The majority of detected storm surges, measured as pure surge residuals, reached heights of only 20 to 50 centimeters, and only about 5 percent of events exceeded one meter. On its face, that might suggest that surges in the Philippines are generally modest. But the picture changes sharply when the researchers examined storm tide, the combined water level produced when the storm surge coincides with the astronomical tide. In approximately 39 percent of events, the storm tide exceeded one meter, nearly eight times the proportion seen for surge alone. The lesson is unambiguous: the timing of a typhoon’s arrival relative to the tidal cycle can transform a manageable rise in water into a genuinely dangerous coastal flood. A 40-centimeter surge arriving at high tide can inundate areas that the same surge at low tide would leave untouched.

The single largest surge in the record was measured at Pasacao, in the province of Camarines Sur, during Typhoon Molave in 2020, when the water level anomaly reached 211 centimeters, more than two meters above the expected tide. Molave, known locally as Quinta, swept across the Bicol region in late October 2020, and the Pasacao observation stands as a stark reminder of what even a storm that is not among the most intense on record can deliver when its winds align favorably with coastal geography. The study’s case analyses of Molave and Super Typhoon Goni, which struck the same general region just days later in early November 2020, highlighted how the orientation of the coastline relative to the storm’s track and the phase of the tide jointly determine the severity of coastal impacts. Two storms hitting within a week of each other, with different tracks and tidal timings, produced markedly different surge outcomes.

Mapping the geographic distribution of the 133 events revealed a pronounced regional pattern that reflects the underlying physics of surge generation. High storm surges clustered along the coasts of Catanduanes, Camarines Sur, Eastern Samar, Cagayan, Batanes, and Palawan. What these disparate locations share is a shallow and wide continental shelf, a bathymetric configuration that favors surge amplification. When storm winds push ocean water across a broad, shallow shelf, friction with the seabed constrains the water’s vertical escape and forces it to pile up at the coast, a mechanism that deep water adjacent to steep coasts does not permit. Eastern Samar’s presence in this list will resonate with anyone familiar with Haiyan, whose catastrophic surge in Leyte Gulf was amplified by exactly this kind of shelf-driven shoaling. Equally telling is where surges were largely absent: Mindanao, the large southern island, registered few significant events, a pattern consistent with its exposure to a less typhoon-frequented portion of the western North Pacific basin.

Beyond its scientific findings, the database itself may prove to be the study’s most consequential product. A validated, observation-based catalogue of historical surge events spanning nearly eight decades provides exactly the kind of ground truth that storm surge forecasters, hazard mappers, and model developers need. Numerical surge models, from the high-resolution unstructured-mesh systems used in operational hurricane forecasting to global reanalyses of extreme sea levels, depend on historical observations for calibration and validation. Extreme value statistics, the branch of statistics used to estimate the probability of rare events such as hundred-year surges, likewise require long, homogeneous observational records to produce trustworthy return-period estimates. By anchoring such analyses in real tide gauge measurements from Philippine waters, the new inventory reduces the reliance on extrapolations from other coastlines and improves the credibility of national hazard assessments, evacuation planning, and coastal infrastructure design.

The study also arrives at a moment when the stakes of surge forecasting in the Philippines are rising. Mean sea level is climbing globally, and even a modest long-term rise raises the baseline upon which every future storm surge builds, effectively converting yesterday’s moderate surge into tomorrow’s damaging flood. The researchers’ use of an annually varying threshold partly reflects this shifting baseline, and their 1947-to-2024 record offers a valuable window into how surge occurrences have unfolded across a period of both changing climate and changing observation technology. The authors acknowledge the National Mapping and Resource Information Authority’s Hydrography Branch for providing the tide gauge data, and the work was carried out without dedicated external funding, a testament to the value of careful analysis of existing national observational assets.

For a country that sits squarely in the most active tropical cyclone basin on Earth, with roughly twenty storms entering its area of responsibility in a typical year, the ability to look back through 77 years of measured sea level data is more than an academic exercise. It is a way of letting the ocean itself testify about which coasts are most exposed, how badly the tide can conspire with the wind, and what the worst-case scenarios look like when the next Molave or Goni forms over the warm waters of the Pacific. The 133 events now catalogued in this inventory are, in effect, 133 rehearsals for future disasters, and the researchers hope that forecasters, planners, and coastal communities across the archipelago will use them to be better prepared when the next rehearsal becomes the real thing.

Subject of Research: Historical storm surge events in the Philippines reconstructed from tide gauge observations

Article Title: Investigating historical storm surge occurrences in the Philippines from tide gauge observations

Article References: Ilagan, A. A., Cabrera, O. C., & Villafuerte, M. Q., II (2026). Investigating historical storm surge occurrences in the Philippines from tide gauge observations. Natural Hazards, 122(18), Article 625. https://doi.org/10.1007/s11069-026-08360-x

Image Credits: AI Generated

DOI: 10.1007/s11069-026-08360-x

Keywords: storm surge, storm tide, Philippines, tide gauge, tropical cyclone, Typhoon Molave, Super Typhoon Goni, sea level, coastal flooding, hazard assessment, UTide, Natural Hazards