The mechanism at the heart of the study is stratosphere–troposphere exchange, or STE, the process by which air crosses the tropopause, the boundary that separates the stratified, ozone-rich stratosphere from the well-mixed troposphere below. Stratospheric ozone constitutes a primary natural source of background tropospheric ozone, and when intrusions of stratospheric air descend deep enough, they can measurably raise ozone concentrations in the lower atmosphere. Most research on this process has focused on the mid-latitudes, where jet streams and storm systems routinely fold the tropopause. What makes the new analysis striking is that it documents an unusually deep intrusion at tropical latitudes, reaching the lower troposphere above a city that sits 2,300 meters above sea level in a mountain-ringed basin.

The story begins with Rossby waves, the vast undulations of the jet stream that encircle the planet. Between 6 and 14 March 2016, an upper-level trough associated with a mid-latitude cyclone amplified over the eastern North Pacific and propagated equatorward toward central Mexico. As the wave grew, the potential vorticity contours that trace the dynamical tropopause began to overturn, a signature of anticyclonic Rossby wave breaking. A filament of high-PV stratospheric air, known as a PV streamer, stretched southward below 20 degrees north latitude. Part of this filament pinched off from the main stratospheric reservoir, forming a cut-off low over Mexico, a closed circulation that kept the disturbance anchored over the region for days.

With the cut-off low in place, the tropopause itself began to fold. The researchers, using the ERA5 reanalysis from the European Centre for Medium-Range Weather Forecasts, tracked the 2 PVU surface, the conventional dynamical marker of the tropopause, as it plunged below 400 hectopascals, well into the middle and lower troposphere. Vertical cross-sections revealed sharp potential vorticity gradients and locally reduced static stability within the intrusion, hallmarks of stratospheric air descending into tropospheric surroundings. The fold persisted for roughly three days, from 9 to 11 March, and was intensified by the subtropical jet, whose winds exceeded 50 meters per second, and by the elevated terrain of central Mexico, which helped steer the descending air toward the surface.

To confirm that air parcels actually crossed the tropopause rather than merely deforming it, the team computed backward trajectories with the Lagrangian Analysis Tool LAGRANTO, initialized over Mexico City between 700 and 100 hectopascals. The results were unambiguous: parcels arriving near 700 hectopascals over the city between 11 and 13 March had originated in the stratosphere above 250 hectopascals, within the fold region. Two distinct descent pathways emerged. One group of parcels plunged rapidly, losing more than 200 hectopascals of pressure in 48 hours, but arrived with diluted potential vorticity, suggesting heavy mixing along the way. A second group descended more gradually, preserving potential vorticity values near 2 PVU aloft and carrying ozone tracer concentrations of roughly 8 times 10 to the minus 8 kilograms per kilogram down to the 750 to 700 hectopascals layer, directly above the city.

Isentropic analysis added a third line of evidence. By interpolating winds, potential vorticity, and ozone onto constant potential-temperature surfaces between 320 and 340 Kelvin, the researchers quantified quasi-horizontal transport along sloping isentropic surfaces, the efficient conveyor that carried ozone-rich air from mid-latitude source regions toward central Mexico. Between 8 and 10 March, ozone-rich air remained confined to the 340 to 360 Kelvin range, above the dynamical tropopause. After 10 March, the signal descended progressively, and by 12 and 13 March air with stratospheric characteristics appeared on the tropospheric side of the tropopause near 315 to 320 Kelvin. The timing matched the surface observations almost perfectly: maximum downward transport occurred approximately one day before the peak in surface ozone mixing ratios.

Satellite observations corroborated the picture from orbit. Total column ozone measurements from the Earth Polychromatic Imaging Camera on the Deep Space Climate Observatory showed an enhancement across Mexico, while tropospheric ozone columns increased over eastern Mexico on 10 and 11 March, coinciding with the development of the fold. Independent measurements from the Ozone Mapping and Profiler Suite on the Suomi National Polar-orbiting Partnership satellite displayed similar large-scale features. The tropospheric signal over central Mexico itself was weaker, which the authors attribute to the high elevation of the terrain and the reduced sensitivity of satellite instruments in the upper troposphere and lower stratosphere.

Crucially, the study does not claim that stratospheric air alone caused the exceedances. The authors are careful to frame the intrusion as a preconditioning mechanism: the descending stratospheric air elevated the background ozone over the region, and local photochemical production from traffic and industry then pushed concentrations over regulatory thresholds. This distinction matters for attribution. In a polluted tropical megacity, stratospheric transport interacts with local precursor emissions, boundary-layer dynamics, and basin circulations, making it extraordinarily difficult to separate the large-scale contribution from local production. The reanalysis-based diagnostics cannot quantify the exact fraction of surface ozone that came from the stratosphere, and the authors acknowledge that ERA5 may smooth out the sharpest small-scale features of folds and mixing layers.

The implications extend well beyond a single event. When the team examined four other major high-ozone episodes in Mexico City, on 25 December 2015, 14 April 2007, 10 April 2019, and 22 February 2024, they found the same dynamical fingerprint: tropopause folds preceding the surface enhancements by a few days, with both shallow and deep intrusions acting as effective pathways for stratospheric ozone. Elevated ozone mixing ratios during the March 2016 event were also detected over Guadalajara, Monterrey, San Luis Potosí, and Torreón, showing that a single breaking Rossby wave can synchronize pollution events across distant metropolitan areas. Cities located in elevated tropical basins appear especially vulnerable, because the altitude brings the lower troposphere closer to the descending stratospheric air, while lower-elevation regions experience weaker surface impacts.

The findings carry a practical message for air-quality forecasting. Rossby wave amplification, PV streamer formation, cut-off low development, and depression of the dynamical tropopause are all large-scale phenomena that numerical weather prediction models can identify days before the associated intrusion arrives. In the March 2016 case, ozone-rich air reached the lower troposphere about a day before the environmental contingency began, suggesting that monitoring potential vorticity on isentropic surfaces and tracking the 1.5 to 2 PVU tropopause could provide genuine early warning of elevated ozone risk during boreal winter and early spring. The authors caution that a single reanalysis-based case cannot establish operational skill, and that multi-event hindcast experiments are needed to quantify achievable lead times and false-alarm rates. But the core insight stands: weather disturbances thousands of kilometers away, in the mid-latitude jet stream, can reach into the tropics and shape the air that millions of city dwellers breathe.

Subject of Research: Stratosphere–troposphere exchange and its influence on surface ozone pollution over tropical Mexico

Article Title: Stratosphere–troposphere exchange and surface ozone pollution over tropical regions: a case study of Rossby wave breaking and tropopause folding

Article References: Lopez-Bravo, C., Caetano, E., & Franco-Díaz, A. (2026). Stratosphere–troposphere exchange and surface ozone pollution over tropical regions: a case study of Rossby wave breaking and tropopause folding. Weather and Climate Dynamics, 7(4), 1995-2013. https://doi.org/10.5194/wcd-7-1995-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1995-2026

Keywords: stratosphere-troposphere exchange, ozone pollution, Mexico City, Rossby wave breaking, tropopause fold, potential vorticity, cut-off low, ERA5 reanalysis, Lagrangian trajectories, isentropic transport, air quality forecasting, subtropical jet

The research, led by Martin Andrews of the Met Office Hadley Centre in the United Kingdom, brought together twelve climate models from eleven modelling centres around the world under the umbrella of the Quasi-Biennial Oscillation initiative, known as QBOi, part of the World Climate Research Programme’s activities on atmospheric processes. The team ran three coordinated experiments covering the period 1979 to 2020. In the first, the models generated their own QBOs freely, as they normally would. In the second, the tropical stratospheric winds were nudged, or gently relaxed, towards the winds observed in the ERA5 reanalysis dataset, effectively forcing every model to carry the real QBO. In the third, the QBO was suppressed altogether by relaxing the winds towards a climatological average, erasing the oscillation entirely. Comparing the three allowed the researchers to isolate, with unusual clarity, what a realistic QBO does and does not deliver in terms of remote atmospheric influence.

The headline target was the Holton-Tan Effect, the classic teleconnection first described in 1980, in which the phase of the QBO modulates the strength of the Northern Hemisphere winter polar vortex, the swirling westerly jet known as the Polar Night Jet that circles the Arctic stratosphere. Observations show that during easterly QBO winters the polar jet weakens, because the zero-wind line separating tropical and extratropical flow shifts poleward, channelling large-scale planetary waves into the vortex where they deposit easterly momentum and slow it down. During westerly QBO winters the waves spread more freely and the vortex strengthens. The observed signal is dramatic: in January the difference between easterly and westerly composites reaches about 20 metres per second in the reanalysis data.

What the models produced was far more modest. In the free-running experiment, the ensemble mean captured a statistically significant but weak Holton-Tan-like response, peaking at only about 2 metres per second, roughly a tenth of the observed magnitude, and it entirely missed the reversal of the signal seen in observations in March. When the models were nudged towards the observed QBO, the response roughly doubled, peaking near 5 metres per second in December and January, and the number of models showing a negative polar jet response during easterly phases rose from five out of eleven to ten out of twelve. Yet even this improved response amounted to only about 20 per cent of the observed signal. Statistical sub-sampling showed that the nudged and free-running distributions of polar jet responses were not significantly different from each other, suggesting that the improvement came mainly from correcting the too-weak amplitude of the simulated QBOs rather than from strengthening the underlying teleconnection mechanism itself.

The study also probed how well the QBO phase influences sudden stratospheric warmings, the dramatic events in which the polar vortex breaks down and stratospheric temperatures can leap by tens of degrees within days. Observations indicate roughly 0.9 warmings per easterly-QBO winter compared with about 0.5 per westerly-QBO winter, and warmings tend to arrive about a month earlier during easterly phases. The models captured only a small and marginally significant increase in warming frequency during easterly phases, and even then only when the analysis was restricted to models with a significant Holton-Tan Effect. More strikingly, neither the free-running nor the nudged experiments reproduced the observed separation in warming timing between the two QBO phases, nor the observed tendency for final warmings of spring to arrive earlier after westerly-QBO winters. The team notes that models with weaker climatological polar vortices produce more warmings overall, a bias that may be muddying the picture.

One genuinely encouraging result concerned predictability. When the nudged ensemble was compared directly with the observed January polar jet over the 42-year period, the correlation was a highly significant 0.61, implying that up to 30 to 40 per cent of the observed year-to-year variability of the January vortex may be attributable to the equatorial stratosphere, including the QBO. In the experiment with the QBO erased, the correlation collapsed to an insignificant 0.06, despite identical sea surface temperatures, sea ice and atmospheric forcings. This is a powerful demonstration that a forecast system carrying the true phase and amplitude of the QBO holds real predictive information about the winter polar vortex, information that vanishes when the oscillation is removed. The authors suggest that the combination of reasonable skill with a too-weak predicted signal may reflect the well-known signal-to-noise paradox that afflicts seasonal-to-decadal forecasting.

The connection to surface weather proved harder to pin down. The North Atlantic Oscillation, the seesaw of pressure between the Azores and Iceland that steers winter storms across Europe, is only weakly correlated with the QBO even in observations, with correlations of about 0.2 to 0.3 in January. In the model ensembles the correlation was smaller still, generally below 0.1 and statistically insignificant, accounting for less than one per cent of the NAO’s variability. Even when the analysis was restricted to the subset of models with a significant Holton-Tan Effect, only a weak positive January correlation emerged, and its significance vanished when an alternative two-station NAO index was used. Because the downward pathway from a strong or weak polar vortex to the tropospheric circulation is itself well captured by the models, the failure appears to lie in the weak QBO-to-vortex link upstream, which starves the surface response of amplitude.

In the North Pacific, the picture was more nuanced. Observations show that the QBO phase shifts the subtropical jet, with easterly-QBO winters producing a weakening of the jet around 30 degrees north and a strengthening near 50 degrees north, a signature of a slight northward displacement. The nudged ensemble captured a significant wind anomaly on the equatorward flank of the jet, something the free-running ensemble failed to produce, and it also reproduced observed patterns of tropical precipitation anomalies over the Pacific that may act as a tropospheric bridge for QBO influences. However, the model response poleward of 35 degrees north actually degraded with nudging, and the entire latitudinal pattern appeared shifted about five degrees too far north, a systematic bias the authors could not fully explain. The confounding influence of the El Nino-Southern Oscillation loomed large here: filtering to only ENSO-neutral winters changed both the sign and the spatial structure of the wind and precipitation signals, underscoring how entangled these tropical drivers are.

The overall message is a careful one. Bias-correcting the QBO in climate models demonstrably helps: it strengthens the polar jet response, improves the representation of tropical precipitation and subtropical wind anomalies, and unlocks genuine predictive skill for the January vortex. But a realistic QBO alone is not sufficient. The models’ internal machinery, the way planetary waves propagate, break and deposit momentum in response to the equatorial winds, remains too feeble, and key observed signatures such as the timing of stratospheric warmings and the March reversal of the Holton-Tan signal remain out of reach. Whether the remaining gaps stem from biases in the extratropical stratosphere or from a fundamentally incomplete representation of the coupling mechanisms is, the authors conclude, a central question for the next generation of experiments. For now, the QBO’s whisper to the mid-latitudes is real, measurable, and only partially heard by the models that try to simulate it.

Subject of Research: The influence of the equatorial Quasi-Biennial Oscillation on extratropical stratospheric and tropospheric circulation in nudged multi-model climate experiments

Article Title: Extratropical teleconnections in a multi-model ensemble nudged towards the observed QBO

Article References: Andrews, M. B., Butchart, N., Anstey, J. A., Bednarz, E., Elsbury, D., García-Franco, J. L., Kumar, V., Palmeiro, F. M., Trencham, N. E., Yoshida, K., Chai, Z., Hong, D.-C., Huang, K., Jaison, A. M., Kawatani, Y., Knight, J. R., Lin, P., Lott, F., Lu, Y., … Xie, J. (2026). Extratropical teleconnections in a multi-model ensemble nudged towards the observed QBO. Weather and Climate Dynamics, 7(3), 1797-1820. https://doi.org/10.5194/wcd-7-1797-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1797-2026

Keywords: Quasi-Biennial Oscillation, stratosphere, polar vortex, Holton-Tan Effect, sudden stratospheric warming, North Atlantic Oscillation, subtropical jet, climate models, teleconnections, QBOi, nudging, ERA5 reanalysis

The research, conducted by Hak-Jun Lee of GeoSystem Research Corporation, Sang-Wook Yeh of Ewha Womans University, and Myong-In Lee of Ulsan National Institute of Science and Technology, focuses on a Pacific sea surface temperature pattern known as a tripole. This pattern, familiar to climate scientists from studies of the Interdecadal Pacific Oscillation, consists of anomalies in three key basins: the Northwest Pacific, the central equatorial Pacific, and the Southwest Pacific. When one region is warmer than average while the other two are cooler, and vice versa, the ocean surface takes on a three-poled structure that can persist for years and influence atmospheric circulation far beyond the tropics.

The challenge the researchers faced is that this tripole pattern does not exist in isolation. El Niño and its cold counterpart La Niña, together comprising the El Niño and Southern Oscillation, or ENSO, imprint their own signature on Pacific sea surface temperatures and dominate the tripole signal. To isolate the part of the variability that operates independently, the team constructed what they call a linear ENSO-independent tripole index, abbreviated EITI. The method is statistically elegant: they computed the tripole index from sea surface temperature anomalies averaged over the three Pacific regions during boreal summer, then subtracted the portion of that index that could be linearly explained by the simultaneous Niño3.4 index, the standard yardstick of ENSO strength measured in the central equatorial Pacific. What remains is a clean measure of Pacific variability that has nothing to do with concurrent ENSO conditions.

Using observed sea surface temperatures from the NOAA Extended Reconstructed Sea Surface Temperature version 5 dataset, together with precipitation data from the Global Precipitation Climatology Project and atmospheric reanalysis from the Copernicus ERA5 product, the researchers examined what happens during the positive phase of the EITI, when the tripole pattern reaches a particular configuration. The results were striking. A positive EITI phase is associated with enhanced precipitation along the East Asian monsoon rainband, meaning wetter conditions across the densely populated monsoon region. The atmospheric machinery behind this response involves two well-known components of the summer circulation: the subtropical jet stream and the western North Pacific subtropical high.

During positive EITI phases, the team found that the subtropical jet shifts equatorward, toward the equator, while the western North Pacific subtropical high strengthens. Both changes matter enormously for the monsoon. The subtropical jet acts as a guide rail for storm tracks and is intimately tied to the position of the meiyu-baiu rainband that stretches from eastern China through Korea and Japan; a shift in the jet translates directly into a shift in where the heaviest summer rains fall. The subtropical high, meanwhile, is the great anticyclonic circulation over the western Pacific that steers moist air masses toward East Asia and controls the timing and intensity of the monsoon onset. When the high strengthens, it pumps more moisture-laden air toward the rainband, amplifying rainfall. The EITI thus provides a single oceanic index that captures the coordinated behavior of these two circulation systems.

But the most consequential finding of the study concerns predictability rather than mechanism. The researchers turned to the Decadal Climate Prediction Project component of the sixth Coupled Model Intercomparison Project, known as CMIP6 DCPP. This international effort coordinates hindcast experiments in which state-of-the-art climate models are initialized with observed ocean and atmosphere conditions and then run forward for several years, allowing scientists to test how far ahead different climate features can actually be predicted. The team evaluated how well the models predicted the winter Niño3.4 index and the summer EITI at various lead times, comparing the forecasts against observations.

The comparison revealed a sharp contrast between the two indices. The boreal winter Niño3.4 index, the classic ENSO measure, is skillfully predicted at a lead time of one year, but its prediction skill declines rapidly as the lead time increases. This mirrors the well-documented spring predictability barrier and the fundamental limits of ENSO forecasting, which rarely extends useful skill beyond a year. The summer EITI, by contrast, remains skillfully predicted up to a lead time of four years. Because the EITI is independent of ENSO, its persistence reflects the slower, longer-lived dynamics of the broader Pacific climate system, including the decadal-scale ocean memory that underlies patterns such as the Pacific Decadal Oscillation and the Interdecadal Pacific Oscillation.

Even more importantly, the link between the EITI and the East Asian summer monsoon survives the forecasting test. The researchers found that the EITI and EASM relationship is most robustly reproduced in the multi-model ensemble, the combined output of many different climate models, and that this relationship remains statistically significant at lead times of up to three years. In other words, when the models collectively predict the state of the ENSO-independent Pacific tripole several years ahead, that prediction carries genuine information about the likely behavior of the East Asian summer monsoon. This is a remarkable result for a climate feature that has historically resisted prediction beyond a single season, and it suggests that the multi-model ensemble approach, which averages out the idiosyncratic errors of individual models, is particularly effective at capturing this slow ocean-atmosphere coupling.

The implications extend well beyond the academic literature. Multi-year lead forecasts of monsoon rainfall would transform water resource management across East Asia, allowing reservoir operators, agricultural planners, and disaster preparedness agencies to anticipate prolonged wet or dry phases years in advance rather than reacting to seasonal forecasts issued only months ahead. The study also adds to a growing body of work showing that components of the climate system other than ENSO, including the Indian Ocean basin mode, the tropical Atlantic, and Pacific decadal variability, contribute substantially to East Asian climate. By explicitly removing the ENSO contribution, the new EITI framework clarifies how much of the monsoon’s variability is governed by these slower, more predictable oceanic patterns.

Caveats remain, as they always do in climate science. The analysis relies on linear regression to strip out the ENSO signal, and nonlinear interactions between ENSO and the tripole pattern could complicate the picture in ways a linear framework does not capture. The robustness of the EITI and EASM relationship also varies among individual models, appearing most reliably only in the multi-model ensemble, a reminder that model biases in simulating Pacific sea surface temperatures and monsoon dynamics still limit how confidently these results can be translated into operational forecasting. Nevertheless, the core message of the study is clear and potentially transformative: the Pacific Ocean stores climate information that persists for years, and by learning to read the ENSO-independent part of that signal, scientists may finally extend the horizon of East Asian summer monsoon prediction from months to years, offering societies across the monsoon domain a far longer window of preparation for the rains that shape their lives.

Subject of Research: ENSO-independent Pacific sea surface temperature variability and its multi-year predictability for the East Asian summer monsoon

Article Title: The influence of linear ENSO-independent Pacific variability on the East Asian summer monsoon and its predictability

Article References: Lee, H.-J., Yeh, S.-W., & Lee, M.-I. (2026). The influence of linear ENSO-independent Pacific variability on the East Asian summer monsoon and its predictability. Climate Dynamics, 64(11), Article 445. https://doi.org/10.1007/s00382-026-08398-4

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08398-4

Keywords: East Asian summer monsoon, ENSO, Pacific tripole, sea surface temperature, CMIP6, decadal climate prediction, subtropical jet, western North Pacific subtropical high, multi-year predictability, Climate Dynamics, Pacific Decadal Oscillation, climate modeling