This study combines objective in situ aircraft turbulence observations with atmospheric fields from the ERA5 reanalysis. Turbulence events are identified from automated EDR reports, while ERA5 is used both to characterize the large-scale atmospheric circulation associated with each event and to evaluate the corresponding turbulence-relevant atmospheric diagnostics.

ERA5 is the fifth-generation global atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). ERA5 provides a physically consistent reconstruction of the global atmosphere with hourly temporal resolution and approximately 31 km horizontal resolution, covering the period from 1950 to the present . Owing to its temporal consistency and comprehensive representation of the atmospheric state, ERA5 has become one of the standard datasets for climatological analyses and analogue-based attribution studies.

Observed turbulence events are identified using automated in situ aircraft reports based on the EDR, which provides an objective and largely aircraft-independent measure of turbulence intensity and it is the standard ICAO metric for reporting aircraft turbulence intensity . Unlike PIREPs, EDR offers a quantitative estimate of turbulence that is largely independent of aircraft type and operational conditions, making it particularly suitable for climatological analyses. Following operational practice, this study focuses on moderate-or-greater turbulence reports (EDR≥0.2 m2/3s-1). This threshold is used exclusively to identify objectively observed turbulence events and should not be interpreted as an operational definition of “extreme” turbulence.

To investigate changes in aviation-relevant atmospheric environments and assess the influence of climate change, we analyse the following atmospheric variables:

500 hPa geopotential height (Z500): used exclusively to identify circulation analogues by characterizing the large-scale atmospheric circulation associated with each event. Z500 provides a robust description of synoptic-scale weather systems, including ridges, troughs, cut-off lows, cyclones and anticyclones, which control the upper-level dynamical environment.

Throughout the analysis, Z500 is used exclusively to identify circulation analogues, whereas the remaining atmospheric variables are evaluated only after analogue selection. Furthermore, VWS, DEF and TI1 are calculated around 250 hPa being representative of commercial flights cruise altitude. ERA5 ensures spatial and temporal consistency across decades, making it possible to identify robust circulation analogues and perform circulation-conditioned attribution analyses .

To evaluate whether observed changes in turbulence metrics are linked to anthropogenic climate change, we implement an analogue-based attribution framework based on ClimaMeter . This approach identifies atmospheric analogues to isolate the contribution of climate change while conditioning the analysis on comparable large-scale circulation patterns . Rather than attempting to reproduce the complete atmospheric state associated with each turbulence encounter, TurboMeter constrains the synoptic-scale circulation and subsequently evaluates whether turbulence-relevant atmospheric diagnostics differ between historical and present-day climates under comparable circulation conditions. The framework consists of the following steps:

Event Selection. Turbulence-prone days are identified using automated in situ reports from commercial aircraft, obtained from the Meteorological Assimilation Data Ingest System (MADIS) archive maintained by the National Oceanic and Atmospheric Administration (NOAA). These reports are based on the Aircraft Communications Addressing and Reporting System (ACARS) and provide estimates of EDR. We retain only quality-controlled reports above 8 km altitude. Full details of the dataset and preprocessing are given in .

Here, we focus on days in 2024 for which EDR≥0.2 m2/3s-1, corresponding to MOG turbulence events, over four domains: North Atlantic (643 reports), North Pacific (425), East Asia (150), and CONUS (1905)

The turbulence dataset from NOAA MADIS is dominated by U.S. airlines, with more than 50 % of reports occurring over CONUS, whereas East Asia is represented by only about 150 reports. Consequently, conclusions for East Asia should be regarded as preliminary and motivate future analyses based on more regionally balanced datasets.

The attribution gauge indicates the relative influence of natural climate variability vs. climate change on the event, providing information on whether the event was strengthened by climate change or influenced by natural variability. This attribution is based on the differences in the distribution of three major modes of climate variability between the past and present periods as in : El Niño–Southern Oscillation (ENSO), Atlantic Multidecadal Oscillation (AMO), and Pacific Decadal Oscillation (PDO). Firstly, the statistical significance of the difference in phase distributions between the two periods is assessed by using a two-tailed von Mises test as in . Then, by considering each mode as equally contributing, we assign a 1/3 weight to each mode such that the possible gauge positions are :

pointing to the right: no significant differences across ENSO, AMO, or PDO, i.e. there is no influence of natural variability;

The rarity gauge quantifies the rarity of the event's synoptic pattern. This is assessed using the analogue quality metric Q, as defined in , which measures the mean Euclidean distance between the event and its best analogues. To contextualize this value, Q is compared to a reference distribution Qa, representing the Euclidean distances between the best analogues of the analogues of the event. The gauge position is determined as follows:

pointing to the left: for both the past and present periods, Q<P75(Qa), that is similar events have occurred in the past.

High percentiles are used to determine gauge thresholds since the analysis focuses on rare events, which are, by definition, rarely observed.

In both gauges, the needle can take four discrete positions serving as ordinal categories reflecting increasing confidence in the attribution or rarity assessment. These values, designed as visual indicators to convey complex diagnostic outcomes in an intuitive way, are also used to classify the event in the following categories as in :

strengthened by human-driven climate change: both gauge indicators are on red;

This method, as its original source-based ClimaMeter , offers a physically interpretable, observationally grounded alternative to model-only attribution studies. It allows us to evaluate whether the atmosphere, under similar synoptic configurations, is now more prone to producing turbulence than it was decades ago. To display the results we use the same format of ClimaMeter thus presenting in the first row the attribution gauges, in the second row the meteorological and turbulence conditions during the event of interest, in the third row the differences between the present and the past, while the bottom row reports the changes in the monthly occurrence in the two periods and the changes observed over airports. Since our events are all upper-level ones, we cannot detect changes at airports' level. However, we decided to maintain the structure to provide a more general format that can be used also to assess changes at low levels in future works.

The North Atlantic represents one of the busiest flight corridors worldwide and is characterized by strong baroclinicity and a persistent upper-level jet stream, making it particularly prone to clear-air turbulence. For the North Atlantic region, 643 MOG turbulence reports (EDR≥0.2 m2/3s-1) from the NOAA MADIS archive were recorded during 2024, with 25 February 2024 corresponding to the day with the highest number of reports (88). As shown in Fig. , 89 % (78/88) of the reports are concentrated over the western North Atlantic, primarily along the eastern coast of North America and extending northeastward towards Newfoundland and the Labrador Sea.

The circulation-conditioned attribution analysis for this event is summarized in Fig. . The observed atmospheric configuration is characterized by a pronounced upper-level ridge extending across the central North Atlantic (Fig. a), associated with a broad jet stream exceeding 50 ms-1 (Fig. b). Enhanced vertical wind shear develops along the southern flank of the jet (Fig. c), producing elevated values of the Ellrod turbulence index (TI1; Fig. d) over the central and eastern portions of the basin, in good agreement with the spatial distribution of the observed turbulence reports (Fig. ).

The historical and present-day analogues reproduce highly comparable synoptic-scale circulation patterns, as evidenced by the relatively small differences in the Z500 field (Fig. e), although not statistically significant over almost all the domain. This indicates that the analogue-selection procedure successfully constrains the large-scale circulation associated with the observed event and that changes in synoptic-scale patterns are not statistically different. In contrast, the upper-level jet exhibits a moderate strengthening over portions of the eastern North Atlantic and south of Iceland (Fig. f), accompanied by coherent increases in vertical wind shear (Fig. g). These dynamical changes translate into a pronounced enhancement of TI1 over the central North Atlantic (Fig. h), with the close spatial correspondence between VWS and TI1 indicating that the strengthened turbulence-favourable environment is primarily associated with enhanced upper-level shear rather than changes in horizontal deformation.

The attribution gauge indicates that the event was predominantly strengthened by climate change, whereas the rarity gauge suggests that similar synoptic configurations remain characteristic of the winter season in both climatological periods. This interpretation is further supported by the seasonal distribution of the analogues (Fig. i), which remains concentrated during December, January and February in both climate periods, indicating that the comparison is performed under similar seasonal circulation regimes. Although airport-scale diagnostics (Fig. j) are of limited relevance for this upper-level event, they are retained to preserve the standard TurboMeter visualization and facilitate future applications to lower-tropospheric hazards.

To further investigate the physical environment associated with the selected analogues, Figs.  and  present complementary diagnostics that are independent of the analogue-selection procedure.

The additional surface diagnostics reveal that the historical and present-day analogues retain highly comparable synoptic-scale circulation patterns, as confirmed by the limited (and not significant) differences in Z500 (Fig. d). In contrast, sea surface temperatures exhibit widespread positive anomalies across much of the North Atlantic (Fig. h), particularly along the Gulf Stream extension and toward the Labrador Sea, where warming locally exceeds 1–2 °C. Surface wind speed also displays moderate positive anomalies over the central North Atlantic (Fig. l), suggesting a slightly more energetic lower-tropospheric environment under present-day climate conditions. Together, these changes indicate that the analogues evolve over a systematically warmer ocean while maintaining nearly identical large-scale circulation.

The surface warming is reflected in the thermodynamic environment (Fig. ). CAPE remains generally low throughout the basin (Fig. a–d), while CIN exhibits only weak and spatially fragmented differences between the two analogues (Fig. e–h), indicating that large-scale convective instability plays only a secondary role for this event. In contrast, total column water vapour displays a coherent increase across the central and western North Atlantic (Fig. l), consistent with the warmer and moister atmospheric background expected under present-day climate conditions. These suggest that analogues develop within a systematically moister atmosphere that can favor convection and then upper-level turbulence. These thermodynamic changes are indeed accompanied by enhanced upper-level wind shear (Fig. g), ultimately producing larger TI1 values (Fig. h).

By considering all the 643 MOG turbulence reports (EDR≥0.2 m2/3s-1) in the North Atlantic region that occurred in 2024 (see the Supplement) we observe that the meteorological conditions in around 8 % (51 over 643) of events have been strengthened by human-driven climate change, 71 % (457 over 643) have been mostly strengthened by human-driven climate change, and the remaining 21 % (135 over 643) have been likely influenced by both human-driven climate change and natural variability. We do not detect any event only driven by natural variability and for all events we have medium-to-high confidence in our claims.

The North Pacific constitutes one of the most dynamically active aviation corridors, where intense upper-level jet streams, strong baroclinicity, and frequent cyclonic activity create favourable conditions for aviation turbulence throughout the cold season. For the North Pacific region, 425 MOG turbulence reports (EDR≥0.2 m2/3s-1) from the NOAA MADIS archive were recorded during 2024, with 20 December 2024 corresponding to the day with the highest number of reports (104). As shown in Fig. , the observed turbulence reports are organized into two distinct clusters: a northern cluster located close to the west coast of North America (27 %, 28/104) and a larger subtropical cluster extending across the central North Pacific (73 %, 76/104), reflecting the broad-scale structure of the upper-level jet.

The circulation-conditioned attribution analysis for this event is summarized in Fig. . The observed atmospheric configuration is characterized by a deep upper-level trough over the central North Pacific (Fig. a), associated with a broad jet stream exceeding 70 ms-1 (Fig. b). Enhanced vertical wind shear develops along the southern flank of the jet (Fig. c), producing a broad region of elevated TI1 values (Fig. d) matching the spatial distribution of the observed turbulence reports (Fig. ).

The historical and present-day analogues reproduce remarkably similar synoptic-scale circulation patterns, with relatively weak differences in Z500 (Fig. e), again confirming that the analogue-selection procedure successfully constrains the large-scale atmospheric circulation. The largest changes appear in the upper-level dynamical diagnostics. Although the jet stream exhibits only moderate strengthening (Fig. f), widespread and statistically significant increases (>50%) in vertical wind shear are observed over large portions of the basin (Fig. g). These changes translate into a pronounced enhancement of TI1 (Fig. h, >50%), whose spatial pattern closely follows that of the vertical wind shear anomalies, indicating that the strengthened turbulence-favourable environment is primarily associated with modifications of the upper-level dynamical structure rather than substantial changes in the jet position itself.

The attribution gauge indicates that the events was influenced by both climate change and natural variability, whereas the rarity gauge classifies the synoptic configuration as rare. The seasonal distribution of the analogues (Fig. i) remains concentrated during the boreal cold season, with both historical and present-day analogues occurring predominantly during December and January. This consistency supports the circulation-conditioned nature of the attribution and indicates that the comparison is performed between analogues representative of the same seasonal circulation regime. Again, although airport-scale diagnostics (Fig. j) are not available for this upper-level event, the panel is retained to preserve the standard TurboMeter layout.

The additional surface diagnostics (Fig. ) confirm that the analogues retain nearly identical large-scale circulation patterns, with only weak differences in the Z500 field (Fig. d). In contrast, sea surface temperatures exhibit widespread positive anomalies across most of the North Pacific basin (Fig. h), with statistically significant warming extending over much of the subtropical and mid-latitude Pacific. Surface wind speeds also display moderate positive anomalies over the central basin (Fig. l), indicating a slightly more energetic lower-tropospheric environment under present-day climate conditions. These results suggest that present-day analogues evolve over a systematically warmer ocean while preserving nearly identical synoptic circulation.

Furthermore, CAPE remains generally modest throughout the basin, with only localized statistically significant increases near the eastern boundary of the domain (Fig. d), in agreement with the spatial distribution of the observed turbulence reports (Fig. ). CIN exhibits comparatively weak and spatially fragmented differences (Fig. h), indicating that large-scale convective instability undergoes only limited modifications between the historical and present-day analogues. By contrast, total column water vapour displays a coherent increase across much of the North Pacific (Fig. l), consistent with the enhanced atmospheric moisture expected under a warmer climate. These results indicate that the enhanced turbulence environment over the North Pacific is primarily associated with changes in the upper-level dynamical structure acting within a warmer and moister atmospheric background. Unlike the North Atlantic case, where the increase in atmospheric moisture is concentrated over specific sectors of the basin, the North Pacific exhibits a more spatially homogeneous thermodynamic response, whereas the dominant contribution to the enhanced turbulence signal remains the widespread increase in vertical wind shear.

By considering all the 425 MOG turbulence reports (EDR≥0.2 m2/3s-1) in the North Pacific region occurred in 2024 (see the Supplement) we observe that there are no events for which the meteorological conditions strengthened uniquely by human-driven climate change, while 19 % (81 over 425) have been mostly strengthened by human-driven climate change, and the remaining 81 % (344 over 425) have been likely influenced by both human-driven climate change and natural variability. We do not detect any event only driven by natural variability and for all events we have medium-to-high confidence in our claims.

East Asia represents one of the most dynamically complex aviation corridors, where strong upper-level jet streams interact with intense baroclinicity, subtropical moisture transport, and frequent convective activity. For the East Asia region, 150 MOG turbulence reports (EDR≥0.2 m2/3s-1) from the NOAA MADIS archive were recorded during 2024, with 12 April 2024 corresponding to the day with the highest number of reports (41). As shown in Fig. , most turbulence reports are concentrated along the Japanese archipelago and the adjacent western North Pacific, closely following the position of the upper-level jet.

The circulation-conditioned attribution analysis for this event is summarized in Fig. . The observed atmospheric configuration is characterized by a broad upper-level ridge extending across East Asia (Fig. a) and a strong subtropical jet exceeding 60 ms-1 (Fig. b). Enhanced vertical wind shear develops along the equatorward flank of the jet (Fig. c), producing elevated TI1 values (Fig. d) over Japan and the adjacent western North Pacific.

The analogues reproduce remarkably similar large-scale circulation patterns, as confirmed by the relatively weak Z500 differences (Fig. e). The upper-level jet also retains nearly the same spatial configuration (Fig. f), although modest strengthening is evident over portions of the western North Pacific. Despite these comparatively small circulation changes, the present-day analogue population exhibits coherent increases in vertical wind shear (Fig. g), while TI1 shows more fragmented changes (Fig. h). This suggests a primary role of enhanced upper-level shear, while the horizontal deformation seems to act in reducing changes in TI1.

The attribution indicates that the event was influenced by both climate change and natural variability, not red, while the rarity gauge classifies the circulation pattern as rare. The analogues remain confined to the spring season (Fig. i), although present-day analogues occur more frequently during April than during May compared with the historical climate, suggesting a modest seasonal redistribution of the circulation patterns associated with turbulence occurrence.

The surface diagnostics (Fig. ) confirm that the analogues retain nearly identical large-scale circulation patterns, while weak changes emerge in sea surface temperature and near-surface winds (Fig. ). Unlike the North Atlantic and North Pacific cases, East Asia does not exhibit a basin-wide warming signal. Instead, SST anomalies remain comparatively weak and spatially heterogeneous, whereas moderate increases in near-surface wind speed are observed over portions of the western North Pacific. These results suggest that surface forcing plays a less dominant role in this region than over the major oceanic flight corridors.

The thermodynamic diagnostics reveal a more structured response than observed over the two oceanic flight corridors. CAPE exhibits localized negative changes (Fig. d), while CIN shows regional changes (Fig. h). Total column water vapour (Fig. l) also is characterized by slightly increased moisture over East Asia and reduced values farther offshore. Thus, these results indicate that the strengthened turbulence-favourable environment over East Asia is associated not only with enhanced upper-level shear but also with regional and localized changes in atmospheric moisture, suggesting a stronger coupling between upper-level dynamics and regional thermodynamic processes than observed over the North Atlantic and North Pacific.

By considering all the 150 MOG turbulence reports (EDR≥0.2 m2/3s-1) in the East Asia region (see the Supplement), we observe that there are no events for which the meteorological conditions have been strengthened uniquely by human-driven climate change, while the 73 % (109 over 150) have been mostly strengthened by human-driven climate change, and the remaining 27 % (41 over 150) have been likely influenced by both human-driven climate change and natural variability. We do not detect any event only driven by natural variability and for all events we have medium-to-high confidence in our claims.

CONUS encompasses one of the most dynamically diverse aviation environments, where strong upper-level jets interact with complex orography, frontal systems, deep convection, and subtropical moisture transport. Consequently, turbulence observed over this corridor may arise from a broad spectrum of interacting dynamical processes, providing a particularly challenging test case for the circulation-conditioned attribution framework.

For the CONUS region, 1905 MOG turbulence reports (EDR≥0.2 m2/3s-1) from the NOAA MADIS archive were recorded during 2024, with 26 March 2024 corresponding to the day with the highest number of reports (332). As shown in Fig. , most turbulence reports are concentrated over the southeastern US, particularly over Georgia and Alabama, immediately north of the Gulf of Mexico, a region characterized by the interaction between subtropical moisture transport and strong upper-level dynamics.

The observed atmospheric configuration is characterized by a deep upper-level trough extending across western North America (Fig. a), producing a strong southwesterly jet over northern Mexico and the central US (Fig. b). The resulting upper-level shear generates an elongated band of enhanced vertical wind shear (Fig. c), which closely coincides with elevated values of TI1 (Fig. d).

The analogues reproduce remarkably similar large-scale circulation patterns, as indicated by the relatively weak differences in Z500 (Fig. e). The upper-level jet also undergoes only moderate modifications (Fig. f), whereas considerably larger increases are observed in vertical wind shear (Fig. g). These dynamical changes translate into widespread positive TI1 anomalies (Fig. h), particularly over the southern US and the Gulf of Mexico, indicating that the strengthened turbulence-favourable environment is primarily associated with enhanced upper-level shear.

The attribution gauge indicates that the event was strongly strengthened by anthropogenic climate change while the rarity gauge classifies the associated circulation pattern as highly exceptional. The analogues remain concentrated during the boreal spring (Fig. i), with present-day analogues occurring slightly more frequently during April than in the historical period, suggesting a modest seasonal redistribution of favourable circulation patterns.

The surface diagnostics (Fig. ) confirm that the analogues retain highly comparable synoptic-scale circulation patterns (Fig. d). In contrast to the predominantly oceanic flight corridors, the CONUS case exhibits coherent warming of the near-surface atmosphere over large portions of the southern US (Fig. h), accompanied by positive sea surface temperature anomalies over the Gulf of Mexico (Fig. l). Together, these changes indicate that the circulation analogues evolve within a warmer land–ocean environment under present-day climate conditions.

The thermodynamical diagnostics reveal the strongest thermodynamic response among the four analysed flight corridors (Fig. ). CAPE exhibits pronounced positive anomalies over the Gulf of Mexico and the southeastern US (Fig. d), indicating increasingly favourable conditions for convection. Consistently, CIN displays localized reductions over portions of the same region (Fig. h), suggesting a weaker inhibition of convective development. Total column water vapour also increases over much of the southern US and the Gulf of Mexico (Fig. l), indicating enhanced atmospheric moisture availability under present-day climate conditions. These indicate that the circulation-conditioned strengthening of the turbulence-favourable environment over CONUS results from the combined influence of enhanced upper-level dynamics and a warmer, moister, and locally more unstable lower atmosphere. Compared with the North Atlantic and North Pacific cases, where the dominant signal is primarily associated with upper-level dynamics, the CONUS event exhibits a substantially stronger coupling between atmospheric dynamics and regional thermodynamic conditions while the large-scale circulation remains remarkably consistent between the historical and present-day analogues.

By considering all the 1905 MOG turbulence reports (EDR≥0.2 m2/3s-1) in the CONUS region that occurred in 2024 (see the Supplement), we observe that more than 27 % (522 over 1905) of events have been strengthened by human-driven climate change, 48 % (922 over 1905) have been mostly strengthened by human-driven climate change, and the remaining 25 % (461 over 1905) have been likely influenced by both human-driven climate change and natural variability. We do not detect any event only driven by natural variability and for all events we have medium-to-high confidence in our claims.