Stratospheric temperature trend is a fingerprint for anthropogenic climate change. Since 1980, observational records have shown a negative trend in the Arctic lower-stratospheric temperature (ALST). However, it remains inconclusive to what extent the observed changes are driven by external forcing versus internal variability, and which modes of interdecadal internal variability significantly affect ALST. Moreover, providing a solid answer to how ALST will change in the near future remains a challenge. Using five large-ensemble simulations, this study shows that both external forcing and internal variability associated with the interdecadal Pacific oscillation (IPO) modulate the ALST changes. Among, the positive-negative phase transition of the IPO from 1980 to 2014 enhances both the upward propagation of planetary waves into the stratosphere and the strength of the Brewer-Dobson (BD) circulation, contributing to the Arctic stratospheric warming. The quantitative results suggest that the phase transition of IPO could offset approximately 15% (4%-26%) of the Arctic lower-stratospheric cooling. Furthermore, the IPO evolution substantially affects the near-term projection of the ALST, introducing large uncertainty to the near-term projection. In conclusion, the findings highlight the combined effect of external forcing and internal variability, especially that relating to IPO, significantly influences ALST changes.

The denitrification process involves the removal of reactive nitrogen from stratospheric air masses through the gravitational sedimentation of nitric acid trihydrate particles in polar stratospheric clouds. This process delays the deactivation of reactive chlorine, thereby prolonging the duration of spring ozone depletion and intensifying its magnitude. Using the three-dimensional atmospheric chemical transport model SLIMCAT with different denitrification schemes, numerical simulation study is conducted on two distinct Arctic stratospheric ozone depletion events occurring from December 2010 to March 2011 and from December 2019 to March 2020. Through comparison of the simulation data with observations from the microwave limb sounder (MLS), the denitrification processes during these two events are thoroughly analyzed, and the model's capability to simulate stratospheric chemical composition and denitrification under different schemes is comprehensively evaluated. Results show that, compared to the thermodynamic equilibrium scheme, SLIMCAT model incorporating the Denitrification by Lagrangian Particle Sedimentation (DLAPSE) scheme, which considers microphysical processes, yields a more accurate representation of denitrification in the lower Arctic stratosphere. During both ozone depletion events, the simulated volume mixing ratio of HNO3 on 456 K isentropic surface in the Arctic region is lower under the thermodynamic equilibrium scheme than under DLAPSE scheme, with an average relative difference of approximately -11%. Consistently, the simulated ClO volume mixing ratio in March is about 10% higher, while O3 volume mixing ratio is about 4% lower. These results indicate that the thermodynamic equilibrium scheme overestimates the denitrification process in the lower stratosphere, which in turn delays the deactivation of ClOx. It leads to an overestimation of ClOx volume mixing ratio and consequently an overestimation of O3 depletion in the Arctic stratosphere. As the temperature in the Arctic winter stratosphere decreases, HNO3 volume mixing ratio simulated by the thermodynamic equilibrium scheme initially increases and then decreases, with the transition occurring at a higher critical temperature than in DLAPSE scheme. This leads to the sedimentation of nitric acid trihydrate particles starting at relatively higher temperatures during the cooling process, resulting in greater removal of HNO3. This phenomenon may explain the overestimation of denitrification in the lower Arctic stratosphere by the thermodynamic equilibrium scheme. Compared to the period from December 2010 to March 2011, the Arctic stratospheric polar vortex from December 2019 to March 2020 exhibits greater stability and intensity, along with more persistent low temperatures. These favorable conditions facilitate the earlier formation and sustained sedimentation of larger nitric acid trihydrate particles in the polar stratosphere, leading to enhanced HNO3 removal and consequently stronger denitrification in the lower stratosphere during the latter period. The findings of this study contribute significantly to improving the simulation of polar stratospheric clouds and ozone in chemical transport models.

Abstract. Stratospheric Sudden Warmings (SSWs) significantly affect surface climate in boreal winter. However, their impacts vary considerably from one event to another: more than one-third of SSWs are not followed by expected tropospheric anomalies. To isolate the forced responses to SSWs, this study analyzes model experiments from the Stratospheric Nudging And Predictable Surface Impacts (SNAPSI) project, where the stratospheric zonal-mean state is nudged using either observations or climatology. The differences between the two experiments are examined within a multi-model ensemble framework. Nudging experiments conducted for the 2018 SSW, which featured significant tropospheric responses, and the 2019 SSW, which showed none, reveal that both SSWs consistently drive a negative Northern Annular Mode over time. The forced tropospheric response is primarily driven by an increase in Arctic surface pressure resulting from poleward mass fluxes in the stratosphere and upper troposphere. The poleward mass fluxes are initially induced by the zonal wind nudging in the middle stratosphere and subsequently by the eddy heat and momentum fluxes in the stratosphere and upper troposphere. This result suggests that while SSWs intrinsically drive tropospheric anomalies, the internal variability in the troposphere strengthens or suppresses the forced anomalies from the stratosphere, which may determine the existence of expected tropospheric anomalies following SSWs.

Thunderstorms over northern South Asia (NSA) constitute a critical transboundary mechanism, facilitating the vertical transport of pollutants from the NSA boundary layer to the Tibetan Plateau (TP) and the stratosphere. The efficiency of this thunderstorm-driven transport depends strongly on storm top height, yet how this height responds to increasing atmospheric moisture under climate warming remains unclear, representing a critical gap for projecting pollutant transport. Using long-term observations from the Tropical Rainfall Measuring Mission (TRMM) satellite and reanalysis data, we systematically examine how South Asian summer monsoon (SASM) moisture transport influences thunderstorm top height over NSA. This study identifies a dual-regime mechanism whereby atmospheric humidity governs cloud top height in thunderstorms. Under this regime, humidity exhibits a strong positive correlation with storm top height across NSA during the pre-monsoon (March-May), whereas a sharp east-west contrast emerges during the SASM. The correlation strengthens over western NSA but weakens over eastern coastal areas, indicating that overly humid conditions (q(850) > 14 g kg(-1)) suppress intense thunderstorm development, favoring convection resembling maritime regimes. Storm-scale multivariate regression identifies lower-tropospheric (850 hPa) moisture as the dominant control, explaining >55% of the variance in inland storm top height. Both humidity-based and height-based approaches consistently yield a strong positive correlation (r > 0.80) between moisture variability and thunderstorm top height. These findings imply that warming-induced increases in atmospheric water-holding capacity may further intensify thunderstorm top height, especially in moisture-sensitive inland regions, with important consequences for future pollutant vertical transport and stratosphere-troposphere exchange.

This study investigates the midlatitude tropospheric teleconnection associated with February Barents-Kara Sea (BKS) sea ice during 1979-2019 and finds evidence for its significant shift around the year 2000. Before 2000, BKS sea ice reduction is associated with an enhanced stratospheric wavenumber 1, but the relationship between BKS sea ice reduction and downstream tropospheric circulation is insignificant. After 2000, BKS sea ice reduction shows a weaker relationship with stratospheric wavenumber 1 but is accompanied by a significant strengthening of the East Asian trough in the troposphere. Both observational and modeling evidences suggest that this shift is partly attributable to the weaker background stratospheric polar vortex after 2000 compared to earlier decades. Before 2000 (under a stronger vortex state), the late-winter lower stratosphere provides favorable environment for planetary wave propagation, promoting the vertical wave propagation triggered by BKS sea ice reduction, which enhances stratospheric wavenumber 1 but weakens the horizontal wave propagation in the troposphere, causing the East Asian trough to be relatively insensitive to sea ice changes. After 2000 (under a weaker vortex state), the lower stratosphere becomes unfavorable for upward wave propagation, and waves triggered by BKS sea ice reduction are largely confined to horizontal propagation in the troposphere. As a result, the East Asian trough becomes more sensitive to sea ice changes, while the stratospheric wavenumber 1 response is insignificant. We propose that the background state of the stratospheric polar vortex plays a crucial role in modulating the causal relationship between BKS sea ice variability and East Asian tropospheric circulation.

This study investigates the responses of upper-level frontogenesis in the middle latitudes to the Arctic Stratospheric Polar Vortex (SPV) changes during winter, and explores the underlying physical mechanisms. Through diagnostics of upper-level frontogenesis function and identification of synoptic-scale upper-level fronts, the results reveal that during weak SPV years, upper-level frontogenesis intensifies significantly in the 30°–50° N latitude band, particularly over North America, the North Atlantic, and Northeast Asia, while decreasing between 50° and 60° N. The physical mechanisms involve three main processes. Firstly, SPV weakening is accompanied by planetary wave divergence in 30°–50° N, which accelerates the westerly jet and increases potential vorticity (PV) gradients in this region, corresponding to enhanced frontogenesis in this area. Conversely, planetary wave convergence around 50°–60° N weakens westerlies and reduces PV gradients, weakening upper-level frontogenesis there. Secondly, SPV weakening induces significant deepening of both the North American and East Asian troughs, enhancing cold air advection in the trough regions, which drives a shift of the thermally direct circulation toward the anticyclonic side in jet entrance areas. On the other hand, the intensified subsidence in the frontal warm sector promotes frontogenesis by enhancing isentropic surface tilting and atmospheric baroclinicity. Lastly, the accelerated westerly flow in middle latitudes (30°–50° N) intensifies geostrophic forcing in jet entrance regions. This enhanced forcing markedly strengthens the thermally direct circulation, further creating favorable conditions for upper-level frontogenesis.

Accurate precipitation nowcasting is crucial for disaster mitigation and socio-economic planning, yet existing methods often struggle with false alarms, missed events, and long range dependency modeling at high spatiotemporal resolution. To address these challenges, we propose FlashBack Memory (FB), a module that dynamically retrieves key historical states and integrates them via an adaptive fusion gate, enhancing the spatiotemporal representation capability of recurrent-based models. We incorporate FB into PredRNN, PredRNNpp, MIM, MotionRNN, and PredRNN-V2, and evaluate on CIKM2017, Shanghai2020, and SEVIR datasets. Experimental results demonstrate that FB significantly improves MSE, MAE, SSIM, and CSI metrics, particularly for high-intensity rainfall and long-sequence predictions, while reducing false alarms and missed events and enhancing temporal consistency and spatial localization. The proposed method provides a general and efficient memory enhancement mechanism, improving the overall performance of recurrent-based precipitation nowcasting models.

The stratospheric temperature has continued to decline over the past half-century, in contrast to the steady increase in the tropospheric temperature. However, the future changes in stratospheric temperature under anthropogenic influences, as well as the specific role of stratospheric interactive chemistry in their attribution and projection, remain uncertain. Here, we show that current climate models generally underestimate (overestimate) the observed temperature trends in the lower (upper) stratosphere. These temperature biases may lead to an overestimation of upper tropospheric warming, particularly in models that lack interactive chemistry. By applying observational constraints, we demonstrate that interactive chemistry models can reduce the uncertainty in future lower stratospheric temperature projections by approximately 15% compared to non-interactive chemistry models. In conclusion, these results highlight the dominant role of anthropogenic forcing in shaping stratospheric temperatures and underscore the importance of incorporating interactive chemistry in global climate models to improve the reliability of projections.

Polar stratospheric clouds (PSCs) play a critical role in stratospheric ozone depletion. Previous studies have shown that the quasi-biennial oscillation (QBO) influences the Arctic stratospheric polar vortex and ozone, yet few studies have thoroughly analyzed the impact of the QBO on Arctic PSC occurrence. This study examines this impact using CALIPSO observations from 2006 to 2021 and SLIMCAT simulations from 1979 to 2022. The results show that the winter PSC coverage area is significantly larger during the westerly QBO (WQBO) phase than during the easterly QBO (EQBO) phase, with a zonal asymmetry in PSC occurrence frequency anomalies. The QBO influences the temperature, water vapour (H2O), and nitric acid (HNO3) in the Arctic stratosphere, which are key factors affecting PSC formation. During the WQBO phase, Arctic stratospheric temperatures show negative anomalies, with the centre of this anomaly biased towards North America. In addition, H2O shows positive anomalies in the Arctic lower stratosphere, mainly due to the stronger polar vortex preventing the transport of high-moisture air at high latitudes to mid-latitudes, causing H2O to accumulate inside the polar vortex. HNO3 shows negative anomalies, primarily caused by denitrification through nitric acid trihydrate (NAT) sedimentation. Sensitivity analyses further indicate that QBO-induced temperature anomalies are the dominant driver of PSC variability, while the direct effect of H2O anomalies on PSCs is relatively small. The reduction of HNO3 mainly affects PSCs in February and March. This work implies that future changes in the QBO may influence ozone by affecting PSCs.

Understanding the mechanisms behind heavy precipitation is crucial for predicting those events. Based on hourly ERA5 reanalysis and rain gauge records, this study investigates the spatiotemporal evolution and anomalous upper-level circulation of heavy regional rainfall events (RREs) in summer over the southern Qilian Mountains, a key center of heavy rainfall in the northeastern Tibetan Plateau. Two distinct types of heavy RREs are identified, accounting for 73.6% of the total summer rainfall. Type 1 is confined to a relatively local spatial scale and has a shorter duration, with a diurnal peak occurring at 2200 LST, whereas Type 2 exhibits widespread propagation from the northwest to the southeast, with a longer duration and a diurnal peak occurring at 0600 LST. Distinct upper-level disturbances contribute to the evolution of these two types of events. For Type 1, a positive anomaly in potential vorticity near the tropopause is observed due to the intrusion of high potential vorticity air from the stratosphere, reducing the stability of the lower troposphere. In contrast, Type 2 is characterized by an eastward-moving warm anomaly, centered at 350 hPa. The upper-level warm anomaly contributes to upper tropospheric divergence and lower tropospheric water vapor convergence. The upper-level warm anomaly may result from the northward extension of South Asia High, as well as the warm anomalies in the middle and lower troposphere. The findings of this study enhance the understanding of the impact of upper-level disturbances on heavy precipitation in the complex terrains of arid and semi-arid areas.

The intensity and position of the westerly jet influence precipitation in Central Asia, while the impact of its structural changes on precipitation remains underexplored. This study defines a Jet Break Index based on ERA5 reanalysis data to examine how jet break events affect summer precipitation in the region. Results show that the index, derived using relative vorticity at 200 hPa, effectively identifies jet break events. The convergence and southward strengthening of Rossby waves as they propagate into Central Asia lead to jet break and trigger tropopause folding events. As stratospheric air intrudes into the troposphere, the resulting disturbance induces ascending (descending) motion in the downstream (upstream) region and is accompanied by positive (negative) vertical moisture advection contributions, thereby enhancing (suppressing) precipitation accordingly. These findings highlight the significant impact of jet break on precipitation anomalies and its implications for forecasting over Central Asia.

Using various observations and a chemistry-climate model, this study investigates the impact of wintertime total column ozone (TCO) on snow cover over the Tibetan Plateau (TP). The results indicate that during anomalously high TP TCO events, the snow cover on the TP is anomalously high, and vice versa. Further analysis reveals that high TP TCO leads to a warmer stratosphere and a colder troposphere. The stratospheric warming induces lower geopotential height in the upper troposphere and lower stratosphere (UTLS) over the TP. The cyclonic circulation in the UTLS associated with this low pressure leads to high potential vorticity (PV) stratospheric air being transported to the western TP. To maintain the conservation of PV, cyclonic vorticity forms above the western TP in the UTLS and can extend into the troposphere, then transporting water vapor from the Bay of Bengal and the Arabian Sea to the TP. The southerlies associated with this cyclonic circulation and positive vertical gradient of zonal wind in the troposphere induce ascending motions over the TP. Increased water vapor and ascending motions lead to more snowfall and cloud formation. The increased cloud enhances the cooling of the TP, leading to condensation process in the southern boundary of the TP and lower geopotential height in the UTLS over the TP. This decreased geopotential height strengthens the snowfall process. The increased snow cover in turn causes a cooling of the TP surface. This positive feedback further amplifies the snow-cover variation. Model simulations suggest that a change of 10 Dobson Unit (DU) in TCO can result in a 1.7% change in snow cover. SIGNIFICANCE STATEMENT: Substantial snow cover exists over the Tibetan Plateau (TP) during winter and exerts important impacts on climate systems and hydrological processes in the TP and surrounding areas. Understanding the formation and changes of snow cover over the TP is of great importance. This study found that wintertime total column ozone (TCO) variations over the TP have important contributions to the snow-cover variations over the TP, with a 10 Dobson Unit (DU) TCO change resulting in a 1.7% change in snow cover over the TP. This result will provide a better understanding of snow-cover variations over the TP and will be helpful in improving the accuracy of snow-cover predictions.

The impact of Antarctic sea ice reduction during early austral winter on the austral winter Antarctic stratospheric polar vortex is investigated using reanalysis data set and model simulations. Both reanalysis data set and model simulations show that the reduction of Antarctic sea ice during early austral winter leads to a northward displacement of the tropospheric mid-latitude jet, resembling the negative phase of the Southern Annular Mode. Meanwhile, the reduction of sea ice induces a weaker Antarctic stratospheric polar vortex during winter, which is accompanied by a weaker polar night jet. Further analysis indicates that the Antarctic sea ice reduction could lead to a greater excitation of Rossby waves and significant positive geopotential height anomalies over the Antarctic continent. The zonal wave 1 and 2 components of geopotential height anomalies are in phase with the climatology, corresponding to enhanced upward propagation of wave activity flux in early austral winter. Meanwhile, the reduction of sea ice in early austral winter could result in a more favorable atmospheric environment for the propagation of planetary waves into the stratosphere. These processes ultimately weaken the Antarctic stratospheric polar vortex and the polar night jet in winter. The reduction of sea ice in the Amundsen Sea sector enhances the upward propagation of planetary wave, while the reduction of sea ice in the Indian Ocean sector has the opposite effect.

The quasi-biennial oscillation (QBO) dynamically interacts with the extratropical atmosphere. However, the relationship between the QBO in austral winter and the Antarctic stratospheric polar vortex in spring remains unclear. In this study, we propose a joint predictor involving the QBO for the Antarctic stratospheric polar vortex and ozone in austral spring. During the westerly QBO phase (WQBO), positive zonal-mean zonal wind anomalies at 20–40° S in the upper stratosphere in July, named the positive extratropical mode, can lead to a stronger Antarctic stratospheric polar vortex and lower ozone concentration in November, with correlations reaching 0.75 and −0.60, respectively. The mechanism is summarized as follows: the positive extratropical mode triggers a secondary circulation, which further alters the environmental conditions for wave propagation in the stratosphere. The resulting anomalous wave divergence leads to a stronger Antarctic stratospheric polar vortex during the austral spring, while during the easterly QBO phase (EQBO), the correlation between the extratropical mode and the strength of the polar vortex is only 0.1. Due to the stronger upward motion in the tropics, which opposes the secondary circulation induced by the extratropical mode, the EQBO cannot sustain the positive anomalous zonal-mean zonal wind until November. Our results highlight that the extratropical mode during the WQBO could serve as a reliable predictor for both the Antarctic stratospheric polar vortex and the Antarctic ozone hole with a 4-month time lag.

The potential for Arctic sea-ice loss, particularly in the Barents-Kara Seas, to induce Eurasian winter cooling remains contentious. Despite a significant correlation between Barents-Kara-Seas sea-ice loss and Eurasian winter cooling in observations, modeling studies suggest minimal causal influence. Through constraining Barents-Kara-Seas sea-ice to different states in ocean-atmosphere-coupled simulations, here we show that ocean-atmosphere coupling enhances the Eurasian cooling induced by historical sea-ice loss, though still weaker than observed, likely due to internal variability and confounding factors. Historical sea-ice loss induces stronger and deeper Arctic warming with ocean-atmosphere coupling than without, associated with a strengthened Siberian High and East Asian Trough, which promotes Eurasian cooling. However, ocean-atmosphere coupling has little influence on the Eurasian cooling response to projected end-of-the-twenty-first-century sea-ice loss. The Eurasian temperature response to historical sea-ice loss is dominated by dynamically-induced cooling, whereas strong thermodynamical warming masks dynamically-induced cooling in response to far-future sea-ice loss.

Sudden Stratospheric Warmings (SSWs) are weather phenomena occurring in polar regions, and have a profound impact on mid‐latitude cold waves. In this paper, within a deep learning framework, we introduce video prediction techniques into SSW events forecasting for the first time. We develop a Global Attention Motion Decoupled Recurrent Neural Network (GMRNN) to better capture the detailed changes of the polar vortex. Through experiments on representative SSW events in 2018, 2019, and 2021, our model can stably predict SSW events 20 days in advance and accurately capture the morphological changes of the stratospheric polar vortex. Furthermore, we compared our model with baseline models, including PredRNN, MotionRNN, and the sub‐seasonal to seasonal (S2S) integrated forecast models from ECMWF, CMA, and ECCC. The results indicate that our model outperforms these models across various evaluation metrics, compare with ensemble prediction results GMRNN's Structural Similarity increased by approximately 11.2%, and the Anomaly Correlation Coefficient increased by approximately 9.5%. The GMRNN model exhibits superior stability and possesses prediction potential over a longer period.

Using reanalysis datasets and the Community Earth System Model (CESM), this study investigates the effects of ozone-climate interactions on the Arctic stratospheric temperature (AST) changes during winter and early spring. Before 2000, the AST increased significantly in early winter (November and December), which is contributed to by ozone-climate interactions. Specifically, ozone-climate interactions lead to a stratospheric state that enhances upward wave propagation and the downwelling branch of Brewer-Dobson circulation. This leads to an adiabatic warming that significantly raises the AST. This dynamical heating overwhelmingly offsets the longwave radiative cooling effect associated with increased ozone during early winter. In contrast, during late winter and spring, cooling trends in the Arctic stratosphere are predominantly driven by reduced shortwave radiation heating associated with stratospheric ozone depletion. This study highlights the effects of ozone-climate interactions on the long-term trend in the AST.

Recent reports on anomalously large ozone holes imply the uncertainty of the Antarctic ozone recovery. This study demonstrates that the sea surface temperature trends during this century hinder this recovery, despite declining ozone-depleting substances. Observations show a significant recovery of the polar mean total column ozone in September during 2000-2021, contrasted by robust ozone loss in the South Pacific middle stratosphere during October-November. Numerical experiments reveal that ozone-depleting substance reduction alone drives significant Antarctic ozone recovery throughout austral spring. However, superimposing observed sea surface temperature linear increments causes significant stratospheric ozone decreases over the South Pacific in October-November and reduces the spring mean recovery by approximately 46%. Both reanalysis data and simulations indicate that sea surface temperature forcing weakens Antarctic stratospheric planetary wave activity, causing regional cooling and nitric acid loss. These changes increase the ratios between the active halogens and the halogen reservoirs, thereby promoting ozone depletion.

Our study reframes our understanding of stratosphere–troposphere interactions, traditionally thought to be confined within individual hemispheres, by introducing a novel cross-hemispheric link. We demonstrate that strong boreal winter Arctic stratospheric polar vortex (APV) boosts the transmission of upper tropospheric waves from Northern Hemisphere’s mid-high latitudes to the equator. Facilitated by the tropical central and eastern Pacific’s ‘westerly bridge’, these waves reach Southern Hemisphere’s mid-high latitudes. The entire process shows a ‘semicircular road’. Waves reaching the Southern Hemisphere affect the circulation through wave-flow interaction, causing a southward swing of the Southern Hemispheric westerly jet center. This displacement weakens the subtropical jet and strengthens the polar jet, resulting in increased subtropical precipitation and decreased mid-latitude precipitation in the Southern Hemisphere during austral summer. Correspondingly, a weak APV may lead to the opposite result. Our findings underscore APV’s broader impact on the tropospheric atmosphere, extending beyond prior knowledge.