The Asian summer monsoon (ASM) plays a key role in the vertical transport of material (including anthropogenic pollutants, aerosols, aerosol precursors, and other important trace gases) across the tropopause, with significant impacts on stratospheric chemistry and dynamics regionally and globally. Additionally, the Asian region is experiencing increasingly frequent and severe weather extremes connected to the ASM that are causing unprecedented damage to public property and loss of life. Predicting localized extreme events with sufficient lead times using numerical weather prediction (NWP) models remains challenging, and recent research suggests that better representation of stratospheric processes in NWP models can help to improve the prediction of monsoon extremes. On the other hand, it is well recognized that the complexities of the underlying mechanisms of stratosphere–troposphere coupling processes are difficult to incorporate in NWP models.

Observational and modelling aspects of the stratosphere–troposphere coupling processes and extreme weather events associated with the Asian summer monsoon were addressed at an international workshop, Stratosphere-Troposphere Interactions and Prediction of Monsoon weather EXtremes (STIPMEX), in Pune, India, from 2 to 7 June 2024. The STIPMEX workshop provided a platform for discussions on dynamical, chemical, radiative, and convective processes of the atmosphere during the ASM and fostered knowledge exchange and collaboration between experts on stratosphere–troposphere interactions and extreme weather prediction. The workshop aimed to promote and improve the inclusion of stratospheric and tropospheric processes in NWP models for better predictability of monsoon extremes. Full information on the STIPMEX workshop including a detailed list of themes and topics as well as an overview of the delivered presentations can be found at https://sparc-extreme.tropmet.res.in/.

This special issue has been initiated to publish the new and original research presented during STIPMEX and make it available to the wider community, and all STIPMEX presenters are encouraged to submit full write-ups of their novel and so far unpublished scientific studies. Submissions of follow-on studies or material representing a product of conference discussions and knowledge exchange are also welcome, as are any other submissions or related and relevant work that fits the scope of the workshop and special issue.

To best accommodate the two overarching STIPMEX themes, (i) dynamical, chemical, radiative, and convective processes in general, with a particular emphasis on recent changes and trends in stratosphere–troposphere coupling and linkages between stratospheric aerosol variability (e.g. due to volcanic eruptions) and the Asian summer monsoon, and (ii) the challenges of forecasting extreme weather events during the Asian summer monsoon, the special issue is organized as an inter-journal special issue of Atmospheric Chemistry and Physics (ACP) and Weather and Climate Dynamics (WCD).

Constraining the circulation in general circulation models by nudging the model variables toward reanalysis provides a rather powerful tool for investigating the sensitivities of predictions and simulations to specific processes or phenomena and enables the impact of model biases to be better quantified. A shared methodology for applying nudging to stratospheric variables has been developed by the World Climate Research Programme (WCRP) Stratosphere–troposphere Process And their Role in Climate (SPARC) Quasi-Biennial Oscillation initiative (QBOi) and Stratospheric Nudging And Predictable Surface Impacts (SNAPSI) activities. This methodology involves nudging (relaxing) only the zonal mean, while atmospheric waves are allowed to evolve freely, though publications that adopt other nudging methodologies or that explore sensitivity to the nudging methodology are encouraged to submit. For SNAPSI the nudging is applied throughout the stratosphere, whereas QBOi applies nudging to a localized region of the equatorial stratosphere. The reliability and robustness of the conclusions obtained from this new approach are further enhanced by the use of coordinated multi-model experiments.

The purpose of the proposed special issue is to provide a common location for reporting results from the SNAPSI and QBOi coordinated experiments utilizing stratospheric nudging, along with results from other related studies that nudge the atmosphere, whether nudging just the zonal mean or the full field. A number of modelling centres have now carried out the SNAPSI and QBOi experiments. The special issue would collect papers analysing these datasets that have been produced by participating modelling centres.

The SNAPSI experiments are designed to study the role of sudden stratospheric warmings (SSWs) in surface predictability; they are hindcast-type experiments, specifying 45-day runs covering three recent SSW events, with a large ensemble size of 50–100 members for each case. Analysis topics include the role of stratospheric variability in surface extremes and predictability and stratosphere–troposphere coupling mechanisms. The QBOi experiments are designed to study the impact of biases in the quasi-biennial oscillation (QBO) on QBO teleconnections and the forcing of the QBO; they are climate-type experiments, specifying 40-year runs with one to three ensemble members. Analysis topics include the QBO teleconnections with the stratospheric polar vortex and the relationship between the QBO and Madden–Julian oscillation (MJO). Analysis of both sets of coordinated experiments, SNAPSI and QBOi, is currently in progress, with papers expected to be submitted in the coming 1–2 years.

The climate research community uses reanalyses widely to understand atmospheric processes and variability in the middle atmosphere, yet different reanalyses give very different results for the same diagnostics. For example, the global energy budget and hydrological cycle, the Brewer–Dobson circulation, stratospheric vortex weakening and intensification events, and large-scale wave activity at the tropical tropopause are known to differ among reanalyses.

The Stratosphere–troposphere Processes And their Role in Climate (SPARC) Reanalysis Intercomparison Project (S-RIP) began in 2013 as a coordinated activity to compare numerous key diagnostics in reanalysis data sets. The objectives of this project were

• to understand the causes of differences among reanalyses,
• to provide guidance on the appropriate usage of various reanalysis products in scientific studies,
• to contribute to future improvements in the reanalysis products by establishing collaborative links between the reanalysis centres and the SPARC community.

Phase 1 of the S-RIP project culminated with the publication of the S-RIP report (https://www.sparc-climate.org/sparc-report-no-10 ) in January 2022 and a very successful special issue (https://acp.copernicus.org/articles/special_issue829.html) in ACP and ESSD with over 50 papers. Phase 1 was very successful in achieving the above objectives, and in doing so it taught us the value and importance of continuing reanalysis intercomparisons and communications between the reanalysis centres and the SPARC community. The above objectives thus remain the primary aims of S-RIP as the project moves into Phase 2.

This special issue is being initiated in the early stages of S-RIP Phase 2. The community is continuing to produce valuable papers including both updates using new reanalyses of diagnostics studied in Phase 1 and evaluation of diagnostics for processes and atmospheric regions that were not emphasized in Phase 1. This special issue welcomes papers both during this transitional period and in the following years of Phase 2 and both updates of work on processes studied in Phase 1 and new studies focused on additional processes and/or atmospheric regions.

The S-RIP project focuses primarily on differences among reanalyses, but studies that include operational analyses and studies comparing reanalyses with observations or model outputs are encouraged. Phase 1 of S-RIP emphasized diagnostics in the upper troposphere, stratosphere, and mesosphere. This special issue will collect research relevant to S-RIP, including broadening of the scope to, for example, evaluation of new reanalyses and of chemical reanalyses; more comprehensive evaluation of processes in the upper stratosphere and mesosphere; evaluation of tropospheric processes such as blocking, jet stream variations, and temperature anomalies; and more comprehensive evaluation of links between the stratospheric, upper tropospheric, and near-surface circulation and implications for extreme weather events.

All researchers are encouraged to submit to this issue regardless of past participation in S-RIP; we further encourage researchers to participate in and help guide S-RIP Phase 2.

The Asian summer monsoon (ASM) plays a key role in the vertical transport of material (including anthropogenic pollutants, aerosols, aerosol precursors, and other important trace gases) across the tropopause, with significant impacts on stratospheric chemistry and dynamics regionally and globally. Additionally, the Asian region is experiencing increasingly frequent and severe weather extremes connected to the ASM that are causing unprecedented damage to public property and loss of life. Predicting localized extreme events with sufficient lead times using numerical weather prediction (NWP) models remains challenging, and recent research suggests that better representation of stratospheric processes in NWP models can help to improve the prediction of monsoon extremes. On the other hand, it is well recognized that the complexities of the underlying mechanisms of stratosphere–troposphere coupling processes are difficult to incorporate in NWP models.

Observational and modelling aspects of the stratosphere–troposphere coupling processes and extreme weather events associated with the Asian summer monsoon were addressed at an international workshop, Stratosphere-Troposphere Interactions and Prediction of Monsoon weather EXtremes (STIPMEX), in Pune, India, from 2 to 7 June 2024. The STIPMEX workshop provided a platform for discussions on dynamical, chemical, radiative, and convective processes of the atmosphere during the ASM and fostered knowledge exchange and collaboration between experts on stratosphere–troposphere interactions and extreme weather prediction. The workshop aimed to promote and improve the inclusion of stratospheric and tropospheric processes in NWP models for better predictability of monsoon extremes. Full information on the STIPMEX workshop including a detailed list of themes and topics as well as an overview of the delivered presentations can be found at https://sparc-extreme.tropmet.res.in/.

This special issue has been initiated to publish the new and original research presented during STIPMEX and make it available to the wider community, and all STIPMEX presenters are encouraged to submit full write-ups of their novel and so far unpublished scientific studies. Submissions of follow-on studies or material representing a product of conference discussions and knowledge exchange are also welcome, as are any other submissions or related and relevant work that fits the scope of the workshop and special issue.

To best accommodate the two overarching STIPMEX themes, (i) dynamical, chemical, radiative, and convective processes in general, with a particular emphasis on recent changes and trends in stratosphere–troposphere coupling and linkages between stratospheric aerosol variability (e.g. due to volcanic eruptions) and the Asian summer monsoon, and (ii) the challenges of forecasting extreme weather events during the Asian summer monsoon, the special issue is organized as an inter-journal special issue of Atmospheric Chemistry and Physics (ACP) and Weather and Climate Dynamics (WCD).

Constraining the circulation in general circulation models by nudging the model variables toward reanalysis provides a rather powerful tool for investigating the sensitivities of predictions and simulations to specific processes or phenomena and enables the impact of model biases to be better quantified. A shared methodology for applying nudging to stratospheric variables has been developed by the World Climate Research Programme (WCRP) Stratosphere–troposphere Process And their Role in Climate (SPARC) Quasi-Biennial Oscillation initiative (QBOi) and Stratospheric Nudging And Predictable Surface Impacts (SNAPSI) activities. This methodology involves nudging (relaxing) only the zonal mean, while atmospheric waves are allowed to evolve freely, though publications that adopt other nudging methodologies or that explore sensitivity to the nudging methodology are encouraged to submit. For SNAPSI the nudging is applied throughout the stratosphere, whereas QBOi applies nudging to a localized region of the equatorial stratosphere. The reliability and robustness of the conclusions obtained from this new approach are further enhanced by the use of coordinated multi-model experiments.

The purpose of the proposed special issue is to provide a common location for reporting results from the SNAPSI and QBOi coordinated experiments utilizing stratospheric nudging, along with results from other related studies that nudge the atmosphere, whether nudging just the zonal mean or the full field. A number of modelling centres have now carried out the SNAPSI and QBOi experiments. The special issue would collect papers analysing these datasets that have been produced by participating modelling centres.

The SNAPSI experiments are designed to study the role of sudden stratospheric warmings (SSWs) in surface predictability; they are hindcast-type experiments, specifying 45-day runs covering three recent SSW events, with a large ensemble size of 50–100 members for each case. Analysis topics include the role of stratospheric variability in surface extremes and predictability and stratosphere–troposphere coupling mechanisms. The QBOi experiments are designed to study the impact of biases in the quasi-biennial oscillation (QBO) on QBO teleconnections and the forcing of the QBO; they are climate-type experiments, specifying 40-year runs with one to three ensemble members. Analysis topics include the QBO teleconnections with the stratospheric polar vortex and the relationship between the QBO and Madden–Julian oscillation (MJO). Analysis of both sets of coordinated experiments, SNAPSI and QBOi, is currently in progress, with papers expected to be submitted in the coming 1–2 years.

The climate research community uses reanalyses widely to understand atmospheric processes and variability in the middle atmosphere, yet different reanalyses give very different results for the same diagnostics. For example, the global energy budget and hydrological cycle, the Brewer–Dobson circulation, stratospheric vortex weakening and intensification events, and large-scale wave activity at the tropical tropopause are known to differ among reanalyses.

The Stratosphere–troposphere Processes And their Role in Climate (SPARC) Reanalysis Intercomparison Project (S-RIP) began in 2013 as a coordinated activity to compare numerous key diagnostics in reanalysis data sets. The objectives of this project were

• to understand the causes of differences among reanalyses,
• to provide guidance on the appropriate usage of various reanalysis products in scientific studies,
• to contribute to future improvements in the reanalysis products by establishing collaborative links between the reanalysis centres and the SPARC community.

Phase 1 of the S-RIP project culminated with the publication of the S-RIP report (https://www.sparc-climate.org/sparc-report-no-10 ) in January 2022 and a very successful special issue (https://acp.copernicus.org/articles/special_issue829.html) in ACP and ESSD with over 50 papers. Phase 1 was very successful in achieving the above objectives, and in doing so it taught us the value and importance of continuing reanalysis intercomparisons and communications between the reanalysis centres and the SPARC community. The above objectives thus remain the primary aims of S-RIP as the project moves into Phase 2.

This special issue is being initiated in the early stages of S-RIP Phase 2. The community is continuing to produce valuable papers including both updates using new reanalyses of diagnostics studied in Phase 1 and evaluation of diagnostics for processes and atmospheric regions that were not emphasized in Phase 1. This special issue welcomes papers both during this transitional period and in the following years of Phase 2 and both updates of work on processes studied in Phase 1 and new studies focused on additional processes and/or atmospheric regions.

The S-RIP project focuses primarily on differences among reanalyses, but studies that include operational analyses and studies comparing reanalyses with observations or model outputs are encouraged. Phase 1 of S-RIP emphasized diagnostics in the upper troposphere, stratosphere, and mesosphere. This special issue will collect research relevant to S-RIP, including broadening of the scope to, for example, evaluation of new reanalyses and of chemical reanalyses; more comprehensive evaluation of processes in the upper stratosphere and mesosphere; evaluation of tropospheric processes such as blocking, jet stream variations, and temperature anomalies; and more comprehensive evaluation of links between the stratospheric, upper tropospheric, and near-surface circulation and implications for extreme weather events.

All researchers are encouraged to submit to this issue regardless of past participation in S-RIP; we further encourage researchers to participate in and help guide S-RIP Phase 2.