The climate research community uses atmospheric reanalysis data sets to understand a wide range of processes and variability in the atmosphere, yet different reanalyses may give very different results for the same diagnostics. The Stratosphere–troposphere Processes And their Role in Climate (SPARC) Reanalysis Intercomparison Project (S-RIP) is a coordinated activity to compare reanalysis data sets using a variety of key diagnostics. The objectives of this project are to identify differences among reanalyses and understand their underlying causes, to provide guidance on appropriate usage of various reanalysis products in scientific studies, particularly those of relevance to SPARC, and to contribute to future improvements in the reanalysis products by establishing collaborative links between reanalysis centres and data users. The project focuses predominantly on differences among reanalyses, although studies that include operational analyses and studies comparing reanalyses with observations are also included when appropriate. The emphasis is on diagnostics of the upper troposphere, stratosphere, and lower mesosphere. This paper summarizes the motivation and goals of the S-RIP activity and extensively reviews key technical aspects of the reanalysis data sets that are the focus of this activity. The special issue “The SPARC Reanalysis Intercomparison Project (S-RIP)” in this journal serves to collect research with relevance to the S-RIP in preparation for the publication of the planned two (interim and full) S-RIP reports

Diagnostics of finite-amplitude local wave activity (LWA) are applied to the 500-hPa geopotential height field to diagnose persistent synoptic weather events of anomalously large wave activity in the Northern Hemisphere. By considering the cyclonic and anticyclonic components of LWA separately, persistent weather systems associated with large-amplitude troughs and ridges are detected. While anticyclonic wave events are predominantly found over Europe and Alaska, cyclonic wave events usually occur over East Asia and northeastern Canada. Those preferred regions correspond to the location of planetary-scale ridges and troughs, which contribute, together with transient anomalies, to the formation of wave events. Although wave events are not blocking events per definition, they are typically associated with increased blocking in their vicinity. Their spatial relationship to blocking, however, varies depending on their cyclonic or anticyclonic nature and the type of wave-breaking signatures. Wave events are also shown to be accompanied by warm or cold temperature extremes, whose spatial pattern depends on the type of events, cyclonic or anticyclonic, and the sector affected. Trends in the frequency of wave events indicate that cyclonic wave events and the associated cold extremes affecting East Asia have become more frequent in recent decades and could be linked to recent trends toward La Niña–like conditions in the Pacific and trends toward the negative phase of Arctic Oscillation.

Abstract. The annular modes characterize the dominant variability of the extratropical circulation in each hemisphere, quantifying vacillations in the position of the tropospheric jet streams and the strength of the stratospheric polar vortices. Their representation in all available reanalysis products is assessed. Reanalysis uncertainty associated with limitations in the ability to constrain the circulation with available observations, i.e., the inter-reanalysis spread, is contrasted with sampling uncertainty associated with the finite length of the reanalysis records. It is shown that the annular modes are extremely consistent across all modern reanalyses during the satellite era (ca. 1979 onward). Consequently, uncertainty in annular mode variability, e.g., the coupling between the stratosphere and troposphere and the variation in the amplitude and timescale of jet variations throughout the annual cycle, is dominated by sampling uncertainty. Comparison of reanalyses based on conventional (i.e., nonsatellite) or surface observations alone with those using all available observations indicates that there is limited ability to characterize the Southern Annular Mode (SAM) in the presatellite era. Notably, prior to 1979, surface-input reanalyses better capture the SAM at near-surface levels than full-input reanalyses. For the Northern Annular Mode, however, there is evidence that conventional observations are sufficient, at least from 1958 onward. The addition of 2 additional decades of records substantially reduces sampling uncertainty in several key measures of annular mode variability, demonstrating the value of more historic reanalyses. Implications for the assessment of atmospheric models and the strength of coupling between the surface and upper atmosphere are discussed.

To highlight the details of stratosphere–troposphere dynamical coupling during the onset of strong polar vortex variability, this study identifies stratospheric vortex weakening (SVW) events by rapid deceleration of the polar vortex and performs composite budget analyses in the transformed Eulerian-mean (TEM) framework on daily time scales. Consistent with previous work, a rapid deceleration of the polar vortex, followed by a rather slow recovery, is largely explained by conservative dynamics with nonnegligible contribution by nonconservative sinks of wave activity. During the onset of such events, stratospheric zonal wind anomalies show a near-instantaneous vertical coupling to the troposphere, which results from an anomalous upward and poleward propagation of planetary-scale waves. In the troposphere, zonal wind anomalies are also influenced by synoptic-scale waves, confirming previous studies. The SVW events driven by wavenumber-1 disturbances show comparable circulation anomalies to those driven by wavenumber-2 disturbances both in the stratosphere and troposphere. The former, however, exhibits more persistent anomalies after the onset than the latter. During both events, tropospheric wavenumber-1 and 2 disturbances project strongly onto the climatological waves, indicating that vertical propagation of planetary-scale waves into the stratosphere is largely caused by constructive linear interference. It is also found that the SVW-related vertical coupling is somewhat sensitive to the stratospheric mean state. Although overall evolution of zonal-mean circulation anomalies are reasonably similar under an initially weak or strong polar vortex, the time-lagged downward coupling is evident only when the polar vortex is decelerated under a weak vortex state. These results are compared with other definitions of weak polar vortex events, such as stratospheric sudden warming events.

The sensitivity of stratospheric polar vortex variability to the basic‐state stratospheric temperature profile is investigated by performing a parameter sweep experiment with a dry dynamical core general circulation model where the equilibrium temperature profiles in the polar lower and upper stratosphere are systematically varied. It is found that stratospheric variability is more sensitive to the temperature distribution in the lower stratosphere than in the upper stratosphere. In particular, a cold lower stratosphere favors a strong time‐mean polar vortex with a large daily variability, promoting frequent sudden stratospheric warming events in the model runs forced with both wavenumber‐1 and wavenumber‐2 topographies. This sensitivity is explained by the control exerted by the lower‐stratospheric basic state onto fluxes of planetary‐scale wave activity from the troposphere to the stratosphere, confirming that the lower stratosphere can act like a valve for the upward propagation of wave activity. It is further shown that with optimal model parameters, stratospheric polar vortex climatology and variability mimicking Southern and Northern Hemisphere conditions are obtained with both wavenumber‐1 and wavenumber‐2 topographies.