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Abstract Atmospheric blocking entails a persistent, anomalous meandering of the jet stream that disrupts the eastward migration of transient eddies in the midlatitudes. Here we analyze a large number of blocking (and blocking-like) events in the Northern Hemisphere winter with the ERA5 reanalysis through the lens of vertically-averaged wave-activity budget. By applying a feature tracking algorithm, large-valued wave-activity anomalies that persist for 4 days or longer at a given location are identified as blocks, and block-centered composites are constructed for the wave-activity budget through the lifecycle of blocks. The identified events share commonly recognized features of blocking. The majority of the persistent events occur in clusters collocated with the quasi-stationary ridge associated with the Atlantic and the Pacific storm track. Frequency of persistent blocks is higher (lower) in regions where the ‘carrying capacity’ of the jet stream is lower (higher). A very low carrying capacity for the transient waves leads to a large population of blocks over Europe. The composite lifecycle of persistent blocks shows that convergence (divergence) of the zonal flux of wave-activity dominates the budget during the onset (decay) phase of the block, while the eddy-induced wind plays a crucial role of suppressing the zonal flux during the maturation period. Our finding broadly supports the ‘traffic jam’ hypothesis of Nakamura and Huang as a common mechanism of block formation, although there is vast diversity in the actual manifestation of individual blocks. It is argued that carrying capacity is suited for estimating blocking probability rather than for making deterministic forecasts of blocking events. 

Abstract Synoptic eddies embedded in a westerly flow undergo downstream developments due to their dispersive nature. This paper examines the finite-amplitude aspects of downstream development with the budget of local wave activity (LWA), including explicit contributions from diabatic heating. LWA captures well individual troughs/ridges and the wave packet, and its column budget affords simplified interpretations. In the LWA framework, (linear) downstream development demonstrated in previous analyses is represented by the LWA advection by the zonal reference flow plus LWA flux induced by the radiation of Rossby waves. In addition, convergence of nonlinear advective LWA flux, baroclinic sources at the lower boundary, meridional redistribution by eddy momentum flux, and diabatic sources and sinks complete the column budget of LWA. When applied to the life cycles of troughs within coherent wave packets in the Southern Hemisphere, the LWA budget reveals that individual troughs grow mainly through downstream development, convergence of nonlinear advective flux by eddies, and diabatic heating. Downstream development and divergence of nonlinear flux also dominate trough decay. Contributions from nonlinear advective eddy flux are large in the presence of a strong ridge either immediately upstream or downstream of the trough. Furthermore, anticyclonic components of advective LWA fluxes associated with the upstream or downstream ridge transfer LWA into or out of the trough. Diabatic contributions are significant when the heating exhibits a tilted vertical structure that gives rise to enhanced vertical gradient in heating. 

ABSTRACT Weather at the mid‐latitudes is governed by cyclones and anticyclones mostly migrating eastward. These weather systems cause the jet streams to undulate; the meandering patterns are known as the Rossby waves. Occasionally, Rossby waves bring forth localised extreme weather phenomena. An example of a finite‐amplitude wave phenomenon is atmospheric blocking, which is often associated with heat waves and droughts. Recent development of a finite‐amplitude local wave activity (FALWA) theory by Nakamura and collaborators enables comprehensive analysis of the dynamics of finite‐amplitude Rossby waves observed in climate data, which helps to understand the drivers of their life cycles. Despite the simplicity of interpretation it brings about, to apply the FALWA diagnostic to climate data requires more involved calculations than the traditional Eulerian framework. This article introduces the open‐source Python packagefalwa,which encapsulates the FALWA diagnostics implemented on gridded climate data presented in the authors' previous publications. It reviews the essence of the FALWA theory, the corresponding components in the package that implement the calculations, and where users can find sample notebooks to start with. It aims to serve as a road map for new users to easily navigate through this package. The latter half of this article documents the practices of the developers, which include the documentation tools, continuous integration practice, and repository maintenance using automated GitHub functionalities. The authors also discuss existing numerical issues and future improvement plans. This open‐source project aims to promote the broader application of FALWA diagnostics on climate data and model outputs by streamlining complex numerical computations. 

Abstract Given the widespread presence of clouds in the midlatitudes, one expects significant effects of condensational and radiative processes on the large-scale circulation of the atmosphere, but these diabatic effects are hard to constrain from observation. The authors propose a simple method to estimate the diabatic effects on the waviness of the jet stream based on the observed column-mean budget of Rossby wave activity. Wave activity in the midlatitudes is maintained by injection due to surface baroclinicity and/or diabatic sources, downstream transport due to advection and wave radiation, and eventual dissipation through mixing and thermal damping. Once the diabatic sources of wave activity are identified from the residual of the budget, one can suppress them and recompute the budget assuming that the transport velocity and damping rate do not change and thereby assess the impact of the diabatic sources. For the Northern Hemisphere, we found significant positive values of the residual in regions coincident with high column cloud water, suggesting that there are diabatic sources of wave activity associated with clouds. In winter, maritime diabatic sources contribute to wave activity over the Atlantic and the Pacific by about 33% and 30%, respectively, while in summer, the numbers are lower. The estimates are based on the assumptions that the perturbed wave sources do not alter the flow and that sources and sinks are geographically separated. For the Southern Hemisphere, this last assumption is questionable, and therefore, the confidence level of the estimates is low. 

Large-scale circulation of the atmosphere in the Earth's extratropics is dominated by eddies, eastward (westerly) zonal winds, and their interaction. Eddies not only bring about weather variabilities but also help maintain the average state of climate. In recent years, our understanding of how large-scale eddies and mean flows interact in the extratropical atmosphere has advanced significantly due to new dynamical constraints on finite-amplitude eddies and the related eddy-free reference state. This article reviews the theoretical foundations for finite-amplitude Rossby wave activity and related concepts. Theory is then applied to atmospheric data to elucidate how angular momentum is redistributed by the generation, transmission, and dissipation of Rossby waves and to reveal how an anomalously large wave event such as atmospheric blocking may arise from regional eddy-mean flow interaction. 

This paper examines the role of wave–mean flow interaction in the onset and suddenness of stratospheric sudden warmings (SSWs). Evidence is presented that SSWs are, on average, a threshold behavior of finite-amplitude Rossby waves arising from the competition between an increasing wave activity A and a decreasing zonal-mean zonal wind [Formula: see text]. The competition puts a limit to the wave activity flux that a stationary Rossby wave can transmit upward. A rapid, spontaneous vortex breakdown occurs once the upwelling wave activity flux reaches the limit, or equivalently, once [Formula: see text] drops below a certain fraction of u REF , a wave-free, reference-state wind inverted from the zonalized quasigeostrophic potential vorticity. This fraction is 0.5 in theory and about 0.3 in reanalyses. We propose [Formula: see text] as a local, instantaneous measure of the proximity to vortex breakdown (i.e., preconditioning). The ratio r generally stays above the threshold during strong-vortex winters until a pronounced final warming, whereas during weak-vortex winters it approaches the threshold early in the season, culminating in a precipitous drop in midwinter as SSWs form. The essence of the threshold behavior is captured by a semiempirical 1D model of SSWs, similar to the “traffic jam” model of Nakamura and Huang for atmospheric blocking. This model predicts salient features of SSWs including rapid vortex breakdown and downward migration of the wave activity/zonal wind anomalies, with analytical expressions for the respective time scales. The model’s response to a variety of transient wave forcing and damping is discussed. 

Abstract This paper examines probability distributions oflocal wave activity(LWA), a measure of the jet stream's meander, and factors that control them. The observed column‐mean LWA distributions exhibit significant seasonal, interhemispheric, and regional variations but are always positively skewed in the extratropics, and their tail often involves disruptions of the jet stream. A previously derived one‐dimensional (1D) traffic flow model driven by observed spectra of transient eddy forcing qualitatively reproduces the shape of the observed LWA distribution. It is shown that the skewed distribution emerges from nonlinearity in the zonal advection of LWA even though the eddy forcing is symmetrically distributed. A slower jet and stronger transient and stationary eddy forcings, when introduced independently, all broaden the LWA distribution and increase the probability of spontaneous jet disruption. A quasigeostrophic two‐layer model also simulates skewed LWA distributions in the upper layer. However, in the two‐layer model both transient eddy forcing and the jet speed increase with an increasing shear (meridional temperature gradient), and their opposing influence leaves the frequency of jet disruptions insensitive to the vertical shear. When the model's nonlinearity in the zonal flux of potential vorticity is artificially suppressed, it hinders wave‐flow interaction and virtually eliminates reversal of the upper‐layer zonal wind. The study underscores the importance of nonlinearity in the zonal transmission of Rossby waves to the frequency of jet disruptions and associated weather anomalies.