The atmosphere’s flow becomes unpredictable beyond a certain time due to the inherent growth of small initial-state errors. While much research has focused on tropospheric predictability, predictability of the middle atmosphere remains less studied. This work contrasts the intrinsic predictability of different layers, with a focus on the mesosphere/lower thermosphere (MLT, ~50-120 km altitude). Ensemble simulations with the UA-ICON model for an austral winter/spring season are conducted with a gravity-wave permitting horizontal resolution of 20 km. Initially small perturbations grow fastest in the MLT, reaching 10% of saturation after 5-6 days, compared to 10 days in the troposphere and two weeks in the stratosphere. However, perturbation energy in the MLT reaches 50% saturation only after about two weeks, similar to the troposphere. Those saturation times are overestimated by up to a factor of two when using a coarser resolution (grid size 160 km), highlighting the need for gravity wave-resolving models. Predictability in the MLT depends on horizontal scales. Motions on scales of hundreds of kilometers are predictable for less than five days, while larger scales (thousands of kilometers) remain predictable for up to 20 days. This scale-dependent progression of predictability cannot be explained by simple scaling for upscale error growth. Vertical wave propagation plays a significant role, with gravity waves transmitting perturbations upward at early lead times and planetary waves enhancing long-term predictability. In summary, the study shows that MLT predictability is scale-dependent and highlights the necessity of high-resolution models to capture fast-growing perturbations and assess intrinsic predictability limits accurately.

A document by Tiffany A Shaw. Click on the document to view its contents.

February-March 2020 was marked by highly anomalous large-scale circulations in the Northern extratropical troposphere and stratosphere. The Atlantic jet reached extreme strength, linked to some of the strongest and most persistent positive values of the Arctic Oscillation index on record, which provided conditions for extreme windstorms hitting Europe. Likewise, the stratospheric polar vortex reached extreme strength that persisted for an unusually long period. Past research indicated that such circulation extremes occurring throughout the troposphere-stratosphere system are dynamically coupled, although the nature of this coupling is still not fully understood and generally difficult to quantify. We employ sets of numerical ensemble simulations to provide statistical characterizations of the mutual coupling of the early 2020 circulation extremes. We find an overall robust coupling between tropospheric and stratospheric anomalies: ensemble members with polar vortex exceeding a certain strength tend to exhibit a stronger tropospheric jet and vice versa. Moreover, members exhibiting a breakdown of the stratospheric circulation (associated with a sudden stratospheric warming) tend to lack periods of persistently enhanced tropospheric circulation. A crucial component in determining the stratospheric vortex strength appears to be the vortex geometry, corresponding to states that promote either reflection or dissipation of planetary waves. Despite these lines of evidence for vertical coupling, our simulations underline the role of internal variability within each atmospheric layer. The circulation extremes during early 2020 may be viewed as resulting from a fortuitous alignment of dynamical evolutions within the troposphere and stratosphere, aided by each layer’s modification of the other layer’s boundary condition.

Sudden stratospheric warmings (SSWs) are impressive fluid dynamical events in which large and rapid temperature increases in the winter polar stratosphere (~0–50km) are associated with a complete reversal of the climatological wintertime westerly winds. SSWs are fundamentally caused by the breaking of planetary-scale waves that propagate upwards from the troposphere. During an SSW, the polar vortex breaks down, accompanied by rapid warming of the polar air column. This rapid warming and descent of the polar air column affects tropospheric weather, shifting jet streams, storm tracks, and the Northern Annular Mode (NAM), including increased frequency of cold air outbreaks over North America and Eurasia. SSWs affect the whole atmosphere above the stratosphere producing widespread effects on atmospheric chemistry, temperatures, winds, neutral (non-ionized) particle and electron densities, and electric fields. These effects span the surface to the thermosphere and across both hemispheres. Given their crucial role in the whole atmosphere, SSWs are also seen as a key process to analyze in climate change studies and subseasonal to seasonal predictions. This work reviews the current knowledge on the most important aspects related to SSWs from the historical background to involved dynamical processes, modelling, chemistry and impact on other atmospheric layers.