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

Surface anticyclones connected to the ridge of an upper-tropospheric Rossby wave are the dynamical drivers of mid-latitude summer heatwaves. It is, however, unclear to which extent an anomalously low zonal phase speed of the wave in the upper troposphere is necessary for persistent temperature extremes at the surface. Here, we use spectral analysis to estimate a categorical phase speed for synoptic-scale waves. A composite analysis of ERA5 reanalysis data reveals how a meridional shift in the Rossby wave packet envelope associated with a change in phase speed alters the geographically phase-locked stationary wave pattern. In both composites for amplified low or high phase speed waves, respectively, the ridges and troughs of these temporal-mean wave trains show enhanced and reduced heatwave frequency. The phase speed of synoptic-scale waves is, hence, crucial for where, but less important for whether heatwaves occur.

Roughly one-third of sudden stratospheric warming (SSW) events lack a strong canonical surface impact, and this can lead to a forecast bust if a strong impact had been predicted. Hence, it is important to predict before the SSW onset if an event will propagate downward. The predictability of the downward impact of SSWs is considered in 7 subseasonal-to-seasonal forecast models for 16 major SSWs between 1998 and 2022, a larger sample size than considered by previous works. The models successfully predict which SSWs have a stronger downward impact to 100hPa, however they struggle to predict which have a stronger tropospheric impact. The downward impact is stronger if the deceleration of the 10hPa winds is better predicted. Downward impact is stronger for split and for absorbing SSWs, and is better predicted in high-top models. In contrast, there is little relationship between SSWs with above-average predictability and the subsequent downward impact.

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.

The Arctic stratospheric polar vortex is an important driver of winter weather and climate variability and predictability in North America and Eurasia, with a downward influence that on average projects onto the North Atlantic Oscillation (NAO). While tropospheric circulation anomalies accompanying anomalous vortex states display substantial case-by-case variability, understanding the full diversity of the surface signatures requires larger sample sizes than those available from reanalyses. Here, we first show that a large ensemble of seasonal hindcasts realistically reproduces the observed average surface signatures for weak and strong vortex winters and produces sufficient spread for single ensemble members to be considered as alternative realizations. We then use the ensemble to analyze the diversity of surface signatures during the 25% weakest and strongest vortex winters. Over Eurasia, only one of three weak vortex clusters yields continent-wide cold conditions, suggesting that the observed Eurasian cold signature could be artificially strong due to insufficient sampling. For both weak and strong vortex cases, the canonical temperature pattern in Eurasia only clearly arises when North Atlantic sea surface temperatures exhibit the tripolar structure in-phase with the NAO. Over North America, while the main driver of interannual winter temperature variability is the El Nino;Southern Oscillation (ENSO), the stratosphere can modulate ENSO teleconnections, affecting temperature and circulation anomalies over North America and downstream. These findings confirm that anomalous vortex states are associated with a broad spectrum of surface climate anomalies on the seasonal scale, which may be obscured by the small observational sample size.

Many indices have been defined to estimate the intensity of a heatwave. However, these indices are often used indiscriminately, without sufficient consideration of their possible different results and of the challenges that this poses to a proper characterization and comparison of events. This study, by comparing four different indices applied to reanalyses data, shows that the choice of heatwave intensity metrics has important effects on the detection of the most intense events for the period 1950-2021, with indices based on cumulative values of a target variable that must be preferred over the ones relying on temporal averages. Under these considerations, one of the given indices is additionally selected for the study of heatwaves of the period 1950-2021, showing that heatwaves that were unlikely before 1986 have become up to ten times more usual and up to three times more intense during recent times.