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Unprecedented extreme Saharan dust (duxt) events have recently expanded northward from subtropical NW Africa to the Atlantic and Europe, with severe impacts on the Canary Islands, mainland Spain and continental Portugal. These six historic duxt episodes occurred on 3–5 and 22–29 February 2020, 15–21 February 2021, 14–17 January 2022, 29 January–1 February 2022, and 14–20 March 2022. We analyzed data of 341 governmental air quality monitoring stations (AQMSs) in Spain (330) and Portugal (11), where PM10 and PM2.5 are measured with European norm (EN) standards, and found that during duxt events PM10 concentrations are underestimated due to technical limitations of some PM10 monitors meaning that they can not properly measure extremely high concentrations. We assessed the consistency of PM10 and PM2.5 data and reconstructed 1690 PM10 (1 h average) data points of 48 and 7 AQMSs in Spain and Portugal, respectively, by using our novel “duxt-r” method. During duxt events, 1 h average PM10 and PM2.5 concentrations were within the range 1000–6000 µgm-3 and 400–1200 µgm-3, respectively. The intense winds leading to massive dust plumes occurred within meteorological dipoles formed by a blocking anticyclone over western Europe and a cutoff low located to the southwest, near the Canary Islands and Cape Verde, or into the Sahara. These cyclones reached this region via two main paths: by deviating southward from the Atlantic mid-latitude westerly circulation or northward from the tropical belt. The analysis of the 2000–2022 PM10 and PM2.5 time series shows that these events have no precedent in this region. The 22–29 February 2020 event led to (24 h average) PM10 and PM2.5 concentrations within the range 600–1840 and 200–404 µgm-3, respectively, being the most intense dust episode ever recorded on the Canary Islands. The 14–20 March 2022 event led to (24 h average) PM10 and PM2.5 values within the range 500–3070 and 100–690 µgm-3 in southeastern Spain, 200–1000 and 60–260 µgm-3 in central Spain, 150–500 and 75–130 µgm-3 in the northern regions of mainland Spain, and within the ranges 200–650 and 30–70 µgm-3 in continental Portugal, respectively, being the most intense dust episode ever recorded in these regions. All duxt events occurred during meteorological anomalies in the Northern Hemisphere characterized by subtropical anticyclones shifting to higher latitudes, anomalous low pressure expanding beyond the tropical belt and amplified mid-latitude Rossby waves. New studies have reported on recent record-breaking PM10 and PM2.5 episodes linked to dipole-induced extreme dust events from North Africa and Asia in a paradoxical context of a multidecadal decrease in dust emissions, a topic that requires further investigation.

In July and August of 2022, unprecedented and long-lasting heatwaves attacked central and eastern China (CEC); and the most affected area was in the Yangtze River (YR) basin. The extreme heatwaves and associated drought and wildfire had significant social impacts, but the underlying mechanisms remain unknown. Observational analysis indicates that the heatwaves were regulated by anomalous anticyclone in the mid-upper troposphere over northern CEC. Specifically, the easterly anomalies at the southern flank of the anticyclone caused air isentropic sliding and transported low moist enthalpy (cold and dry) air to the YR basin, contributing to anomalous sinking motions and extreme heatwaves. In comparison, heatwaves were more serious in August than in July due to stronger upper-level anomalous anticyclone and associated easterlies. Importantly, different mechanisms were responsible for the heatwaves in the two months. In July, the relatively weaker anticyclonic anomaly over northern CEC was dominated by the forcing of diabatic heating over northwestern South Asia (NWSA), corresponding with the record-breaking rainfall in and around Pakistan. In August, a powerful anticyclonic condition for the CEC heatwaves originated from an extreme silk road pattern (SRP), superposing the effect of NWSA diabatic heating due to persistent downpour. We notice that another upstream anticyclonic node in the SRP also created heatwaves in Europe. Therefore, the CEC extreme heat was actually associated with other concurrent extremes over the Eurasian continent through large-scale atmospheric teleconnections in 2022.

Mediterranean-type climates are defined by temperate, wet winters, and hot or warm dry summers and exist at the western edges of five continents in locations determined by the geography of winter storm tracks and summer subtropical anticyclones. The climatology, variability, and long-term changes in winter precipitation in Mediterranean-type climates, and the mechanisms for model-projected near-term future change, are analyzed. Despite commonalities in terms of location in the context of planetary-scale dynamics, the causes of variability are distinct across the regions. Internal atmospheric variability is the dominant source of winter precipitation variability in all Mediterranean-type climate regions, but only in the Mediterranean is this clearly related to annular mode variability. Ocean forcing of variability is a notable influence only for California and Chile. As a consequence, potential predictability of winter precipitation variability in the regions is low. In all regions, the trend in winter precipitation since 1901 is similar to that which arises as a response to changes in external forcing in the models participating in phase 5 of the Coupled Model Intercomparison Project. All Mediterranean-type climate regions, except in North America, have dried and the models project further drying over coming decades. In the Northern Hemisphere, dynamical processes are responsible: development of a winter ridge over the Mediterranean that suppresses precipitation and of a trough west of the North American west coast that shifts the Pacific storm track equatorward. In the Southern Hemisphere, mixed dynamic–thermodynamic changes are important that place a minimum in vertically integrated water vapor change at the coast and enhance zonal dry advection into Mediterranean-type climate regions inland.

Atmospheric blocking represents an important aspect of the mid-latitude weather variability, but the different processes contributing to its formation and maintenance are not yet fully understood. This study investigates the role that diabatic processes, in particular the release of latent heating in strongly ascending airstreams, play in the dynamics and spatio-temporal variability of blocking in a detailed 38-year global climatological analysis. The results show that the formation and (re-)intensification of blocking are often preceded by latent heating connected to upstream baroclinic developments. While the importance of latent heating varies considerably between individual blocking events and different regions, in particular between ocean and continents, latent heating is generally most important during onset and in more intense and larger blocks. The episodic nature of latent heating during the blocking life cycle, associated with a series of transient cyclones approaching the blocking, can contribute to both the high- (fast onset and fluctuation in intensity and size) and low-frequency (maintenance and quasi-stationarity during maturation phase) properties of blocking anticyclones and provide the required flow amplification in addition to dry-dynamical interaction between synoptic eddies and blocking. This amplification results from a combination of the direct injection of anticyclonic air into the upper-troposphere within cross-isentropic ascending airstreams, setting up large-scale anticyclonic PV anomalies, and the advection of PV by the enhanced divergent outflow at the tropopause (indirect effect). This divergent outflow on the western flank of the blocking anticyclone interacts with the upper-level PV gradient and leads to a westward amplification of the ridge, diminishing the tendency for dissipation and the eastward advection by the background flow, thus contributing to blocking stationarity. Taking into account such diabatic mechanisms in blocking dynamics will be important to improve predictions of blocking and assess future changes in the extratropical large-scale circulation.

The anticyclonic high-pressure systems over the southern-hemisphere, subtropical oceans have a significant influence on regional climate. Previous studies of how these subtropical anticyclones will change under global warming have focused on austral summer while the winter season has remained largely uninvestigated, together with the extent to which the dominant mechanisms proposed to explain the multi-model-mean changes similarly explain the inter-model spread in projections. This study addresses these gaps by focusing on the mechanisms that drive the spread in projected future changes across the Coupled Model Intercomparison Project Phase 5 and 6 archives during both the summer and winter seasons. The southern hemisphere anticyclones intensify in strength at their center and poleward flank during both seasons in the future projections analyzed. The inter-model spread in projected local diabatic heating changes accounts for a considerable amount of the inter-model spread in the response of the South Pacific anticyclone during both seasons. However, model differences in projected zonal-mean tropospheric static stability changes, which in turn influence baroclinic eddy growth, are most influential in determining the often-strong increases in sea level pressure seen along the poleward flank of all the anticyclones during both seasons. Increased zonal-mean tropospheric static stability over the subtropics is consistent with the poleward shift in Hadley cell edge and zonal-mean sea level pressure increases. The results suggest that differences in the extent of tropical-upper-tropospheric and subtropical-lower-tropospheric warming in the southern hemisphere, via their influence on tropospheric static stability, will largely determine the fate of the anticyclones over the coming century.

Persistent open ridges and blocking highs (maxima) of 500-hPa geopotential height (Z500; PMZ) adjacent in space and time are identified and tracked as one event with a Lagrangian objective approach to derive their climatological statistics with some dynamical reasoning. A PMZ starts with a core that contains a local eddy maximum of Z500 and its neighboring grid points whose eddy values decrease radially to about 20 geopotential meters (GPMs) smaller than the maximum. It connects two consecutive cores that share at least one grid point and are within 10° of longitude of each other using an intensity-weighted location. The PMZ ends at the core without a successor. On each day, the PMZ impacts an area of grid points contiguous to the core and with eddy values decreasing radially to 100 GPMs. The PMZs identified and tracked consist of persistent ridges, omega blockings and blocked anticyclones either connected or as individual events. For example, the PMZ during 2–13 August 2003 corresponds to persistent open ridges that caused the extreme heatwave in Western Europe. Climatological statistics based on the PMZs longer than 3 days generally agree with those of blockings. In the Northern Hemisphere, more PMZs occur in DJF season than in JJA and their duration both exhibit a log-linear distribution. Because more omega-shape blocking highs and open ridges are counted, the PMZs occur more frequently over Northeast Pacific than over Atlantic-Europe during cool seasons. Similar results are obtained using the 200-hPa geopotential height (in place of Z500), indicating the quasi-barotropic nature of the PMZ.

A statistical link is presented between atmospheric blocks in the Euro-Atlantic sector and the frequency of regional-scale heavy precipitation events in Europe. Changes in the odds of European 1-, 3- and 5-day accumulation heavy precipitation in the presence of a block are investigated for different geographical locations of blocks and for summer and winter. The results show a significant modulation of the odds of heavy precipitation events during blocking episodes over the North-Atlantic and Europe. Blocks located further east have only limited effects on the odds of heavy precipitation events over Europe. The spatial patterns are very diverse and are strongly dependent on the location of the blocks and on the season. Generally, the odds of heavy precipitation events are reduced in the area of the blocking anticyclone and increased in the areas southwest to southeast of it and in some cases also north of it. Often areas with increased odds of heavy precipitation coincide with the location of the storm track.

Northwestern North America has experienced an exceptional heatwave in late June 2021 with many new temperature records across western Canada, Oregon and Washington states. Here we use a recent atmospheric reanalysis and a conditional approach based on dynamical adjustment to assess and quantify the influence of atmospheric circulation and other driving factors to the heatwave magnitude during the June 28–30 period. A blocking anticyclone, enhanced low‐level moisture and clear‐sky downward long‐wave radiation are shown to be the main factors of the heatwave persistence and magnitude. The heatwave magnitude is mainly attributable to internal variability with climate change being an additional factor (10%). Consequences of a similar atmospheric circulation anomaly in different phases of the Pacific Decadal Oscillations and in a warmer world at different global warming levels (1, 2, 3, and 4°C) are explored based on a single model initial‐condition large ensemble. Plain Language Summary Gathering robust statistics and performing extreme event attribution for very rare heat extreme events, such as the 2021 Northwestern North American heatwave, remain challenging due to incomplete sampling of weather data (∼100 years) challenging the application of extreme value theory and caveats related to the use of imperfect climate models in estimating likelihood changes between worlds with and without human influence. Here we use the dynamical adjustment method to quantify the key factors responsible for the magnitude and persistence of the heatwave. Dynamical adjustment aims to identify the causal factors that led to the heatwave with an approach conditional on the observed atmospheric circulation during the event. We find that natural variability is the main driver of the heatwave extreme magnitude with a small contribution from climate change. We also find that the heatwave spatial pattern mainly comes from the atmospheric circulation‐related component (the dynamic component). We investigate a possible contribution due the strong negative phase of the Pacific Decadal Oscillation observed in June 2021 and find it to be rather small. Finally, we ask whether future climate change can make a future similar event even more extreme. We find that the dynamic component would increase by 4°C in a 2°C warmer global climate. Key Points A blocking ridge, enhanced moisture and clear‐sky downward long‐wave radiation are the main causes of the heatwave magnitude and duration The circulation‐induced heatwave component is the main driver of the heatwave pattern and magnitude with a minor role for climate change A similar blocking event in a 2°C warmer climate would lead to a 4°C increase of the circulation‐induced heatwave component

Subtropical anticyclones dominate the subtropical ocean basins in summer. Using the multimodel output from phase 5 of the Coupled Model Intercomparison Project (CMIP5), the future changes of the subtropical anticyclones as a response to global warming are investigated, based on the changes in subsidence, low-level divergence, and rotational wind. The subtropical anticyclones over the North Pacific, South Atlantic, and south Indian Ocean are projected to become weaker, whereas the North Atlantic subtropical anticyclone (NASA) intensifies, and the South Pacific subtropical anticyclone (SPSA) shows uncertainty but is likely to intensify. Diagnostic analyses and idealized simulations suggest that the projected changes in the subtropical anticyclones are well explained by the combined effect of increased tropospheric static stability and changes in diabatic heating. Increased static stability acts to reduce the intensity of all the subtropical anticyclones, through the positive mean advection of stratification change (MASC) over the subsidence regions of the subtropical anticyclones. The pattern of change in diabatic heating is dominated by latent heating associated with changes in precipitation, which is enhanced over the western North Pacific under the “richest get richer” mechanism but is reduced over subtropical North Atlantic and South Pacific due to a local minimum of SST warming amplitude. The change in the diabatic heating pattern substantially enhances the subtropical anticyclones over the North Atlantic and South Pacific but weakens the North Pacific subtropical anticyclone.

The thermodynamic processes and synoptic circulation features driving lower-tropospheric temperature extremes in the high Arctic (>80°N) are investigated. Based on 10-day kinematic backward trajectories from the 5% most intense potential temperature anomalies, the contributions of horizontal and vertical transport, subsidence-induced warming, and diabatic processes to the generation of the Arctic temperature anomaly are quantified. Cold extremes are mainly the result of sustained radiative cooling due to a sheltering of the Arctic from meridional airmass exchanges. This is linked to a strengthening of the tropospheric polar vortex, a reduced frequency of high-latitude blocking, and in winter also a southward shift of the North Atlantic storm track. The temperature anomaly of 60% of wintertime extremely warm air masses (90% in summer) is due to transport from a potentially warmer region. Subsidence from the Arctic midtroposphere in blocking anticyclones is the most important warming process with the largest contribution in summer (70% of extremely warm air masses). In both seasons, poleward transport of already warm air masses contributes around 20% and is favored by a poleward shift of the North Atlantic storm track. Finally, about 40% of the air masses in winter are of an Arctic origin and experience diabatic heating by surface heat fluxes in marine cold air outbreaks. Our study emphasizes the importance of processes in the Arctic and the relevance of anomalous blocking—in winter in the Barents, Kara, and Laptev Seas and in summer in the high Arctic—for the formation of warm extremes.