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A unique chain connects the flow over the North Atlantic and the development of cyclones within the Mediterranean basin. One typical mechanism includes several successive processes: upper-level flow perturbations upstream cause Rossby wave breaking (RWB) events along the jet stream, which in turn develop into potential vorticity streamers. These streamers reach the Mediterranean, and through increased baroclinicity they enhance cyclonic activity in the region. Using ERA5 reanalysis data and rain gauge measurements, we provide a systematic analysis connecting wintertime North Atlantic storm track regimes and Eastern Mediterranean cyclones and rainfall. To do so, we use different detection algorithms for each element in the chain (RWBs, streamers and cyclones). A cluster analysis of upper tropospheric eddy kinetic energy reveals a favorable configuration of the storm track where North Atlantic storms are able to propagate farther northeast. This results in upper-level potential vorticity streamers forming more eastward alongside above-average precipitation over the Levant. Meanwhile, other latitudinal positions of the storm track (southward or northward) were found to hinder cyclonic activity in the region and reduce rainfall there. The intense rainy winter of 1991–1992 is brought as a test case to exemplify this mechanism in its extreme. We show that the rain-enhancing storm track regime was prominent throughout most of this season, alongside frequent streamers in the Eastern Mediterranean. [ABSTRACT FROM AUTHOR]

Rossby wave-breaking events describe the last stage in the life cycle of baroclinic atmospheric disturbances. These breaking events can strongly influence large-scale circulation and are also related to weather extremes such as heat waves, blocking, and extreme precipitation events. Nonetheless, a complete understanding of the synoptic-scale dynamics involved with the breaking events is still absent. For example, it is not clear how well the theoretical life cycle experiments, which use a specified initial perturbation with a single zonal wavenumber and a prescribed simplified initial zonal jet, capture the life cycle of real-atmosphere weather systems. Here we combine a storm-tracking technique together with a wave-breaking detection algorithm to examine how upper-level wave breaking and surface weather systems are related in the North Atlantic during winter. These datasets allow us to examine whether upper-level wave breaking and low-level weather systems always occur simultaneously and if we can identify preferred relations between the surface weather system type (cyclone or anticyclone) and the type of the upper-level breaking event (cyclonic or anticyclonic wave breaking denoted CWB or AWB, respectively). We find that in the North Atlantic, most weather systems are associated with AWB and/or CWB at some point during their lifetime, while only few cyclones and anticyclones do not involve any upper-level wave breaking (roughly 11 % and 15 %, respectively). Our results imply that composites of cyclones and anticyclones involve a mixture of different types of life cycles, depending on whether they involve CWB or AWB, as well as their position relative to the Rossby wave-breaking (RWB) center. Moreover, the system characteristics (including actual and relative positions, intensities, and displacements) differ depending on the associated breaking type. We distinguish between "same-pairing" cases (i.e., cyclone with CWB and anticyclones with AWB) and "opposite-pairing" cases (i.e., cyclones with AWB and anticyclones with CWB). Compositing the cyclones and anticyclones based on this criterion, we find that in similar pairings the surface system is positioned so that its associated upper-level winds would enhance the breaking (the anomalous circulation is in the same direction as the background shear), but, for opposite pairings, the upper-level winds associated with the surface system do not act to enhance the breaking which occurs in the direction of the background shear. A better understanding of the different life cycles of real-atmosphere cyclones and anticyclones and the upper-level breaking they involve is important for exploring the relation between storm tracks and slowly varying weather regimes and how they are mediated by RWB events. [ABSTRACT FROM AUTHOR]

Rossby wave breaking events describe the last stage in the life-cycle of baroclinic atmospheric disturbances. These breaking events can strongly influence the large-scale circulation, and are also related to weather extremes such as heat waves, blockings, and extreme precipitation events. Nonetheless, a complete understanding of the synoptic-scale dynamics involved with wave breaking events is still absent. Here we highlight the fundamental relation between low-level weather systems and upper-level wave breaking events in the North Atlantic region, by combining a storm-tracking technique together with a wave breaking detection algorithm. We show that Anticyclonic Wave Breaking (AWB) events are associated with a strong upper-level ridge and a low-level anticyclone to its east, which are both located in the anticyclonic side of the jet. During the breaking, a strong cyclone is often found to the north-northwest of the anticyclone, while a weaker one is often found to its south-southeast. Time evolution composites centered around anticyclones during AWB show that as the downstream trough wraps anticyclonically to the east and south of the ridge, the upper-level jet weakens to the south, hence the upper-level jet becomes more split into an upstream poleward tilted jet, and a downstream zonal jet. In addition, the Sea Level Pressure (SLP) anomalies rotate in an anticyclonic manner relative to each other, such that the initially zonally oriented low-high SLP anomaly dipole become meridionally oriented by the end of the life-cycle (low-above-high). On the contrary, Cyclonic Wave Breaking (CWB) events are associated with a strong upper-level trough and a low-level cyclone to its east, which are both located in the cyclonic side of the jet. An additional anticyclone is often found to the northeast of the cyclone. Time evolution composites centered around cyclones during CWB show that as the downstream ridge wraps cyclonically to the east and north of the trough, the zonal and more southward shifted upper-level jet weakens further to the north. A relative cyclonic rotation is observed at low-levels, such that a high-above-low SLP anomaly dipole is found by the end of the life-cycle. A simple kinematic interpretation is suggested for the poleward and equatorward shifted jets associated with AWB and CWB events, respectively. Anomalous life-cycles of anticyclones during CWB events and cyclones during AWB events are also discussed. [ABSTRACT FROM AUTHOR]

Atmospheric temperature distributions are often identified with their variance, while the higher-order moments receive less attention. This can be especially misleading for extremes, which are associated with the tails of the probability density functions (PDFs), and thus depend strongly on the higher-order moments. For example, skewness is related to the asymmetry between positive and negative anomalies, while kurtosis is indicative of the "extremity" of the tails. Here we show that for near-surface atmospheric temperature, an approximate linear relationship exists between kurtosis and skewness squared. We present a simple model describing this relationship, where the total PDF is written as the sum of three Gaussians, representing small deviations from the climatological mean together with the larger-amplitude cold and warm temperature anomalies associated with synoptic systems. This model recovers the PDF structure in different regions of the world, as well as its projected response to climate change, giving a simple physical interpretation of the higher-order temperature variability changes. The kurtosis changes are found to be largely predicted by the skewness changes. Building a deeper understanding of what controls the higher-order moments of the temperature variability is crucial for understanding extreme temperature events and how they respond to climate change. [ABSTRACT FROM AUTHOR]