[1] The Active Red Sea Trough (ARST) is an infrequent weather phenomenon that is associated with extreme precipitation, flash floods, and severe societal impacts in the Middle East (ME). Using reanalysis (ERA-Interim) and observational precipitation (Aphrodite and stations) data, we investigate its underlying dynamics, geographical extent, and seasonality. Twelve ARST events affecting the Levant have the same dynamical characteristics as those associated with a major flood in Jeddah (Saudi Arabia) on 25 November 2009. Hence, the Jeddah flooding was caused by an ARST, which implies that ARSTs can affect a much larger part of the ME than previously assumed. We present an ARST concept involving six dynamical factors: (1) a low-level trough; the Red Sea Trough (RST), (2) an anticyclone over the Arabian Peninsula; the Arabian Anticyclone (AA), (3) a transient midlatitude upper trough, (4) an intensified subtropical jet stream, (5) moisture transport pathways, and (6) strong ascent resulting from tropospheric instability and the synoptic-scale dynamical forcing. We explain the ARST as the interaction of a persistent stationary wave in the tropical easterlies (i.e., the RST) with a superimposed amplifying Rossby wave, resulting in northward propagating moist air masses over the Red Sea. Our findings emphasize the relevance of the AA, causing moisture transport from the Arabian and Red Seas. The particular topography in the Red Sea region and associated low-level circulation makes the ARST unique among tropical-extratropical interactions. The ARST seasonality is explained by the large-scale circulation and in particular the seasonal cycle of the semipermanent quasi-stationary RST and AA.

Extreme precipitation in the arid Middle East can cause flash floods with dramatic societal impacts. This study investigates the synoptic-scale dynamics of three extreme precipitation events that occurred in Saudi Arabia in autumn, winter and spring. Using ERA-Interim reanalysis, soundings and observational precipitation data, we study precipitation characteristics, the synoptic circulations, moisture transport pathways and forcing mechanisms for upward motion. All three cases involved strong tropical-extratropical interactions whereby midlatitude forcing instigated an incursion of tropical moisture over the Arabian Peninsula that fuelled the heavy rainfall. In each case, a midlatitude upper-level trough, associated with anticyclonic Rossby wave breaking, intruded into the subtropics. The phase relationship between this trough and the tropical low-level circulation was consistent with wave amplification through baroclinic growth. Eulerian and Lagrangian analyses reveal moisture transport from nearby and remote tropical regions, leading to above-normal tropospheric moisture content over Saudi Arabia. The autumn case (November 2009) showed a transient midlatitude upper-level trough that interacted with the climatological Red Sea Trough near the surface, being an Active Red Sea Trough' event. The winter case (January 2005) resembled tropical plume-like characteristics and demonstrated the coupling of a midlatitude cyclone and the equatorial low-pressure zone over Africa, an intensified subtropical jet stream, and pronounced moisture fluxes at middle and upper levels. The spring case (April-May 2013) involved a quasi-stationary cut-off low and persistent advection of low-level moist air masses, partly from the south Indian Ocean through cross-equatorial flow. The forcing of ascent was associated with low-level moisture convergence and decreased static stability (autumn case), dynamical lifting (winter case), strong surface sensible heating (spring case), and orographic lifting (all cases), favouring the build-up and release of potential instability. We discuss the three cases from a seasonal perspective and present a synthesis of their common key synoptic features

Extreme precipitation events in the otherwise arid Middle East can cause flooding with dramatic socioeconomic impacts. Most of these events are associated with tropical‐extratropical interactions, whereby a stratospheric potential vorticity (PV) intrusion reaches deep into the subtropics and forces an incursion of high poleward vertically integrated water vapor transport (IVT) into the Middle East. This study presents an object‐based identification method for extreme precipitation events based on the combination of these two larger‐scale meteorological features. The general motivation for this approach is that precipitation is often poorly simulated in relatively coarse weather and climate models, whereas the synoptic‐scale circulation is much better represented. The algorithm is applied to ERA‐Interim reanalysis data (1979–2015) and detects 90% (83%) of the 99th (97.5th) percentile of extreme precipitation days in the region of interest. Our results show that stratospheric PV intrusions and IVT structures are intimately connected to extreme precipitation intensity and seasonality. The farther south a stratospheric PV intrusion reaches, the larger the IVT magnitude, and the longer the duration of their combined occurrence, the more extreme the precipitation. Our algorithm detects a large fraction of the climatological rainfall amounts (40–70%), heavy precipitation days (50–80%), and the top 10 extreme precipitation days (60–90%) at many sites in southern Israel and the northern and western parts of Saudi Arabia. This identification method provides a new tool for future work to disentangle teleconnections, assess medium‐range predictability, and improve understanding of climatic changes of extreme precipitation in the Middle East and elsewhere.

Abstract. Extreme precipitation events (EPEs) frequently cause flooding with dramatic socioeconomic impacts in many parts of the world. Previous studies considered two synoptic-scale processes, Rossby wave breaking and intense moisture transport, typically in isolation, and their linkage to such EPEs in several regions. This study presents for the first time a global and systematic climatological analysis of these two synoptic-scale processes, in tandem and in isolation, for the occurrence of EPEs. To this end, we use 40-year ERA-Interim reanalysis data (1979–2018) and apply object-based identification methods for (i) daily EPEs, (ii) stratospheric potential vorticity (PV) streamers as indicators of Rossby wave breaking, and (iii) structures of high vertically integrated horizontal water vapour transport (IVT). First, the importance of these two synoptic-scale processes is demonstrated by case studies of previously documented flood events that inflicted catastrophic impacts in different parts of the world. Next, a climatological quantification shows that Rossby wave breaking is associated with >90 % of EPEs over central North America and the Mediterranean, whereas intense moisture transport is linked to >95 % of EPEs over many coastal zones, consistent with findings of atmospheric river-related studies. Combined Rossby wave breaking and intense moisture transport contributes up to 70 % of EPEs in several subtropical and extratropical regions, including (semi)arid desert regions where tropical–extratropical interactions are of key importance for (heavy) rainfall. Odds ratios of EPEs linked to the two synoptic-scale processes suggest that intense moisture transport has a stronger association with the occurrence of EPEs than Rossby wave breaking. Furthermore, the relationship between the PV and IVT characteristics and the precipitation volumes shows that the depth of the wave breaking and moisture transport intensity are intimately connected with the extreme precipitation severity. Finally, composites reveal that subtropical and extratropical EPEs, linked to Rossby wave breaking, go along with the formation of upper-level troughs and cyclogenetic processes near the surface downstream, reduced static stability beneath the upper-level forcing (only over water), and dynamical lifting ahead (over water and land). This study concludes with a concept that reconciles well-established meteorological principles with the importance of Rossby wave breaking and intense moisture transport for the formation of EPEs. Another conclusion with major implications is that different combinations of Rossby wave breaking and intense moisture transport can reflect a large range of EPE-related weather systems across climate zones and can thus form the basis for a new classification of EPE regimes. The findings of this study may contribute to an improved understanding of the atmospheric processes that lead to EPEs and may find application in climatic studies on extreme precipitation changes in a warming climate.