Red Sea Trough Research Articles

The sources of extreme precipitation predictability; the case of the ‘Wet’ Red Sea Trough

Extreme precipitation events inflict detrimental socio-economic impacts in the Eastern Mediterranean. These are mainly associated with Mediterranean cyclones or the ‘Wet’ Red Sea Trough (WRST). The region's weather forecasters consider the second challenging to forecast, even just a few days in advance. Here, we study the dynamic and thermodynamic factors influencing the intrinsic predictability of WRST events. With this aim, we combine insights from traditional atmospheric analysis techniques, Lagrangian air-parcel backward trajectories, and dynamical systems theory. The latter describes atmospheric states via their local dimension (d) and inverse persistence (θ), which inform us of the intrinsic predictability of the atmosphere in phase space. We compare WRST events of low (upper decile of d and θ) with high (lower decile of d and θ) predictability. We argue that low-predictability events display a significantly different atmospheric pattern. Moreover, the low-predictability events show significantly higher daily precipitation rates, more extensive spatial spread, and greater precipitation variability among events than more predictable ones. On average, low predictability events are initiated by two distinct moisture sources with different water vapor content. We conclude that the dynamical systems framework may become a valuable tool to improve the forecast of extreme precipitation events associated with the WRST by providing a priori information on their intrinsic predictability. We foresee successfully implementing such a framework for other extreme weather events and regions.

Synoptic characteristics of the spatial variability of spring dust storms over Saudi Arabia

Statistical and synoptic studies of spring dust storms over the Arabian Peninsula (AP) were performed using surface observations from 27 surface stations and meteorological data from the NCEP/NCAR reanalysis data set for the period 1978-2008. The study showed that, spatially, the northern and eastern AP are the regions most affected by dust storms and that, temporally, the study period can be divided into two subperiods: before and after 1995, with a pronounced increase before 1995 and a smooth increase (decrease) after 1995 with respect to dust (dust storms) types. The synoptic study reveals three main atmospheric systems: frontal systems over the northern region, Red Sea Trough ()-related systems over the western region, and thermal low systems over the eastern region. Additionally, the synoptic study shows that all the atmospheric systems are associated with a favorable pressure (geopotential) gradient area and that the shape and strength of the maximum wind and upper-layer atmospheric regimes are suitable for completely integrating the atmospheric layers. Moreover, the southern thermal low is a common synoptic component of dust-related atmospheric systems, but its effect is particularly pronounced in the atmospheric system of the eastern region.

Atmospheric conditions leading to buildup of benzene concentrations in urban areas in Israel

Benzene is a carcinogenic air pollutant and its concentration in the atmosphere is derived from the rate of its emission and the atmospheric conditions that cause it to be dispersed and transported. An analysis of the daily benzene concentrations for a period of six years measured in the three largest population centers in Israel (Tel-Aviv, Jerusalem and Haifa) revealed that practically all values were lower than the Israeli 24-h air quality standard. Even though, daily elevated benzene occurrences (here defined as being higher than 30 percent of the 24-h standard) were found to be associated with four out of the 22 synoptic weather types prevailing over the Eastern Mediterranean, more than half of them occurring during the winter and the transitional seasons. These weather types are characterized by a persistent south-easterly dry flow over Israel.During nighttime the very high frequency of the radiative inversion characterizing these 4 synoptic types trap the benzene near the surface, resulting in elevated concentrations. At daytime, a significantly higher relative frequency of weak winds was found for the four relevant synoptic types, as compared to their overall relative frequency. These calm conditions contribute to the observed buildup in the benzene concentrations.The very high prevalence of both radiative inversion during the nights and weak winds in daytime for the four synoptic circulation types did not alter the strength of the synoptic classification in predicting the prevalence of these elevated benzene occurrences. The annual frequency of benzene elevated concentrations is about 30 percent, mostly associated with the presence of the two modes of the Red Sea Trough (eastern and central axis). This points at the strength of the synoptic classification to serve as a valuable predictor of elevated concentrations.

Identification and classification of the wet Red Sea Trough over Israel

AbstractThe Red‐Sea Trough (RST) extends toward the Levant from the south and is mostly associated with dry conditions. This study analysed the rarer type, rain‐producing RSTs, referred to as ‘wet RSTs’ (WRST), using data from 1958 to 2019. A WRST archive was established, based on rain data from a study region covering the arid part of Israel, the region where this system is most active. The study also aimed to find an atmospheric proxy for rainfall, to replace precipitation estimates obtained from climate models. A WRST day was defined as a day when the RST was identified and rainfall of ≥0.1 mm was observed by at least two of the 39 rain stations used. The annual occurrence of WRSTs is 16.4 days, ~16% of all RSTs, peaking in the winter. The relative contribution of WRSTs to the annual rainfall ranges from 16% in the northern part of the study region to 50% in the south. The tropical air intrusion was quantified through trajectory analysis and the results revealed that in most WRST days, 72%, the fraction of air originating from the tropics was less than 10% and only in 3.7% of the days did it exceed 50%. Attempts to extract an atmospheric proxy which identifies a WRST did not yield significant separation between dry and wet RSTs, because of significant differences among several WRST types. Therefore, identification of future changes in WRSTs will still require the use of rainfall data offered by climate models. We automatically classified the WRST days into four types. Two types are associated with 850 hPa negative temperature anomalies and the other two with higher‐than‐normal temperatures, resembling ‘tropical‐like’ RSTs, which are known as ‘active RSTs’ and tropical plumes. A predictive equation was extracted to quantify the tropical air intrusion and replace the use of trajectory analysis in future studies.

Associations of Weather Regimes and Ambient Temperature with the Timing of Birth in Southern Israel

BACKGROUND AND AIM: Previous studies demonstrated associations between exposure to ambient temperature and timing of birth, but did not consider the role of weather regimes as an independent factor or as a modifier of the effect of temperature. We aimed to investigate associations of weather regimes and ambient temperature with the timing of birth in Southern Israel. METHODS: We conducted a case-crossover study of 60,761 singleton live births in Southern Israel during 2004-2014. The case exposure period was the week before the birth date and the control exposure period was 28 days later. Ambient temperature was estimated using a spatio-temporal model at 1 km resolution in Israel. We fit separate conditional logistic regression models for preterm and term births using weekly mean temperatures divided into five quintiles, adjusting for seasonality. Additionally, we created similar models for assessing the associations between weather regimes and the timing of birth for each day preceding the delivery up to 6 days, adjusted for seasonality and temperature. RESULTS: Higher temperature during the week before the birth was associated with the timing of term birth, with an adjusted odds ratio of 1.18 (95% CI: 1.07-1.31) for the highest quintile compared with the lowest, but no association for preterm births. The presence of a Persian Trough was consistently associated with timing of term births, with the strongest association on the date of birth (odds ratio = 1.07, 95% CI: 1.02–1.12) compared with Red Sea Trough. In addition, the association between temperature and term birth was stronger during weeks with a Persian Trough (odds ratio per 1 °C increase = 1.46, 95% CI: 1.44-1.49). CONCLUSIONS: These findings demonstrate a possible role for weather regimes in determining the timing of birth beyond their influence on ambient temperature. Keywords: Ambient temperature, Weather regimes, Timing of birth

Extreme weather and societal impacts in the eastern Mediterranean

Abstract. Gaining a holistic understanding of extreme weather, from its physical drivers to its impacts on society and ecosystems, is key to supporting future risk reduction and preparedness measures. Here, we provide an overview of the state of the art, knowledge gaps and key open questions in the study of extreme weather events over the vulnerable eastern Mediterranean. This region is situated in a transition zone between subtropical and mid-latitude climates. The large-scale atmospheric circulation and its interaction with regional synoptic systems (i.e., Cyprus Lows, Red Sea Troughs, Persian Troughs, “Sharav” Lows) and high-pressure systems mainly govern extreme weather. Complex orographic features further play an important role in the generation of extreme weather. Most extreme weather events, including heavy precipitation, cold spells, floods and windstorms, are associated with Cyprus Lows or active Red Sea Troughs, whereas heat waves are related with either Persian Troughs and sub-tropical high-pressure systems in summer or the Sharav Low during springtime. In future decades, heat waves and droughts are projected to significantly increase in both frequency and intensity. Changes in heavy precipitation may vary in sign and magnitude depending on the scale, severity and region of interest. There are still relatively large uncertainties concerning the physical understanding and the projected changes of cold spells, windstorms and compound extremes, as these types of events received comparatively little attention in the literature. We further identify knowledge gaps that relate to the societal impacts of extreme weather. These gaps mainly relate to the effects extreme weather may have on mortality, morbidity and infrastructure in the eastern Mediterranean. Research is currently limited in this context, and we recommend strengthening the database of analyzed case studies. We trust that this can only be suitably accomplished by inter-disciplinary and international regional collaboration (in spite of political unrest).

Diagnostic study of diabatic heating and potential vorticity during a case of cyclogenesis

On 16-17 November, 2015, north and middle regions of Saudi Arabia were hit by a case of cyclogenesisassociated with heavy rainfall. This work presents a diagnostic study of this heavy rainfallcase based on the analysis of diabatic heating and potential vorticity. The synoptic analysis investigate that the important dynamical factors that causes this case are the northward extension of Red Sea Trough, anticyclone over the Arabian Peninsula, a travailing midlatitude upper trough, moisture transport pathways and strong upward motion arising from tropospheric instability. The calculation of diabatic heating by the thermodynamic equation illustrate that the contribution of vertical temperature advection and the adiabatic term are opposite to each other during the period of study. The largest contribution of the horizontal cold advection occurs during the first two days while the largest contribution of the horizontal warm advection occurs during the maximum development days. The dynamics of the studied case are also investigated in terms of isobaric Potential Vorticity. It is found that the location of the low-level Potential Vorticity anomaly and the Potential Vorticity generation estimates coincides with the heating region, which implies that condensation supports a large enough source to explain the existence of the low-level Potential Vorticity anomaly.

Formation of cyclones over the East Mediterranean within Red‐Sea Troughs

AbstractThe Red‐Sea Trough (RST) is a lower‐level trough extending from the tropical low‐pressure to the Levant. Its annual occurrence in the Levant is 20%, mostly in the autumn. It produces dry weather, but occasionally causing local showers and floods. During winter, the dominant synoptic system over the eastern Mediterranean is the Cyprus low (CL). Previous studies showed that some CLs form within pre‐existing RSTs, through a tropical–extratropical interaction.This study is the first comprehensive climatological framework of such formation events, analyzing their occurrence, seasonality, and the resulting rainfall in Israel. The study identified and analyzed CLs formed within the domain 31°–35°N, 30°–36°E within an RST that was detected during the preceding 24 hr. We used the 6‐hourly ERA‐Interim database, with 0.75° × 0.75° resolution, during 1979–2017, and identified 104 formation events, which constitute 10% of the CLs. Most formation events occur during fall and early winter, similarly to the RST. Eighty four percent of them formed during the evening or night, and almost two thirds of the CLs disappeared temporarily at noon and regenerated afterwards. This is attributed to the sea/land diurnal oscillation. Most of the formed CLs were shallow, with little rain, but some became major storms, like ‘Alexa’, which caused extreme snowing in Jerusalem, in December 2013. The formation scenarios were divided into four categories, according to the location and propagation of the 500‐hPa trough. The first is characterized by a closed cyclone approaching from southwest, often connected to active RSTs. In the second, a trough is deepening from the northern sector, possibly a polar intrusion. In the third, the most populated, a trough is approaching from west. The fourth category contains events with no upper‐level support. Composite maps indicate that upper‐level positive vorticity advection triggered CL formations, on top of pre‐existing above‐normal lower‐level temperatures, imparted by the RST.

Speleothem growth phases in the central Eastern Desert of Egypt reveal enhanced humidity throughout MIS 5

Speleothem deposits in nowadays arid environments are important climate archives, as they indicate phases of enhanced precipitation and can precisely be dated by uranium-series dating. So far only very few of such archives have been found in the today hyper-arid Saharo-Arabian Desert (SAD). Therefore, the study at hand fills a gap that exists for speleothem climate archives in Northeast Africa. A new record from Saqia Cave (Central Eastern Desert, Egypt) documents for the first-time speleothem growth in Egypt for all sub-stages of MIS 5 and singular phases during MIS 6. Most important growth phases occur during periods of strong increase and maximum orbitally-forced northern hemisphere insolation, but also during phases of low insolation, which are in general attributed to aridity in North Africa. Here, at least semi-arid climate conditions are proposed for periods of low insolation during stadials of MIS 5. This suggests an impact of different possible sources of precipitation, apart from large scale shifts of the African monsoon, such as Red Sea Troughs, tropical plums and a larger spatial extent of the Mediterranean winter-rainfall zone.Concerning the spatial rainfall pattern in Egypt, we propose a considerable east to west rainfall gradient in Egypt for the Last Interglacial and suggest a stronger impact of variable moisture sources in the Eastern Desert near the Mediterranean and Red Sea in comparison to the more continental Western Desert of Egypt. This reveals more favorable pre-conditions for an enhanced land use potential in the past. Therefore, the more sustaining wetter climate in the Eastern Desert point to a recurring feasible dispersal corridor for Homo sapiens from the tropical climate into the temperate climate regime throughout MIS 5. Such a more humid climate provided an ideal basis for long-term, favorable environmental conditions east of the Nile Valley, creating a kind of contiguous landscape corridor that may have been attractive to humans and wildlife alike, linking the East African tropical climate regime with that of the temperate zone in Northeast Africa and Eurasia.

Self‐organizing map classification of the boundary layer profile: A refinement of Eastern Mediterranean winter synoptic regimes

AbstractThe boundary layer (BL) profile over the coastal plain of Israel, Eastern Mediterranean (EM), varies considerably during winter. Although in the context of air pollution, the characteristics of the BL height (BLH) was intensively investigated, a quantitative classification of the BL profile regimes has not been performed. Here, we seek to reveal the dominant, recurring regimes of the BL profiles, their quantitative characteristics and links to regional synoptic‐scale patterns. An objective unsupervised classification of winter BL radiosonde profiles is performed for the first time by multi‐parameter self‐organizing map (SOM) analysis. The analysis uses high‐resolution, 12:00‐UTC data of wind, temperature, humidity and pressure measurements during December to February 2007–2018, and yields 30 distinct profile regimes. Composite analysis using ERA5 reanalysis suggests strong association between the profile regimes and synoptic weather systems and highlights four groups: (a) Deep winter cyclones with strong westerly wind and precipitation; (b) Strong surface anticyclones and Red Sea troughs (RST) with a mid‐tropospheric ridge, moderate dry easterly wind and extreme temperatures. (c) Moderate pressure gradients under shallow cyclones, anticyclone to the west and RST to the east of Israel. (d) Active RSTs, accompanied by upper‐tropospheric trough/cutoff low and heavy precipitation. For the first time, general objective classification observes the active RST without requiring specific criteria. Consistent with previous knowledge, the new classification exhibits distinct categories of thermal stability, BLH and turbulence. Importantly, we show that the automatic objective classification of profile data from a single station can be a sensitive discriminator of winter synoptic regimes in the EM, and therefore explains the variability of the BL profile. It facilitates the study of the interaction between the BL and the free troposphere and may improve the prediction of air pollution or future BL profile regimes based on long time series from historical data or climate models.

Synoptic pattern of the Red Sea trough associated with spring dust over the northern and western Arabian Peninsula

From 1979 to 2006, the Red Sea trough (RST) in spring was objectively detected and classified into three categories (short, moderate and long) depending on its northward extension. The results indicated that most short RSTs appeared during a cold spring (larger by 7% in March than May), while most long RSTs appeared during a warm spring, where reached 71.7% in April and May. The dust events during the study period were identified using a threshold value of the Total Ozone Monitoring Satellite (TOMS) aerosol index (AI). The identified dust cases were used to classify each RST category into two classes, dust RSTs and non-dust RSTs. The synoptic characteristics of the RSTs associated with dust events that influenced the northern and western Arabian Peninsula (AP) were studied using each dust category (i.e., comparing dust and non-dust cases) and were compared between the different categories (i.e., comparing the dust cases among short, moderate and long categories). The synoptic study shows that the condition favorable for transporting dust westward into the AP occurs when an interaction is produced between the low pressure over the southern AP and Sudan low and forms a wavy zonal pressure gradient. When the RST is relatively strong, i.e., the situation has a pronounced wavy zonal and meridional pressure gradient area, it produces conditions favorable for transporting dust around the Red Sea. Commonly, dust RSTs are atmospherically integrated systems with pronounced circulation (downward motion in the eastern AP and upward motion in the western AP), a vertical westward trough tilt, horizontal tropical–extratropical interactions and north–south orientations of pressure/geopotential isolines.

Major Changes in Extreme Dust Events Dynamics Over the Arabian Peninsula During 2003–2017 Driven by Atmospheric Conditions

AbstractMineral dust emitted from the arid regions of the Arabian Peninsula (AP), in particular Kingdom of Saudi Arabia (KSA), is an important contributor to the northern hemispheric aerosol loading. Dust events over AP are frequent and persistent throughout the year, with a peak in occurrence and intensity between March and August. This study examines the variability of dust events that occurred over AP during 2003–2017 using long‐term dust optical depth retrieved from MODIS satellite measurements. Dust profiles were derived from CALIOP satellite measurements to assess the vertical distributions of the dust events. Our analysis suggests that the formation of extreme dust events is primarily associated with strong high‐pressure systems that occur over AP. Northerly Shamal winds transport dust aerosols from surrounding arid regions to the AP, amplifying the dust aerosol loading across the country. CALIOP dust profiles show that the dust aerosols reach up to 4 km height above sea level, with peak concentrations at an altitude around 1.5 km. Dry deposition of dust is the dominant removal process over the Arabian Gulf, and wet deposition of dust is the dominant removal process over the Red Sea. A total of 49 extreme dust events occurred during the period between 2003 and 2017, over a combined total of 207 days. We observe a significant increase in the frequency of dust events between 2007 and 2012, peaking in 2012, followed by a marked decrease in the following years. We attribute this sudden decline in dust activity after 2012 to enhanced winter rainfall, after noticing increased soil moisture and vegetation coverage during the dust seasons that followed wetter winters. A strengthened Red Sea trough in the middle and the upper altitudes produced favorable conditions for strong moisture convergence and increased rainfall over AP, which may have dampened dust activity during 2013–2017.

The climatological impact of the upper-tropospheric Rossby wave propagation on the Red Sea trough in winter

There is a prominent feature in the climate of the northeast Africa that, once activated, may cause heavy rainfall occurrence in large parts of the eastern Mediterranean (EM) and the Arabian Peninsula. The feature is Red Sea Trough (RST) which is an inverted trough of low pressure at lower troposphere wave over the northeast Africa and the Red Sea. This study investigates the way Rossby wave propagation in the upper troposphere may change the circulation of the lower troposphere and thus the surface weather in the northeast Africa, the Red Sea and the EM regions on a monthly time scale. The interaction between the wave activity flux at the 300 hPa level and the lower tropospheric circulation is investigated with consideration of the meteorological quantities at different levels of the troposphere in a wide region from the western North Atlantic to the EM. For this, two critical phases of the wave activity flux are defined. The critical positive (negative) phase corresponds to the months during which the amount of wave activity entering the region of study is higher (lower) than the climatological mean plus (minus) one standard deviation. The results show that during the critical positive phase, in the northeast Africa, the mean meridional circulation is strengthened throughout the troposphere. Further consequences of the positive phase are the strengthening and interconnection of the divergence cores of the Atlantic and Mediterranean storm tracks, strengthening of the RST and thus locally-unstable conditions leading to higher monthly precipitation in large parts of the study region.

Changes in the characteristics of \u2018wet\u2019 and \u2018dry\u2019 Red Sea Trough over the Eastern Mediterranean in CMIP5 climate projections

The Eastern Mediterranean resides on the border between the temperate and semi-arid and arid climate zones, and is thus influenced by both mid-latitude and sub-tropical weather systems. Precipitation and extreme weather in this region are mainly associated with either Cyprus Lows or the “wet” Red Sea Troughs. Current regional climate projections indicate that the region may become warmer and drier in future decades. Here, we analyze the influence of enhanced greenhouse gas forcing on the climatological properties of the ‘wet’ and ‘dry’ Red Sea Trough (WRST & DRST, respectively). With this aim, a regional synoptic classification and a downscaling algorithm based on past analogs are applied to eighteen rain stations over the main ground water basins in Israel. The algorithms are applied to the NCEP/NCAR reanalysis data for 1986–2005 and to eight CMIP5 model simulations for the historical (1986–2005) and end of the century (2081–2100) climate conditions according to the RCP8.5 scenario. For the historical period, the CMIP5 models are largely able to represent the characteristics of the Red Sea Trough. Based on the multi-model mean, significant changes are found for WRST and DRST for the late XXI Century. First, an increase in the meridional pressure gradient is found for both the WRST and the DRST, implying stronger horizontal winds. Furthermore, a significant decrease in the occurrence of the WRST (− 20%) and a significant increase in the frequency of the DRST (+ 19%) are identified. Accordingly, the persistence of the WRST decreases (− 9%), while for DRST increases (+ 9%). The decline in the frequency of WRST occurs primarily in the transition seasons, while the increase for DRST is found throughout the wet season. In total, the daily rainfall associated with the WRST system is projected to significantly decline (− 37%) by the end of the XXI century. These results document the projected changes in a dominant synoptic system in this area, which can facilitate a better estimation of the arising challenges, e.g., related to shortage of water resources and associated political unrest, reduced agricultural potential, and increased air pollution and forest fires. Such a pathway can ultimately foster novel mitigation strategies for water resources management and regional climate change adaptation.

The variability of the Siberian high ridge over the Middle East

AbstractAn objective method was developed to detect local cells of the Siberian high (SH) over the Middle East, and mean sea level pressure (SLP) data from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis dataset were used to specify the centers of local SH cells and associated ridges. The results indicated that approximately 25.8% of winter SH cells influence the Middle East, and this influence diminishes from December to February. Synoptically, the area between the Black and Caspian Seas represents a remote area for sensing the influences of the SH on the Middle East. Although the SH is considered a shallow system, the variability of the effectiveness of the SH throughout the Middle East was estimated from the integration of atmospheric systems at the whole‐atmosphere scale (from the surface to 200 hPa). Further, this study demonstrated that the westward extension of the SH over the Middle East is associated with weak cyclogenesis in the area between the Black and Caspian Seas; that is, the atmospheric systems are favourable for strengthening the upper‐layer ridge in this area. However, the eastward retreat of the SH extension from the Middle East is associated with strong cyclogenesis in the area between the Black and Caspian Seas; that is, the atmospheric systems are unfavourable for strengthening the upper‐layer ridge in this area. This study revealed the importance of the SH in the generation of the pressure gradient over the Arabian Peninsula and the associated development of dust storms and the Red Sea Trough.

Climate swings in the northern Red Sea over the last 150,000 years from εNd and Mg/Ca of marine sediments

The εNd and Mg/Ca compositions of terrigenous material from a meridional transect of seven marine sediment cores located between the northern Red Sea and the Gulf of Aqaba, together with modern dust sample compositions, reveal the temporal and spatial changes in precipitation and dust sources over the last ∼150 kyrs, and are further used to discuss the late Quaternary climate history of this region. Two recent glacial periods (Marine Isotopes Stages (MIS) 6 and 2) are characterized by low εNd values associated with a Saharan dust source (∼−12), while Mg/Ca ratios suggest increased transport of dust containing authigenic clays during the last deglacial, which most likely originated from the Arabian Peninsula. By contrast, MIS5e sediments are characterized by an abrupt, basin-wide shift of εNd values toward the local Arabian Nubian Shield end member (∼−2) that are exposed around the Red Sea. This shift stems from intense fluvial events and is coeval with other regional paleo-precipitation records that all point to an extensive northward shift of precipitation from the tropics during MIS5e. Considering model simulations and regional precipitation dynamics, we conclude that although the African summer monsoon probably intensified and extended somewhat northward during MIS5e, it was not the driver of significant precipitation, and the main contribution of precipitation to the northern Red Sea was due to increased frequency and possible intensity of tropical plumes and Red Sea Troughs that triggered massive fluvial floods within the Red Sea catchment. Considering MIS5e as an analogue to a future warmer world, this currently hyperarid region could be exposed over the coming decades to an increased impact of extreme rainfall events, with critical implications for water availability and infrastructure sustainability.

Main atmospheric regimes of the long winter Red Sea trough

In this study, the Sudan low and the Red Sea trough (RST) are objectively detected using the mean sea level pressure data from the NCEP/NCAR reanalysis data set. The results show that the RST exists for 37.45% of winter and that approximately 64% of RSTs are classified as long RSTs. Most of these long RSTs are generated at night and decrease in abundance from December to February; thus, the generation timing is affected by high water-land temperature contrasts. Additionally, this study demonstrates that the Sudan low predominantly forms over the South Sudan region (70.59%) and to a lesser extent over the Sudan region (26.06%). The empirical orthogonal function (EOF) statistical analysis of long RST events indicates that the first ten modes represent 82% of the cases and that the first four modes represent 57.86% of the cases. For simplicity, this synoptic study focuses on the first four modes because they reliably represent the long RST modes. The synoptic study of the four modes demonstrates that as the mode percentage increases, the main dominant synoptic regimes that are associated with the modes and that affect the Red Sea region decrease in number and intensify in strength. Thus, the highest percentage modes are those with the lowest numbers, and strong atmospheric systems are distributed over large areas of the northern Red Sea. Furthermore, this study shows that the relative positions of the main synoptic components consisting of the northern European low-pressure system/Azores high-pressure system or the Siberian high-pressure system cause the long RSTs to be oriented east of or over the Red Sea, respectively. In addition, the study shows that the maximum wind core area expands eastward under strong mode conditions and shrinks westward from the Red Sea under weak mode conditions.

Synoptic features associated with the winter variability of the subtropical jet stream over Africa and the Middle East

Spatial, vertical and temporal variations in the subtropical jet stream over Africa and the Middle East were studied from 1958 to 2016 using NCEP/NCAR reanalysis meteorological data. The results of this study showed that the jet stream was located at a pressure level of 200 hPa more than 87% of the time, and it was located in the meridional zone between 20°N and 35°N in more than 83% of these cases. Additionally, the annual winter distribution of the jet stream illustrated two distinct trends: the first trend ended in 1987 and had a high rate of decrease, and the second trend began in 1988 and had a small rate of increase. Furthermore, synoptically, the study showed that the southern shift of the jet stream intensified the low-pressure systems over Africa and the Mediterranean, in addition to producing favorable conditions for the northward extension of the Red Sea Trough. However, the northward shift was related to the intensified Azores high-pressure system over Africa and the southern Mediterranean. Moreover, the temporal variations of the jet stream indicated that the intensification of the high-pressure system (i.e., the Azores and Siberian highs) tended to decrease the number of jet stream cases at 200 hPa or vertically shifted them downward to 250 hPa.

Automatic identification and classification of the northern part of the Red Sea trough and its application for climatological analysis

AbstractThe Red Sea trough (RST) is a low‐pressure trough extending from south towards the Levant. Unlike previous synoptic classifications covering all systems that affect the region, our algorithm focuses on the RST alone. It uses sea‐level pressure (SLP) and relative geostrophic vorticity for identifying the existence of an RST and classifying it to one of three types, according to the location of the trough axis with respect to 35°E longitude. The following conditions were imposed to assure the existence of an RST: (a) north to south SLP drop across the Levant, (b) average positive sea‐level relative vorticity over the region of interest, (c) existence of a distinct and continuous trough axis from the south towards the region of interest and (d) absence of any pronounced closed cyclone near the Levant. The algorithm was applied on the NCEP/NCAR reanalysis, 2.5° × 2.5° resolution and the ERA‐Interim, 2.5° × 2.5° and 0.75° × 0.75° resolutions. An evaluation of the algorithm against subjective identification, based on the NCEP reanalysis, showed an agreement of 93% for RST identification and 79% for correct classification. The use of fine resolution data may insert noise that reduce the identification rate of RSTs but improves the axis locating. The autumn is the main season of RST, with a maximum in November and a consistent decrease towards the July minimum. The annual frequency varies among the data sources between 17.6 and 24.6%. The trough axis is shown to have a diurnal oscillation; towards the eastern coast of the Mediterranean at nighttime and eastward, inland, at noontime. No consistent long‐term trend was found for the period 1979–2016, during which the global warming was persistent. This automated algorithm is flexible in the sense that it is not confined to any predetermined spatial resolution and is applicable to operational forecast model as well as to climate model outputs.

Analyses of the winter low‐level jet over the southern Red Sea using the Weather Research and Forecasting model

AbstractThe three‐dimensional structure of the Red Sea Low‐Level Jet (RSLLJ) that occurs over the southern Red Sea during the winter season is examined using the Weather Research and Forecasting (WRF) model. The causes of the jet formation are investigated, and the simulations confirm that the jet is a terrain‐induced phenomenon. It initiates as a gap flow as the jet forms north of the strait of Bab el‐Mandab as a result of a hydraulic effect when a stably stratified cool layer is channeled through the mountainous gap at low levels over the strait into the southern Red Sea. The jet intensity is enhanced by the Red Sea trough over the southern Red Sea. The cool dense air mass south of the strait is supported by a generally persistent synoptic regime that includes a high‐pressure system over most of the eastern Arabian Peninsula and the Sudan low‐pressure system over Sudan. The jet extends more than 500 km northward to slightly north of 17°N from the strait around 12.5°N, with a width of 100–150 km over the southern Red Sea. The jet core lies at 500–600 m above sea level between 14°N and 16°N, with wind speeds exceeding 25 m/s. The north–south pressure gradient along the strait, as well as the depth of the stratified cool layer south of the strait, are directly responsible for the jet length and speed over the southern Red Sea, with stronger pressure gradients and deeper cool layers resulting in longer and stronger jets. Sea/land breezes also directly influence the spatial scale, intensity, and diurnal variation of the jet, with land breezes causing faster and more constricted jets at night and divergence due to sea breezes leading to relatively slower jets during the daytime.