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  • View in gallery

    Map of study region for the objective identification of RST situations: (a) target areas for ARST identification according to mean PRWT and CAPE values and (b) target areas for ARST identification according to locations of the troughs in H1000 and H500 patterns.

  • View in gallery

    Composite patterns for all (44) cases with ARST (a) H1000 (gpm; contour interval is 10), (b) H500 (gpm; contour interval is 20), (c) PWRT (mm; kilograms of water per square meter), and (d) 850–500 hPa lapse rate (° km−1) for the ARST cases identified.

  • View in gallery

    As in Fig. 2 [except for (d)], but for composite patterns for the 24 cases with the highest PRWT: (a) H1000, (b) H500, (c) PRWT, and (d) CAPE (J kg−1; contour interval is 200).

  • View in gallery

    As in Fig. 3, but for the 24 cases with the lowest PRWT.

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A Conceptual Model for the Identification of Active Red Sea Trough Synoptic Events over the Southeastern Mediterranean

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  • 1 Department of Geophysics and Planetary Sciences, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel
  • | 2 Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania
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Abstract

A phenomenon characterized by a tongue of low pressure extending northward from the southern Red Sea [Red Sea Trough (RST)] toward the eastern Mediterranean Sea (EM) is analyzed. In general, the RST is associated with hot and dry weather, resulting from east-southeasterly flows in the lower troposphere. In some cases, the RST is found to be accompanied by an upper-tropospheric trough extending from the north over the EM. Such conditions are associated with unstable stratification, favoring the development of mesoscale convective systems. This kind of RST has been defined as an “active” RST (ARST). The ARST phenomenon represents a serious threat to human society in the northeastern Africa–southeastern Mediterranean region, being in some cases associated with devastating floods. In this study, a conceptual model of the ARST phenomenon is discussed, and then an algorithm for the identification of ARST events is presented. The identification algorithm has been applied to a multiyear NCEP–NCAR reanalysis data archive for both RST and ARST events. From the results of a composite analysis of several different atmospheric flow parameters associated with ARST events, the key features associated with ARST events are identified. The results from the analysis of the composite patterns support the suggestion that high amounts of moisture transported from tropical Africa in the form of an atmospheric river to the Red Sea–EM play a key role in determining the intensity of the ARST events.

Corresponding author address: Simon O. Krichak, Tel Aviv University, Geophysics and Planetary Sciences, Ramat Aviv, Tel Aviv, 69978 Israel. E-mail: shimon@cyclone.tau.ac.il

Abstract

A phenomenon characterized by a tongue of low pressure extending northward from the southern Red Sea [Red Sea Trough (RST)] toward the eastern Mediterranean Sea (EM) is analyzed. In general, the RST is associated with hot and dry weather, resulting from east-southeasterly flows in the lower troposphere. In some cases, the RST is found to be accompanied by an upper-tropospheric trough extending from the north over the EM. Such conditions are associated with unstable stratification, favoring the development of mesoscale convective systems. This kind of RST has been defined as an “active” RST (ARST). The ARST phenomenon represents a serious threat to human society in the northeastern Africa–southeastern Mediterranean region, being in some cases associated with devastating floods. In this study, a conceptual model of the ARST phenomenon is discussed, and then an algorithm for the identification of ARST events is presented. The identification algorithm has been applied to a multiyear NCEP–NCAR reanalysis data archive for both RST and ARST events. From the results of a composite analysis of several different atmospheric flow parameters associated with ARST events, the key features associated with ARST events are identified. The results from the analysis of the composite patterns support the suggestion that high amounts of moisture transported from tropical Africa in the form of an atmospheric river to the Red Sea–EM play a key role in determining the intensity of the ARST events.

Corresponding author address: Simon O. Krichak, Tel Aviv University, Geophysics and Planetary Sciences, Ramat Aviv, Tel Aviv, 69978 Israel. E-mail: shimon@cyclone.tau.ac.il

1. Introduction

Among the various synoptic processes that take place over northeastern Africa and the southeastern Mediterranean Sea region (MR) that contribute significantly to precipitation, an important place is occupied by a phenomenon characterized by a tongue of low pressure extending northward from the southern Red Sea toward the eastern Mediterranean (EM) (Ashbel 1938; El-Fandy 1948), that is, the Red Sea Trough (RST) system. The RST attains its largest amplitude in the lower troposphere, and is strongly influenced by the topography of the surrounding region (El-Fandy 1950; Krichak et al. 1997a,b). The RST is regarded as an extension of the African monsoon trough, a large-scale subtropical–tropical low pressure thermal synoptic system (El-Fandy 1948). Interannual variation in the intensity and location of the African monsoon trough may be linked to those of the intertropical convergence zone, which is located between 5° and 10°N in the winter and 20° and 25°N in the summer (at 0° longitude). An algorithm for automatic identification of the RST events on the basis of gridpoint data on the 1000-hPa isobaric surface has been suggested and implemented by Tsvieli and Zangvil (2005).

In general, the RST over the southeastern MR is associated with hot and dry weather, resulting from an east-southeasterly flow in the lower troposphere. Such conditions correspond to a “nonactive” RST (Kahana et al. 2002). In some cases, the RST is found to be accompanied by an upper-tropospheric trough extending from the north over the EM. Such conditions are associated with unstable stratification, favoring the development of mesoscale convective systems. This kind of RST has been defined as an “active” RST (ARST; Dayan and Sharon 1981; Goldreich 2003; Saaroni et al. 1998; Kahana et al. 2002, 2004; Ziv et al. 2005; Tsvieli and Zangvil 2005).

Although ARST events usually lead to reasonably intense precipitation over the region, in some rare situations they are associated with heavy, torrential rains and devastating floods. The ARST is a major cause of flash floods in the arid southern Levant (Ashbel 1938; El-Fandy 1948; Dayan and Sharon 1981; Kahana et al. 2004). Another synoptic-scale process, referred to as a Syrian low, is characterized by a Mediterranean midlatitude cyclone that deepens while approaching Syria (Kahana et al. 2004). The upper- and lower-level troughs that are found to be associated with Syrian lows are located somewhat to the east of those associated with ARSTs. As with ARSTs, Syrian lows often coincide with intense rains and floods in the southern Levant region. The frequency of Syrian lows is significantly lower than that of ARSTs (Kahana et al. 2004), however. The current analysis is limited to the investigation of the ARSTs.

The ARSTs and associated extreme rainfall are often not predicted well, apparently because of several factors such as a lack in our understanding of the physical process, the limited amount of observational data for the area, and the small horizontal scale (5–10 km) of individual mesoscale convective systems (Cotton 1990). Kahana et al. (2004) have suggested that a synoptic approach be used for the identification of the ARST-type synoptic events that cause intense floods in the Negev Desert. Their method is based on the evaluation of several atmospheric parameters, such as the ratio of meridional wind to zonal wind, the relative vorticity and wind speed on the 500-hPa surface over the Negev Desert, the 850–500-hPa temperature gradient over the northern Red Sea, the 500-hPa geopotential height over upper Egypt, and the sea level pressure gradient between 25° and 30°N. Because the focus of Kahana et al. (2004) was on the identification of the synoptic conditions that cause major floods over the Negev Desert during 1965–94, they used a dataset with relatively coarse resolution. As a result, their study was limited in the extent to which it could enhance our understanding of the physical mechanisms that cause the ARST formation.

Our method for identifying ARSTs takes a broader physical perspective because it takes into account processes that occur both in the tropics and midlatitudes and because it includes the impact of moist processes. The approach is based on consideration of the ARST-associated extreme precipitation events as being similar to those over the eastern Pacific Ocean and west coast of North America caused by the intrusions of moist tropical air masses. The precipitation events have been attributed to the effect of atmospheric rivers (AR)—that is, narrow (<1000 km wide) and relatively long (>2000 km) bands of enhanced poleward water vapor flux (Zhu and Newell 1998; Ralph et al. 2004, 2005, 2006; Bao et al. 2006; Neiman et al. 2008), characterized by high values of the vertically integrated water vapor content (i.e., precipitable water, or PRWT). A critical PRWT value of 20 mm (kilograms of water per square meter) has been suggested for the identification of AR events (Zhu and Newell 1998). Also, in the Mediterranean region, the intrusion of moist tropical air masses has been detected. (Alpert and Shay-El 1994; Turato et al. 2004; Pinto et al. 2009; Krichak et al. 2004). The AR is typically viewed in the framework of warm conveyor-belt systems (Browning 1990; Carlson 1991; Eckhardt et al. 2004; Ziv et al. 2010) typically originating in the cyclone’s warm sector where there is a strong meridional energy transport. Its development is typically characterized by extremely high values of convective available potential energy (CAPE; e.g., Emanuel 1993).

In this study, a conceptual model of the ARST phenomenon is proposed and then an algorithm for the identification of ARST events is presented. The model uses physical variables that represent air masses originating over the tropics (PRWT and CAPE) in addition to those associated with midlatitude dynamical processes. As discussed above, this approach allows for a broader physical perspective than those used previously, since it takes into consideration both tropical and midlatitude processes. The identification algorithm has been applied to a multiyear data archive for both RST and ARST events. From the results of a composite analysis of several different atmospheric flow parameters associated with ARST events, the key features of ARST events are identified.

2. Data used

The primary aim of the analysis is to propose an approach for the investigation of the physical mechanisms responsible for ARST development in the current climate and in expected future climate conditions that are based on projections of climate change that are available from multiyear simulations with coupled atmosphere–ocean global climate models typically characterized by very coarse (200–300 km) spatial resolution.

To perform this research, we use 6-hourly gridded data from the multiyear dataset of the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis project (NNRP; Kalnay et al. 1996) for 1961–2000. The NNRP data are available for the entire globe with 2.5° × 2.5° spatial resolution and 6-h temporal resolution. The spatial resolution of the data archive appears to be sufficient for the aim of this investigation.

The NNRP archive is constructed through a consistent assimilation and forecast model procedure, which allows for the incorporation of all available observations. The data assimilation system includes the 28-level global spectral model with T62 horizontal resolution. The precipitation data are obtained from the model runs. Note here that, although the application of gridded reanalysis-based data for climate evaluations over the Euro-Mediterranean region has become a widely accepted strategy, there are significant limitations to the data because of important changes in the global observation system during the twentieth century, as well as global warming–associated regional trends in temperature, water vapor content, kinetic energy, and so on. Nevertheless, for the short-time-scale RST and ARST events, these limitations are for the most part not relevant. These data have been used to obtain CAPE according to the algorithm of Emanuel (1993).

3. Conceptual model of ARST and algorithm for its identification

We consider the ARST as a synoptic-scale, lower-tropospheric RST centered over the Red Sea–EM accompanied by an AR system that forms over the same region, arising as a consequence of the establishment of a midtropospheric cutoff low or intense trough over the North Africa–southeastern Mediterranean region. In such situations, taking place during the cool (typically October–April) Mediterranean season, the ARST development may be seen as arising from the interaction between a lower-tropospheric northward inflow of warm, humid air of tropical origin and a midtropospheric southward inflow of cold and dry air from midlatitudes. The low-level flow from the tropics is characterized by mesoscale features associated with unstable stratification and a high concentration of air moisture, all embedded within the larger synoptic-scale pattern. Such synoptic conditions are typically characterized by high amounts of energy available for convection (i.e., high CAPE), which may be directly related to the high lapse rates in the lower troposphere, indicating greater potential for severe weather.

In accordance with this model of the phenomenon, we suggest an algorithm for the identification of ARST events. This algorithm is based on the analysis of three-dimensional atmospheric gridpoint data over the northeastern Africa–Red Sea region. For the lower-troposphere case, that is, RST events, the algorithm is very similar to that of Tsvieli and Zangvil (2005). According to the new algorithm for ARSTs, the development takes place over the Red Sea–EM when an RST is accompanied by two additional flow features. These are 1) high amounts of CAPE due to the southward midtropospheric flow over the EM and 2) tropical moisture transport in the form of an AR-type system transporting warm air from equatorial Africa toward the EM in narrow elongated bands with enhanced amounts of air moisture. The suggested procedure for ARST identification is based on simultaneous evaluation with the gridded data archive of the following variables: 1000-hPa geopotential height (H1000), 500-hPa geopotential height (H500), CAPE, and PRWT over the EM and northeastern Africa. The algorithm is applied for each 6-h time level of the NNRP data during 1961–2000. The occurrence of ARST events is defined to have taken place when

  1. the CAPE or PRWT exceed particular threshold values over a target area that covers the EM (28°–32°N, 32°–38°E) (see Fig. 1a),
  2. a northward-oriented 1000-hPa trough (H1000) extends from northeastern Africa to the EM within a target area of (22.5°–32.5°N, 25°–45°E) (see Fig. 1b), and
  3. a midtropospheric 500-hPa trough (H500) is detected over one of the following two target areas (25°–30°N, 15°–35°E or 30°–35°N, 25°–40°E) (see Fig. 1b).
Fig. 1.
Fig. 1.

Map of study region for the objective identification of RST situations: (a) target areas for ARST identification according to mean PRWT and CAPE values and (b) target areas for ARST identification according to locations of the troughs in H1000 and H500 patterns.

Citation: Journal of Applied Meteorology and Climatology 51, 5; 10.1175/JAMC-D-11-0223.1

The third condition corresponds to the southward-penetrating midtropospheric trough and/or a cutoff low over the EM. The size and location of the H1000 target area are similar to those used by Tsvieli and Zangvil (2005), which allows for a comparison with their results on RST formation. The following area-averaged critical values of CAPE and PRWT are adopted on the basis of results of a synoptic analysis that focuses mainly on the data for the autumn–early winter season: CAPE_crit = 130 J kg−1 and PRWT_crit = 21 mm.

It must be indicated here that these critical values as well as the coordinates of the chosen target area must be seen only as tentative values to allow us to evaluate our conceptual model. It is clear that the implementation of our approach for the purpose of weather prediction would require a more sophisticated strategy for the determination of the critical values.

4. Results

Evaluation of the efficiency of the new algorithm is complicated by a lack of reliable synoptic analyses of ARST events during most of the analysis period. The dates identified with the algorithm are given in Table 1. According to data by D. Edry (2011, personal communication) for the time period beginning in 1980, the algorithm allows for the identification of about 90% of his list of ARST events in Israel. Thus, a small fraction of ARST events are not detected, most likely because of the already noted simplistic choice of the critical parameter values for the procedure. Nevertheless, the number of ARST cases is sufficient for an analysis of the phenomenon.

Table 1.

ARST cases identified on the basis of 6-hourly NNRP data for 1961–2000.

Table 1.

An analysis of the results has been performed. It is found that the maximum frequency of RST events (days month−1) over northeastern Africa is October (2.6), November (4.0), December (5.3), and January (4.9). These frequencies are somewhat different from those of Tsvieli and Zangvil (2005), who obtained values of 10.2, 7.9, 3.8, and 3.4, respectively. The differences in the results may be attributed to the sizes and locations of the target areas, as well as (including the effect of long-term trends) to the time period [40 vs 11 yr in Tsvieli and Zangvil (2005)] of the analysis.

To evaluate the processes that drive the development of ARST events, composite calculations were performed using all 48 ARST events for 1961–2000 of the multiyear dataset. The H1000 and H500 patterns (Figs. 2a,b, respectively) are characterized by northward- and southward-oriented troughs, respectively. The composite PRWT pattern for the 44 ARST events (Fig. 2c) indicates a broad zone of high PRWT values that extend from tropical Africa along the Red Sea coast to the EM. A narrow tongue with high PRWT values appears to be among the main factors that determine the intensity of the precipitation over the Red Sea basin during the ARST events. The composite pattern for air temperature lapse rate (Fig. 2d) demonstrates the existence of a zone with high instability in the lower troposphere over the northern part of the Arabian Peninsula and the EM–Middle East forming as a consequence of cold-airmass transport in the midtroposphere.

Fig. 2.
Fig. 2.

Composite patterns for all (44) cases with ARST (a) H1000 (gpm; contour interval is 10), (b) H500 (gpm; contour interval is 20), (c) PWRT (mm; kilograms of water per square meter), and (d) 850–500 hPa lapse rate (° km−1) for the ARST cases identified.

Citation: Journal of Applied Meteorology and Climatology 51, 5; 10.1175/JAMC-D-11-0223.1

To distinguish between strong and weak ARST events, the ARST dates have been identified and then sorted into two groups composed of the 24 highest and lowest area-averaged PRWT values over the Fig. 1b target area. The H1000, H500, PRWT, and CAPE patterns for the two cases are shown in Figs. 3a–d and 4a–d, respectively. The events with high PRWT values are associated with a well-developed lower-tropospheric trough, that is, an RST (Fig. 3a); a midtropospheric cutoff low system over the east-central MR (Fig. 3b); strong northward air moisture transport from tropical Africa into the EM (Fig. 3c); and a narrow zone with high CAPE values along the Red Sea (Fig. 3d). This is in contrast with the low-PRWT ARST cases in which the southward-oriented upper-air trough is much weaker (Figs. 4a–d). These differences between the weak and strong ARST events reveal the importance of the southward mid- and upper-tropospheric flow from midlatitudes for the intense ARST events. The fact of large CAPE values (Figs. 3d, 4d) associated with the ARST events supports the suggestion [also in Krichak and Alpert (1998)] that high amounts of air moisture play a key role in determining the intensity of the ARST events over northeastern Africa and the eastern MR.

Fig. 3.
Fig. 3.

As in Fig. 2 [except for (d)], but for composite patterns for the 24 cases with the highest PRWT: (a) H1000, (b) H500, (c) PRWT, and (d) CAPE (J kg−1; contour interval is 200).

Citation: Journal of Applied Meteorology and Climatology 51, 5; 10.1175/JAMC-D-11-0223.1

Fig. 4.
Fig. 4.

As in Fig. 3, but for the 24 cases with the lowest PRWT.

Citation: Journal of Applied Meteorology and Climatology 51, 5; 10.1175/JAMC-D-11-0223.1

5. Discussion

The ARST phenomenon represents a real threat to human society in the northeastern Africa–southeastern Mediterranean region. ARST events are often associated with devastating floods. Among such cases is, for example, that of 1–5 November 1994, which was accompanied by heavy floods in Egypt, Israel, Italy, France, and other countries of the region (e.g., Buzzi and Tartaglione 1995; Lionetti 1996; Krichak and Alpert 1998). In that event more than 500 people lost their lives and large areas were inundated (Obasi 1997).

Since the studies of Ashbel (1938) and El-Fandy (1948), the RST has been a well-known feature associated with a high risk of torrential rains over northeastern Africa. The results from the analysis that is presented here support the suggestion that, similar to ARs, narrow elongated bands with enhanced amounts of air moisture transported from tropical Africa to the Red Sea–EM play a key role in determining the intensity of the ARST events over the MR. The physically based method that is presented here has been developed to allow us to investigate the physical mechanisms responsible for the formation of ARST events, with the additional aim of using this method to evaluate the role of ARSTs in future climates that are based on the climate change projections available from multiyear simulations with coupled atmosphere–ocean global climate models. For this reason, we have used the coarse resolution of the NNRP dataset. It appears reasonable to apply a similar method in weather forecasting that is based on the results from numerical weather prediction models. It must be pointed out, however, that additional evaluation of the method with data of significantly higher resolution (than that available in the NNRP) will be necessary for its possible implementation in weather prediction. The future realization of this work using the data from the recently released National Aeronautics and Space Administration Modern-Era Retrospective Analysis for Research and Applications (MERRA; Rienecker et al. 2011) appears to be promising.

Acknowledgments

The financial support for this work has been provided by the U.S.–Israel Binational Science Foundation (BSF) under Research Grant 2008436 and the U.S. National Science Foundation (NSF) under Grants ATM-0649512 and AGS-1036858. Discussions supported through the European Cooperation in Science and Technology (COST) Earth System Science and Environmental Management (ESSEM) Action ES0905 “Basic concepts for convection parameterization in weather forecast and climate models” and European Science Foundation (ESF) Program “Mediterranean Climate Variability and Predictability” (MedCLIVAR) as well as partial support from a research grant for 2012–13 from the Water Authority of the Ministry of Infrastructures of Israel are also acknowledged. We also acknowledge the use of the NCEP–NCAR reanalysis data provided by the NOAA/OAR/ESRL PSD (Boulder, Colorado; obtained online at http://www.esrl.noaa.gov/psd/). The CAPE values have been computed using an NCL script by C. Bruyere (http://www.ncl.ucar.edu/Document/Functions/wrf.shtml). The helpful comments by two reviewers are greatly appreciated.

REFERENCES

  • Alpert, P., , and Y. Shay-El, 1994: The moisture source for the winter cyclones in the Eastern Mediterranean. Israel Meteorological Research Paper 5, 20–27.

  • Ashbel, D., 1938: Great floods in Sinai Peninsula, Palestine, Syria and the Syrian desert, and the influence of the Red Sea on their formation. Quart. J. Roy. Meteor. Soc., 64, 635639.

    • Search Google Scholar
    • Export Citation
  • Bao, J.-W., , S. A. Michelson, , P. J. Neiman, , F. M. Ralph, , and J. M. Wilczak, 2006: Interpretation of enhanced integrated water vapor bands associated with extratropical cyclones: Their formation and connection to tropical moisture. Mon. Wea. Rev., 134, 10631080.

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