A review on the Mechanism and synoptic patterns of the Red Sea Trough (RST) and its effects on Iran's precipitations

The RST is a large-scale subtropical–tropical thermal low-pressure and synoptic system regarded to an extension of the African monsoon trough, northward over the Red Sea region toward the Eastern Mediterranean (El-Fandy 1948; Vries et al, 2013). The RST is attributed to the local topography and thermal forcing factors (Krichak et al., 1997a). Synoptically, the position of the RST is strongly influenced by the Siberian and Azores high systems (Baseer et al., 2019). The RST 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 investigating to be accompanied by an upper-tropospheric trough extending from the north over the EM. These conditions are associated with unstable stratification, favoring the development of mesoscale convective systems. This kind of RST has been defined as an ‘‘active’’ RST (Tsvieli and Zangvil 2005). ARST events usually lead to reasonably intense precipitation over the eastern Mediterranean; in some rare situations, they are associated with heavy, torrential rains and devastating floods. It can occur in late autumn and to a lesser incidence in early winter and spring. The ARST is a rare climate phenomenon that is the main cause of floods in the arid and semi-arid region of eastern and southern of Mediterranean (Kahana et al. 2004; Krichak et al., 2012; Vries et al, 2013). Some researches show the ARST preference for autumn by coinciding favorable latitudinal positions of the African Monsoon and the subtropical jet (STJ) stream (Dayan et al. 2001). Based on Conceptual model of ARST and algorithm for its identification by Krichak et al., 2012, According to the new algorithm for ARSTs, the occurrence of ARST events is defined to have taken place when:The CAPE or PRWT exceed particular threshold values over a target area that covers the EM (28°–32°N, 32°–38°E).1) 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).2) A mid-tropospheric 500hPa trough is detected over one of the following two target areas (25°–30°N,158–35°E or 30°–35°N, 25°–40°E).The ARST is associated with extreme precipitation, in the Middle East (ME) and some region in the west and southwest of Iran. Generally, it causes flood events in some years, many studies, focused on these cases and most of them explained western cyclones and RST are the main cause of them. On 25 November 2009, Jeddah (Saudi Arabia) was dramatically hit by heavy precipitation and consequent flash floods and on 21 November, 2011this event occurred in Lilac-Behbahan. In this research attempted extract difference between these two ARST synoptic patterns also, we review previous literature addressing ARST associated dynamics.

We tried to investigate the extreme precipitation events over the Jeddah region on 25 November 2009, southwestern Iran (Lilac-Behbahan) on 21 November 2011. To perform this research, we used ERA-Interim reanalysis data and precipitation observations. 6-hourly gridded data from the multiyear dataset of the European Center for Medium-Range Weather Forecasts (ECMWF- ERA-Interim) are employed for 2009 and 2011. We utilize the daily mean and 6 hourly analysis and forecasted variables at pressure and surface levels, as well as vertically integrated quantities derived from pressure levels. The ERA-Interim data are available for the entire globe with 0.5˚ * 0.5˚ spatial resolution and 6-h temporal resolution. Precipitation observational data from stations of Iran are collected from Meteorological Organization and data of 30 stations of Saudi Arabia are available via http://www.meteomanz.com. We followed these ARST events by these dynamical factors (de Vries et al., 2013):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, and6) Strong ascent resulting from tropospheric instability and the synoptic-scale dynamical forcing.

The result shows the spatial extent of Arabian Anticyclone is one major reason for the distribution of precipitation in Saudi Arabia and the southwest of Iran. Also, the position of the axis of upper trough and its rotation is another reason and it's oriented to lower latitude, as whatever southward leading of upper trough to lower latitude is greater and wavelength of the trough is more, accordingly, the possibility of precipitation increased in Saudi Arabia. Moreover, the location of a stream jet can be effective on extreme precipitation. On the other hand, a streak of subtropical jet extended southeast of upper trough, and this status is so significant.