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

The 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.