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King Abdulaziz University (2021)

VARIABILITY OF THE WINTER SUBTROPICAL JET STREAM AND ITS IMPACT ON THE PRECIPITATION OF SAUDI ARABIA

Asiri, Mana Ali

Titre : VARIABILITY OF THE WINTER SUBTROPICAL JET STREAM AND ITS IMPACT ON THE PRECIPITATION OF SAUDI ARABIA

Auteur : Asiri, Mana Ali

Université de soutenance : King Abdulaziz University

Grade : Master Thesis 2021

Résumé
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. Furthermore, the surface observations of rain from 15 stations over the northern Arabian Peninsula (AP) and meteorological data from the NCEP/NCAR reanalysis data set were used to study the winter atmospheric factors and subtropical jet stream (STJ) characteristics controlling the rain situation over the northern AP. The statistical study shows that January has the highest amount of winter rain when the STJ is at 200 hPa and is located in the zone between 25°N and 30°N. Synoptically, the northern AP winter rain forms when the Red Sea trough (RST) extends northward and is located to the east of the upper trough, while the rain decreases when the Siberian high intensifies the pressure gradient around the RST decreases and the STJ becomes zonal or exits over the Middle East. Moreover, the study shows that the Mediterranean cyclone plays a secondary role in the generated rain situations, mainly producing the favored atmospheric conditions for the northward extension of the RST. The results demonstrated that the pressure/geopotential gradient is more important than the relative position of the atmospheric systems in the development of the rain situation. The temporal and vertical variations in the STJ indicate that the rain decreases as the core of the low-pressure difference shifts southward far from the northern Red Sea, the magnitude of the Siberian high difference is comparable to the low-pressure negative difference, while the Azores difference is relatively weak

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Page publiée le 6 mai 2022