Synoptic features associated with the winter variability of the subtropical jet stream over Africa and the Middle East
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ORIGINAL PAPER
Synoptic features associated with the winter variability of the subtropical jet stream over Africa and the Middle East Mana A. Asiri1 · Mansour Almazroui1 · Adel M. Awad1 Received: 18 February 2019 / Accepted: 18 December 2019 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract 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.
1 Introduction The subtropical jet stream (STJ) is a tight band of fast-moving westerly wind that is frequently found around the 200hPa level on either side of the equator (Krishnamurti 1961; Peixoto and Oort 1992; Bluestein 1993; Archer and Caldeira 2008). Although clearly identifying the ambiguous jet stream barriers is difficult (Koch et al. 2006), in general, the jet wave has three phase patterns, i.e., approximately over Japan, the eastern sea board of the USA and eastern Europe, with powerful winds above 25 m/s at 200 hPa (Krishnamurti 1961; Strong and Davis 2007; Archer and Caldeira 2008; Abish et al. 2015). Most important extratropical teleconnection types essentially represent jet stream variability (Wittman et al. 2005; Monahan and Fyfe 2006). Therefore, changes in the jet stream position, depth, or altitude can cause variations in the frequency and depth of synoptic-scale disturbances (Holton Responsible Editor: E.-K. Jin * Adel M. Awad [email protected] 1
Department of Meteorology, King Abdulaziz University, P.O. Box 80208, Jeddah 21589, Saudi Arabia
and Hakim 2013). Additionally, jet streams inhibit the formation and expansion of hurricanes, which preferentially develop in low-shear weather areas (Gray 1968; Vecchi and Soden 2007). Much of the interest in jet stream variability is based on its dynamical importance in synoptic-scale cyclogenesis (Sutcliffe 1939, 1947; Nakamura 1992; Baehr et al. 1999; Wernli et al. 2002) and in dif
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