عنوان مقاله [English]
In general, the southwest Asia is one of the regions with positive anomalous values of tropopause folding frequency compared to the annual average of the northern hemisphere. The frequency of folding in this region in warm seasons is higher than in cold seasons. This research was carried out with the aim of statistical and dynamical analysis of the atmospheric processes associated with the tropopause folds in the southwest Asia during 2000–2015. Identification of tropopause folding is based on the algorithm developed by Sprenger et al. (2003) and Gray (2003) and refined by Škerlak et al. (2014). The detected folds are divided into three categories as shallow, medium, and deep based on their vertical extensions.
The time series analysis of all types of tropopause folds shows that the frequency of folding events has an increasing trend during the period of study. The most frequent folding type is as shallow or medium in the summer season but as deep in the winter. The geographical distribution of the correlation coefficients between the monthly mean folding frequencies and some relevant dynamical quantities indicates that baroclinic instability mechanism plays the main role in the occurrences of tropopause foldings in the winter, while the effects of thermodynamic factors are dominant in the summer.
Dynamical study of tropopause folds in both winter and summer seasons was conducted using January 2001 and 2004 as well as June 2007 and 2015 data sets. Results show that the winter tropopause foldings are associated with the formation of intense baroclinic waves in mid-levels of troposphere, strengthening of jet streams in upper levels and subsequently the formation of surface cyclones. Also, together with the seasonal displacement of the jet, the positions of tropopause folds are moving about 10 to 15 degrees latitudinally.
The analysis of horizontal wave activity flux in January 2004, reveals the presence of a strong wave source (divergence of wave activity flux) in the Western Mediterranean and, at the same time, a wave sink (convergence of flux) over Europe. In January 2001, the wave activity flux was weakened and divided into two branches, one located in middle latitudes and the other in subtropical regions that transmitted wave energy to the southwest Asia. Furthermore, the strong equatorward wave propagation in this month indicates the anticyclonic wave breaking. In the two June months, as the baroclinic waves were weakened, the intensities of wave activity flux as well as the convergence and divergence centers in the southwest Asia were decreased compared to the January cases.
Comparing the above results, it can be deduced that in winter, intense baroclinic wave packets in middle latitudes cause the strength of the subtropical jet, and consequently intensification of wave breaking which are associated with the occurrences of deep tropopause folds in the southwest Asia. In summer, the weakening of baroclinic activities leads to the reduction of deep folding frequency and the folds are formed mainly as shallow type at high levels.