جستجوی مقالات مرتبط با کلیدواژه « فعالیت موج » در نشریات گروه « زمین شناسی »

In this study, the “MPI-ESM-LR” model output from phase 5 of the Coupled Model Intercomparison Project (CMIP5) is used to assess the response of the North Atlantic (NA) and Mediterranian storm tracks to climate change. Historical scenario is used for the past and RCP8.5 scenario is used as the projection for the future period. The conservation of wave activity is used as a diagnostic tool to investigate the eddy activity dynamics. A pair of large centers of divergence-convergence for the horizontal wave activity flux (wave packet) in the NA region forms the signatures of the NA storm track. The NA storm track has a double-branch structure consisting of northern and southern branches. The Mediterranian storm track is identified by a pair of positive/negative centers of wave activity flux in the west/east of the Mediterranian sea. The convergence area extends from the eastern Mediterranian and north-eastern Africa to the Middle-east and western and south-western parts of Iran. The dynamical analysis of the MPI-ESM-LR results shows that the response of the upper-tropospheirc wave activity, propagation and breaking in the northern and southern branches of the NA storm track to global warming, determines the changes of eddy activity in the northern and southern latitudes in its downstream sectors from Europe to Siberia and from Mediterranean Sea to Southwest Asia, respectively. In winter, intensity and number of wave packets decrease in both northern latitudes from the northern branch of the NA storm track to the Scandinavia and Siberia and southern latitudes in the southern branch of the NA storm track, the Mediterranian storm track and the Northern Africa region, while the central branch of wave activity in the middle and eastern NA and its downstream wave packets in the central Europe–Black Sea–Caspian Sea turns out to be the dominant path for the storm activity in the future. Moreover, the eastward flux of wave activity decreases in both the northern and southern latitudes, while it gets stronger in middle latitudes. These results indicate that the double-branch structure of the storm track in the NA and its downstream region in Europe and west Asia will turn to a single-branch pattern at the end of 21st century. Furthermore, the wave breaking and wave packets maxima associated with both the NA and Mediterranian storm tracks and the central branch of wave activity in the Europe–Black Sea will also undergo an eastward shift. Corresponding to that, the tongue of high values of wave activity in the central Mediterranean will also move to the eastern Mediterranian and the tongue of low values of wave activity in the middle-east will disappear. This leads to a considerable increase in penetration of both the Mediterranian and Black Sea wave packets and wave activity to Iran which may result in higher synoptic wave activity in this country in a warming climate.

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.

With regard to the importance of the Mediterranean region as a highly active region in the Northern Hemisphere winter, many studies have been devoted to the weather and climate of this region in general and the impact it receives from the North Atlantic storm track in particular. In 2014, Ahmadi-Givi et al. studied the interaction between the North Atlantic and Mediterranean storm tracks in winter 2005–2004 from the perspective of Rossby-wave propagation and introduced two different kinds of interaction which are referred to as the first and second kind of interaction in what follows. In this paper, based on the latter two kinds of interaction, we aim to identify and better understand the mechanisms of the impact of the North Atlantic storm track by investigating the way Rossby wave-packets are transferred to the Mediterranean region in the extended winter of 2012–2011. To this end, the Hovmoller diagrams, the wave envelope detection, wave activity diagnostics and the energetics are employed. For this extended winter, in total, five marked cases of wave-packet propagation from the North Atlantic to the Mediterranean region are detected. Results show that these five cases can be put into three categories. In the first category, there are two cases in which the wave packets reach Europe, in the upstream of the Mediterranean storm track and penetrate to lower latitudes and the southern branch mentioned in the work by Lee in 2000 and the first track mentioned in the work by Hoskins and Hodges in 2002. Therefore, these cases behave in the same manner of the first kind of interaction. In the second category, there are two cases in which the wave packets pass through the northern part of Europe, then propagate southeastward and join the southern branch along the second track introduced by Hoskins and Hodges in 2002, farther downstream from the first category. Therefore, the cases of the second category behave in the same manner of the second kind of interaction. In the third category, there is only one case in which the wave packet enters the upstream of the Mediterranean storm track and the southern branch in the west of Mediterranean region. However, wave activity diagnostics show that most of the wave activity propagates eastward in the Northern Europe along the second track with little penetration to the lower latitudes. Having signatures of both the first and second kind of interaction, the third category thus exhibits a mixed behavior. Detailed diagnostics are presented for two cases, one from each of the first and second categories. Overall, the fact that four of five cases are compatible with the two kinds of interaction adds to the credibility of this classification. It is also significant that on average, there is monthly one case of wave-packet propagation from the North Atlantic to the Mediterranean region. Finally, it is hoped that this work can be applied to the long-term data sets available, like that of NCEP/NCAR, in order to determine the statistical behavior of the wave packets and the interaction of the two storm tracks.