عنوان مقاله [English]
The recent studies suggest that the predictability of the surface weather, in medium- and long-range time scales, is influenced by the dynamic coupling of the tropospheric and stratospheric circulations. Therefore, it is very important to study the details of coupling mechanisms, which is the main goal of the present study with the main focus on the southwest Asia. The NCEP/NCAR reanalysis data are used, including temperature, geopotential height and horizontal wind components at different pressure levels from 1968 to 2015. The region of study includes Iran and extends westward to the Mediterranean Sea. The iso-potential vorticity surface of 2 PVU (a PVU equal to ) is taken to represent the dynamical tropopause and the potential temperature, geopotential height and pressure are interpolated at the 2 PVU surface.
First, with a method similar to that of Thompson et al. (2002), the dates corresponding to the occurrence of stratospheric polar vortex were identified and classified into cases of strong and weak polar vortices. The anomalies of pressure, geopotential height and potential temperature were then computed with respect to the long-term mean for the cases of polar vortex. Analysis of the results shows that in the southwest Asia, associated with the cases of strong vortex increase in pressure and decreases in potential temperature and geopotential height are observed at the tropopause. Further, in cases of weak vortex, the southwest Asia is mainly in the surf zone and less influenced by the polar vortex compared to the cases of strong vortex which enclose a significant portion of the region. The fact that a larger part of the southwest Asia is located within the inner part of the strong vortices makes their impact on the tropopause more marked.
In the second part of the study, the focus is on the most extreme cases of strong and weak events in the long-term period which occurred respectively in February 1974 and November 2009. In addition to presenting the details of the life cycle of the two cases, the time-lagged correlation coefficients were computed between the tropopause quantities in the region and the geopotential height anomaly at 10hPa between 60°N and 90°N degrees, which is the index introduced by Thompson et al. (2002). The time series of the correlation coefficients averaged over Iran reveals a meaningful delay of about eight to ten days for the impact of the strong polar vortex felt on the tropopause in Iran. Given that this time delay is within the limit of medium-range prediction, further studies on the impact of polar vortex are needed to improve the predictability of the surface weather in the region.