عنوان مقاله [English]
The weather and climate are affected significantly by the storm tracks, which play an important role in midlatitude atmospheric dynamics. There are indications that the Mediterranean region and its downstream areas like the western Asia are affected by the North Atlantic (NA) storm track. As NAO is the most important factor affecting weather in Europe and the Mediterranean region by altering the location and direction of storms, the NA and Mediterranean storm tracks are influenced by this teleconnection. On the basis of the monthly index of the NAO, the critical positive (negative) months, denoted by NAO+ (NAO-), are defined as the ones with the NAO index greater (less) than the long-term mean of the NAO index, plus (minus) one standard deviation. The aim of this research is to study the impacts of the NAO on the NA and Mediterranean storm tracks from the energetic point of view by the JRA-55 reanalysis dataset and to compare the results with those previously obtained by the NCEP/NCAR reanalysis dataset. To this end, composite maps of the most important terms of the time tendency equation of eddy kinetic energy (EKE), as well as the baroclinic generation term, were computed for the critical positive and negative months of the NAO using both datasets in 1959–2017 period for the winter.
Based on the both dataset’s results, although though baroclinicity plays the most important role in increasing EKE in the western NA, it is not very important in Western Europe and the Mediterranean region. Instead, convergence of total energy flux is the dominant factor in producing EKE in the latter areas. Interestingly, the ageostrophic flux has a greater role in strengthening the Mediterranean storm track compared to the NA storm track.
Results of the JRA-55 are generally consistent with the results of the NCEP/NCAR, and the two datasets lead to almost similar patterns for energetics but some differences are also observed in terms of magnitude and extension of the main centers. In almost all the fields, the JRA-55 results show higher values with more details. The JRA-55 results display stronger and more extended maxima for the NA and Mediterranean storm tracks in both phases. The difference between the storm tracks in the two datasets, in terms of EKE amplitudes, is more clear in the negative phase of NAO. In NAO-, the maxima of the NA storm track is about 10% stronger in the JRA-55 results than that of the NCEP/NCAR. Also, both the northern and southern branches of the NA storm track are more intense with a greater eastward extension in the JRA-55. In a similar way, the Mediterranean storm track obtained from the JRA-55 dataset is also more intense with a greater extension, especially over the east and south of the Mediterranean and its downstream regions like the Middle East, than the corresponding results by the NCEP/NCAR. In the negative phase, the difference between the maximum of divergence over the Mediterranean Sea and the maximum of convergence over the Red Sea is larger in the JRA-55. In NAO- this can play an important role in making the strength and spread of the Mediterranean storm track in the JRA-55 greater than those of the NCEP/NCAR. In addition, the impact of NAO phases on the intensity of the divergence (convergence) center over the Mediterranean (Red) Sea is greater when the JRA-55 data are being used.
Unlike the previous results on the NA storm track as being stronger in the positive phase of the NAO, results of the JRA-55 dataset do not exhibit a noticeable difference between the two phases. In addition, although being consistent with the NCEP/NCAR results in the way the Mediterranean storm track is related to the NA storm track in each phase, however the JRA-55 results show weaker linkage between the two storm tracks in NAO- when compared to previous studies.