An energetic study of the relation between NAO and the large-scale tropospheric flow in South West Asia



Previous studies show that the North Atlantic Oscillation (NAO) is the dominant mode of variability in the northern hemisphere winter with marked climatic effects on its downstream regions. In this article the effects of some important forcing terms in the time tendency equation of the Eddy Kinetic Energy (EKE) in critical positive months (CPM) and critical negative months (CNM) of the NAO are studied using NCEP/NCAR reanalysis data. The data covered span the years 1950-2005 for the winter months (December to February). The critical months are defined on the basis of the monthly index of NAO and include 29 CPM and 33 CNM. The selected forcing terms include baroclinic conversion (BCC), barotropic conversion (BTC), convergence of total energy flux (CTF) and ageostrophic flux (CAF). The ensemble mean of vertical average of these forcing terms as well as the baroclinic generation (BCG) term, representing the generation of available potential energy, are computed over an area from 0 to 90E and 20N to 70N for CPM and CNM. In addition to the usual ensemble mean, to avoid cancellation, separate averages are also taken of the positive and negative values of the foregoing quantities.
The results indicate that there is no considerable difference in the amount of EKE between CPM and CNM in the Mediterranean region. However, moving eastward, the values of EKE become greater in the CPM than in the CNM in such a way that the difference between the two reaches its maximum over the South West of Iran. In the CPM, all of the computed forcing terms are larger than in the CNM. This is particularly true for the extreme. The largest values of BCG are observed in high latitudes with two distinct centers of maxima for the CPM and CNM. In the Mediterranean region, the average over the positive values of the BCG shows greater values in CPM, whereas the average over the negative values shows negligible differences between CPM and CNM. Despite the greater generation of available potential energy in the Mediterranean region in CPM, it seems that the other terms act in such a way as to make the mean EKE nearly equal in the CPM and CNM.
In the Mediterranean region and the Middle East, the BTC is the dominant forcing term in EKE production. The comparison of the distribution of BTC and the situation of 300-hPa subtropical jet shows that the two maxima centers of BTC (in the west of Iran and the central Mediterranean) are located in the north of the subtropical jet. The minimum values of BTC are observed in the Red Sea in the south of the subtropical jet in winter. The interesting point is that the magnitude of the BTC term in CPM is greater than CNM, coinciding with a stronger subtropical jet. This fact points to a direct relation between the magnitude of BTC and the intensity of the subtropical jet.
The maps of the ensemble mean of the negative and positive values of energy flux show the same pattern in CNM while some differences are observed in CPM. In the central Mediterranean region, there is energy divergence in both phases of NAO which is stronger in CPM. The direction of energy flux vectors indicates that energy radiated from the central Mediterranean region is transferred southeastward to an energy flux convergence over northeastern Africa and the Red Sea. This convergence is stronger and more extensive in CPM. Converting EKE to the zonal mean kinetic energy by the BTC in this area can be responsible for the observed stronger subtropical jet in CPM. In other words, the presence of a strong energy divergence in the central Mediterranean can be a significant source of energy for eastward travelling tropospheric disturbances in CPM.
The pattern of BCG shows considerable conversion of zonal mean available potential energy to eddy available potential energy in cyclogenesis centers in the Mediterranean region as well as in the west and east of Iran. The comparison of BCG in the two phases of NAO indicates that the connection of the Mediterranean storm track to the Atlantic storm track is weaker in CPM compared to the CNM. This means that in CPM, the Mediterranean storm track can act as a distinct center of action much in the same way as the Pacific and Atlantic storm tracks.