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