Momentum and heat sources of the mesosphere and lower thermosphere regions 70–110 km

The dynamics of the mesosphere/lower thermosphere (MLT) region between 70 and 110 km is examined through use of a climatological model of eastward (ν) and northward (ν) prevailing winds derived from global radar observations. The continuity equation is utilized to calculate the vertical wind, ω, from ν. The Coriolis acceleration ν and net adiabatic heating/cooling N2ω are calculated, where ƒ = Coriolis parameter and N2 =bouyancy frequency squared, and are associated with the mean momentum and heat flux divergences due to perturbation motions about the mean state. The latter are interpreted as originating from a combination of gravity waves, tides, and planetary waves undergoing transience, dissipation and non-linear interactions, and unorganized eddy motions (synoptic-scale disturbances). The quantities u, ν, ω, ƒν and N2ω are decomposed into annual mean, annual and semiannual components. Common features of all these fields include: (a) a dynamical regime below the mesopause which is clearly an upward extension of the mesospheric jet system; (b) a circulation system above about 90 km with characteristics and apparent dynamical origins different from those below the mesopause and (c) significant hemispheric asymmetries. In addition, a well-defined annual mean circulation is revealed. An analogy is drawn between this system and the characteristics and processes governing the tropospheric annual mean circulation. Possible numerical modeling experiments which draw upon our results, and which may enhanced our understanding of MLT dynamics, are proposed.