Aspects of the energetics of the upper stratosphere during the January-February 1973 major sudden warming

Data from the Nimbus 5 selective chopper radiometer have been used to study certain features of the dynamics and energetics of the January-February 1973 sudden warming. Daily values of the energetics of the upper stratosphere during a sudden warming have been calculated. Analyses of the rates of change of temperature and geopotential wave amplitude give no support to the suggestion that the disturbance suffers a time lag with increasing height. Maximum rate of warming is shown to descend from the upper stratosphere. Interaction between travelling and quasi-stationary waves did not appear to be important in the development of the warming. Two main results arise from an analysis of the latitudinal distributions of the energetics. Firstly there is good evidence to suggest that the amplification of the temperature wave between 10 and 2mb results from a transfer of eddy potential energy from the 50–10mb layer, and not from the lower stratosphere or troposphere below 50mb. However, energy propagation above 2mb is initiated by a large increase in energy flux through the 50mb level. Secondly, despite considerable uncertainty in the kinetic energy budgets, it appears that eddy kinetic energy increases in high latitudes as a result of convergence of the vertical energy flux but increases in middle latitudes as a result of barotropic conversion from the zonal flow. The westerly zonal flow is decelerated in mid-latitudes and initially accelerated in high latitudes by divergence and convergence of large poleward eddy momentum transport. Eventually the effect of the induced mean meridional circulation leads to easterly acceleration in high as well as middle latitudes. Some strong resemblances are found in the numerically simulated zonal flow changes of some model warmings which show a meridional lag in the onset of easterly acceleration.