A Spectral Analysis of the Energetics of the Stratospheric Sudden Warming of Early 1957

Abstract

The spectral forms of tile energy equations for zonal and eddy kinetic energies and zonal and eddy available-potential energies are used to measure energy changes and energy conversions at 50 mb during the period 25 January?9 February 1957. The warming, which was of the bipolar type, could he divided into two phases, a first phase in which the meridional temperature gradient had its usual poleward direction at high latitudes and a second phase in which the gradient was reversed. During the first, or amplifying phase, the eddy energy increased and the energy of the zonal flow decreased, the decrease in zonal kinetic energy outweighing that of zonal available potential energy. The energy flow corresponded to that of a baroclinic instability. Zonal available-potential energy was transformed to eddy available-potential energy, eddy available-potential energy to eddy kinetic energy, and eddy kinetic energy to zonal kinetic energy. Indirect meridional circulations with descending motion in the middle latitude warm belt and ascending motions in the tropics and at high latitudes transformed zonal kinetic energy to zonal available-potential energy. During the second stage the eddy energy diminished and the zonal available potential energy increased. The energy flow reversed except that eddy kinetic energy continued to be converted to zonal kinetic energy. Despite this conversion the zonal kinetic energy continued to decline, presumably because of energy transfer to the troposphere. An attempt is made to infer the normal energy regime of the winter stratosphere. It is hypothesized that in the lower stratosphere the energy store is maintained against the depleting effect of radiation by an upward flux of ultra-long-wave energy from the troposphere. Warmings of the type studied here may represent a baroclinic amplification of the presumably topographically-induced long waves. However, there are facets of the warming which make it unlikely that it corresponds to previously studied baroclinic instabilities. In the relatively small stratospheric mass remaining above 30 mb, zonal available-potential energy is generated radiatively, and a different energy regime must exist.