Monthly-mean diabatic circulations in the stratosphere

Abstract

Three-dimensional fields of net radiative heating rate have been calculated from temperature, ozone and water vapour data observed by the LIMS instrument. Monthly-mean distributions of net heating rates are presented for the months November 1978–May 1979; particular attention is paid to the distribution of net heating rates at two isentropic levels in the northern hemisphere, one near 30 km in the mid-stratosphere, the other near the stratopause. The seasonal evolution of the heating rate is discussed in relation to the vortex position. We also consider standard deviations of the daily radiative heating distributions from the monthly-mean fields; large variability is found in the dynamically active months of the winter. An interhemispheric comparison of the net heating rate in November in the northern hemisphere with May in the southern hemisphere reveals similarities in the heating distribution at the lower level but differences at the higher level, where the southern hemispheric distribution is far more zonal than that of the northern hemisphere. The continuity equation may be used to determine the divergent horizontal flux of mass at isentropic levels. Over long periods the transient component is found to be dominated by the diabatic component, so that the net radiative heating distributions can be used to obtain this divergence. Near the stratopause we find an anti-correlation between the northward components of divergent and geostrophic winds. The zonal-mean diabatic circulation has also been calculated and found to agree with the rather broad range of results of several other recent studies. Knowledge of the three-dimensional heating rates and tracer distributions has been applied to the problem of tracer transport by mean and eddy motions across isentropic surfaces. The mean advection is found to be dominant, a consequence of the eddy motions being almost adiabatic. The implication of this result is that it is the zonal-mean diabatic circulation which is responsible for any systematic cross-isentropic tracer transport, the eddies playing only a minor role.