The double peak in upwelling and heating in the tropical lower stratosphere.

Abstract

The processes responsible for double peak latitudinal structures in the time averaged tropical lower stratospheric upwelling, centred near 70hPa and 20º N–S, previously noted in ERA-Interim and other reanalysis and model datasets, are considered. It is demonstrated that the structure of the wave force resolved by ERA-Interim consistently balances the angular momentum transport associated with the double peak. Analysis of the corresponding structures in diabatic heating rates from ERA-Interim indicates that the peaks arise predominantly from meridional structure in ozone concentrations and the associated absorption of both shortwave and longwave radiation. Additional smaller contributions arise from local absorption of longwave radiation emitted from the relatively warm layers above and below, as well as from cloud-related radiative effects and non-radiative diabatic heating. The temperature at 70 hPa is slightly higher near 20º N–S than at the Equator, opposite to what would be expected if the latitudinal structure in radiative heating was associated with local relaxation. It is proposed on the basis of this analysis that the primary cause of the peaks in upwelling is the externally imposed (i.e., non-relaxational) part of the radiative heating field. The dynamical plausibility of this hypothesis is investigated in a companion paper.