Abstract

A striking feature of the upper part of the tropical tropopause layer is that temperatures exhibit a marked annual cycle (rather that the semiannual cycle one might expect from the fact the sun passes over the Equator twice in a year). This temperature cycle has its largest amplitude around 70 hPa. Part of the explanation could come from the annual cycle in ozone having a significant radiative influence in this region. We will first give a brief overview of radiative exchange in the atmosphere with some results from a radiation code set up to do a fixed dynamical heating calculation. We will then quantify the ozone contribution to the temperature using a seasonally evolving fixed dynamical heating (SEFDH) method. We find that the ozone cycle produces an annual cycle in temperature that is consistent with previous work and also argue that water vapour has a non-negligible influence. We also quantify the relative influences of water vapour and ozone on temperatures in the region below the cold point at 100 hPa. The SEFDH approach is limited by the fact that the feedbacks on temperature arising from circulation changes cannot be studied. To go beyond SEFDH , experiments are done using an idealised zonally symmetric general circulation model in which the zonally symmetric dynamics in the stratosphere can respond to the radiative effect of changes in the trace gases. Such a model has the advantage that the atmosphere can be constrained to only include certain aspects of the dynamical and radiative feedback in the stratosphere. With an annual cycle in ozone, the model produces a smaller amplitude annual cycle in temperature with less latitudinal structure than the SEFDH calculation.