Quantifying the Contribution of Vertical Mixing to the Tape Recorder Signal

The tropical tropopause layer (TTL) is a region with multiscale dynamics that acts as a gateway for constituents entering the stratosphere which then impact the global radiative balance. The temperature structure of the TTL is important for setting the amount of water vapor arriving at the base of the stratosphere and forming the aptly-named tape recorder. Relying on vertical advection to carry most of the water vapor signal, the tape recorder can also be used to understand the importance of other dynamical processes such as vertical mixing, e.g. due to breaking gravity waves or overshooting cloud tops. In this study, a simple lag-correlation method is used to compare the tape recorder signal in pressure and potential temperature coordinates. We also investigate the importance of vertical mixing by using a one-dimensional advection-diffusion-dilution model and a variety of scenarios to quantify the relationship between processes in different coordinates. We use daily satellite data from MLS and HALOE, along with ERA-interim and varying resolution outputs from chemistry-climate models (CCMs). All datasets show that effective diffusion is small when calculated in potential temperature coordinates; however the heating rates in ERA-i and some CCMs are larger than what is approximated in observations. In pressure coordinates, diffusion plays an important role in changing the phase of the tape recorder with height, in particular for ERA-i and some models. This suggests that vertical mixing is acting on the tape recorder's slope and that diffusion is partially imbedded within the larger heating rates of ERA-i and certain CCMs. For all datasets, seasonally-varying diffusion produces the most accurate tape recorders. By quantifying the annually-varying influence of vertical mixing on the TTL, this study may benefit models working to predict seasonal changes and trends in this region in relation to the Brewer-Dobson circulation.