Modelling the stratospheric residual circulation under historical and future climates

This thesis deals with the advective part of the global Brewer-Dobson circulation (BDC), also known as the stratospheric residual circulation. The residual circulation cannot be directly measured and is relatively poorly constrained in observations and models. While the processes that drive the residual circulation are relatively well understood, our knowledge of how these processes can manifest in response to an evolving climate state is insufficient. This research addresses several topics related to the representation of the stratospheric residual circulation in models under historical and future climates. The residual circulation is evaluated in a suite of “specified dynamics or nudged” chemistry-climate simulations relaxed towards reanalysis data. It is found that nudging does not constrain the mean strength of the residual circulation while it does tightly constrain the interannual variability in the lower stratosphere. To investigate climate change effects on the residual circulation, model simulations are performed using a 4xCO2 experiment separated into three components: a rapid adjustment due to the radiative effects of CO2; a global mean sea surface temperature (SST) response; and the local deviations of SST from the global mean. It is shown that the global SST explains most of the increased in circulation in the lower stratosphere, while both the rapid adjustment and global SST are important in the upper stratosphere. This means there are two characteristic timescales in the response of the residual circulation to increased CO2, with the relative importance of each timescale being height dependent. Lastly, the utility of different observational-based indirect proxy measures for the residual circulation in the tropical lower stratosphere are evaluated within a chemistry-climate model framework. It is shown that a temperature-based measure captures variability in the residual circulation on interannual timescales, whereas tropical mean ozone concentrations, when lagged, exhibit a close out-of-phase relationship with tropical upwelling across seasonal, interannual and multi-decadal timescales. Overall, this work has progressed our understanding pertaining to mechanistic and diagnostic processes related to the stratospheric residual circulation in numerical simulations.