Improving the simulation of historical surface temperature in UKESM


Many CMIP6 models, including UKESM1, exhibit a substantial cold surface temperature bias in the latter part of the 20th century. This has raised questions about the ability of such models to simulate important climate processes such as aerosol-cloud interactions and in particular reduces confidence in the model’s ability to simulate future climate, although we argue this is possibly mis-guided. In this talk I will present the investigation we undertook to investigate the underlying cause of this bias in UKESM1. We suggest an overly strong negative historical aerosol effective radiative forcing due to an excess of sulphur dioxide (SO2) is a leading contributor to this bias and detail improvements we’ve made to SO2 removal processes. I will introduce a new configuration of UKESM1, UKESM1.1 which we developed building on these improved sulphur cycle processes as well as other improved tunings based on the now wide-ranging evaluation of UKESM1. I will describe the new configuration and present an overview of results from a 6-member historical ensemble as well as some early results from a set of future projections conducted using UKESM1.1.