UKESM1.1: Development and evaluation of an updated configuration of the UK Earth System Model

Abstract

Abstract. Many CMIP6 models exhibit a substantial cold bias in global mean surface temperature (GMST) in the latter part of the 20th century. An overly strong negative aerosol forcing has been suggested as a leading contributor to this bias. An updated configuration of UKESM1, UKESM1.1, has been developed with the aim of reducing the historical cold bias in this model. Changes implemented include an improved representation of SO2 dry deposition along with several other smaller modifications to the aerosol scheme and a retuning of some uncertain parameters of the fully coupled Earth System Model. The Diagnostic, Evaluation and Characterization of Klima (DECK) experiments, a 6-member historical ensemble and a subset of future scenario simulations are completed. In addition, the total anthropogenic effective radiative forcing (ERF), its components and the effective and transient climate sensitivities are also computed. The UKESM1.1 pre-industrial climate is warmer than UKESM1 by up to 0.75 K and a significant improvement in the historical GMST record is simulated with the magnitude of the cold bias reduced by over 50 %. The warmer climate increases ocean heat uptake in the northern hemisphere oceans and reduces Arctic sea ice in better agreement with observations. Changes to the aerosol and related cloud properties are the key drivers of the improved GMST simulation despite only a modest change in aerosol ERF (+0.08 Wm-2). The total anthropogenic ERF increases from 1.76 Wm-2 in UKESM1 to 1.84 Wm-2 in UKESM1.1. The effective climate sensitivity (5.27 K) and transient climate response (2.64 K) remain largely unchanged from UKESM1 (5.36 K and 2.76 K respectively).

The simulation of mineral dust in the United Kingdom Earth System Model UKESM1

“…Further work might possibly have resulted in an alternative choice of settings. For example, in the UKESM1.1 model configuration dust was retuned to give better agreement with observations of AODs, though at the expense of agreement with other observations of size distribution and of concentrations in some areas (Mulcahy et al, 2022).…”

Climate Projections Very Likely Underestimate Future Volcanic Forcing and Its Climatic Effects

“…Atmospheric stratification, wind and humidity affect volcanic plume dynamics and SO 2 injection height (e.g., Mastin, 2014), but SO 2 height is commonly prescribed in modeling studies of volcanic forcing (e.g., Timmreck et al., 2018). To account for meteorological controls on plume dynamics, we have developed UKESM‐VPLUME, which couples the UK Earth System Model (UKESM; Mulcahy et al., 2023) with Plumeria (1‐D eruptive plume model; Mastin, 2007, 2014) (details in Supporting Information ). We use version 1.1 of UKESM with fully‐coupled atmosphere‐land‐ocean and interactive stratospheric aerosols.…”

Climate Projections Very Likely Underestimate Future Volcanic Forcing and its Climatic Effects

“…Atmospheric stratification, wind and humidity affect volcanic plume dynamics and SO 2 injection height (e.g., Mastin, 2014), but SO 2 height is commonly prescribed in modelling studies of volcanic forcing (e.g., Timmreck et al, 2018). To account for meteorological controls on plume dynamics, we have developed UKESM-VPLUME, which couples the UK Earth System Model (UKESM; Mulcahy et al, 2023) with Plumeria (1-D eruptive plume model; Mastin, 2007Mastin, , 2014 (details in Supplementary Information). We use version 1.1 of UKESM with fully-coupled atmosphere-land-ocean and interactive stratospheric aerosols.…”

“…UKESM-VPLUME is a plume-aerosol-chemistry-climate modeling framework that couples the 1-D eruptive plume model Plumeria (Mastin, 2007(Mastin, , 2014 with version 1.1 of UK Earth System Model (UKESM1.1; Mulcahy et al, 2023).…”