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Changes in the monsoon season rainfall over Peninsular Malaysia by the mid-21st century are examined using multi-model ensemble data from the CMIP6 HighResMIP experiments. We examine simulations of the present and future climate simulations run under a high emission scenario of greenhouse gases from the Shared Socioeconomic Pathways (SSP5-8.5). The combined effects of horizontal and vertical resolutions on the projected changes in monsoon rainfall and associated environmental fields are investigated by comparing the ensemble mean of the projected changes utilizing appropriate multi-model groupings. The results indicate a projected decrease (by up to 11% near Mersing of eastern Johor, for the period 2031–2050 relative to 1981–2000) in monsoon precipitation along the southeastern coast of Peninsular Malaysia during the northeast monsoon season associated with the projected weakening of the monsoon flow during boreal winter. For the northwestern regions (e.g. Perak) often affected by severe floods, a significant increase in precipitation (by up to 33%) is projected during the southwest monsoon season, partly driven by the projected strengthening of the cross-equatorial flow and the weakened low-level anti-cyclonic shear of winds in boreal summer. However, the magnitudes and signal-to-noise ratios of the projected changes vary considerably with respect to different horizontal and vertical resolutions. Firstly, models with relatively high horizontal and vertical resolutions project a more significant decrease in precipitation during the northeast monsoon season. Secondly, for the southwest monsoon season, models with relatively high horizontal resolutions project larger magnitudes of increases in precipitation over the northern region, while smaller increases are found in simulations with relatively high vertical resolutions. Generally, reduced ensemble spread and increased signal-to-noise ratios are found in simulations at higher horizontal and vertical resolutions, suggesting increased confidence in model projections with increased model resolution.

In June 2019 the Raikoke volcano, located in the Kuril Islands northeast of the Japanese archipelago, erupted explosively and emitted approximately 1.5 Tg ± 0.2 Tg of SO2 and 0.4–1.8 Tg of ash into the upper troposphere and lower stratosphere. Volcanic ash is usually neglected in modelling stratospheric climate changes since larger particles have generally been considered to be short-lived particles in terms of their stratospheric lifetime. However, recent studies have shown that the coagulation of mixed particles with ash and sulfate is necessary to model the evolution of aerosol size distribution more accurately. We perform simulations using a nudged version of the UK Earth System Model (UKESM1) that includes a detailed two-moment aerosol microphysical scheme for modelling the oxidation of sulfur dioxide (SO2) to sulfate aerosol and the detailed evolution of aerosol microphysics in the stratosphere. We compare the model with a wide range of observational data. The current observational network, including satellites, surface-based lidars, and high-altitude sun photometers means that smaller-scale eruptions such as Raikoke provide unprecedented detail of the evolution of volcanic plumes and processes, but there are significant differences in the evolution of the plume detected using the various satellite retrievals. These differences stem from fundamental differences in detection methods between, e.g. lidar and limb-sounding measurement techniques and the associated differences in detection limits and the geographical areas where robust retrievals are possible. This study highlights that, despite the problems in developing robust and consistent observational constraints, the balance of evidence suggests that including ash in the model emission scheme provides a more accurate simulation of the evolution of the volcanic plume within UKESM1.

The paper constitutes part 2 of a study performing a first systematic inter-model comparison of the atmospheric responses to stratospheric sulfate aerosol injections (SAI) at various latitudes as simulated by three state-of-the-art Earth System Models – CESM2(WACCM6), UKESM1.0, and GISS-E2.1-G. We use a set of five sensitivity experiments with constant annual injections of SO2 in the lower stratosphere at either 30° S, 15° S, 0°, 15° N or 30° N. We identify the similarities and differences in the simulated responses amongst the models as well as demonstrate the role of biases in the climatological circulation and specific aspects of the model microphysics and chemistry in driving the inter-model differences. Building on part 1 (Visioni et al., 2022), we explain the simulated differences in the aerosol spatial distribution between the models: CESM2 shows significantly faster shallow branches of the Brewer Dobson circulation facilitating transport of the relatively larger-sized aerosol to higher latitudes; UKESM shows a relatively isolated tropical pipe and older tropical age-of-air confining the relatively smaller-sized aerosols to the tropics; and the two GISS versions with either bulk or modal aerosol microphysics show elevated sulfate levels at higher latitudes as the result of smaller aerosol sizes and relatively stronger horizontal mixing (thus very young stratospheric age-of-air). We then elucidate the role of these factors in driving the stratospheric responses to SAI. We find a large spread in the magnitudes of the tropical lower stratospheric warming amongst the models, which can be partially attributed to the differences in aerosol distribution and sizes. Regarding the stratospheric ozone responses, we find a good agreement in the tropics between the models with modal microphysics, with lower stratospheric ozone changes consistent with the SAI-induced modulation of the large-scale circulation and the resulting changes in transport. In contrast to the relative agreement at low latitudes, we find a large inter-model spread in the Antarctic ozone responses that can largely be explained by the differences in the simulated latitudinal distributions of aerosols as well as the degree of implementation of heterogeneous halogen chemistry on sulfate. Finally, we also find large differences in stratospheric water vapour responses amongst the models, with CESM2 and GISS with modal microphysics both showing significant increases in stratospheric water vapour under SAI consistent with the increase in cold point temperatures that were largely not reproduced in UKESM. For the GISS runs with bulk microphysics, the SAI simulations show contrastingly different stratospheric responses to the models using the modal aerosol treatment, including the absence of lower stratospheric warming as well as significant reductions in stratospheric water vapour and ozone. The results point towards the importance of detailed treatment of aerosol processes, although some problems in halogen chemistry in GISS are identified that require further attention. Overall, our results contribute to an increased understanding of the underlying physical mechanisms as well as the sources of uncertainty in model projections of climate impacts from SAI.

This study investigates the ability of 20 model simulations which contributed to the CMIP6 HighResMIP to simulate precipitation in different monsoon seasons and extreme precipitation events over Peninsular Malaysia. The model experiments utilize common forcing but are run with different horizontal and vertical resolutions, which allows an investigation of the impact of model resolution on the simulation of precipitation. The impact of resolution on the models’ ability to simulate precipitation and associated environmental fields is assessed by comparing multi-model ensembles at different resolutions with three observed precipitation datasets and four climate reanalyses. Model simulations with relatively high horizontal and vertical resolution exhibit better performance in simulating the annual cycle of precipitation and extreme precipitation over Peninsular Malaysia and the coastal regions. Improvements associated with the increase in horizontal and vertical resolutions are also found in the statistical relationship between precipitation and monsoon intensity in different seasons, especially for the simulated precipitation at finer vertical resolutions during the southwest monsoon seasons. However, the increase in vertical resolution can lead to a reduction of annual mean precipitation compared to that from the models with low vertical resolutions, associated with an overestimation of moisture divergence and underestimation of lower-tropospheric vertical ascent in the different monsoon seasons. This limits any improvement in the simulation of precipitation in the high vertical resolution experiments, particularly for the Southwest monsoon season.