The monsoon hydroclimates in HadGEM3 model configurations GA3.0 and GA4.0: Impact of remote versus local circulations errors and horizontal resolution

Abstract. State-of-the-science general circulation models (GCMs) are the primary tools for making policy-relevant climate calculations. Yet, these models face challenges in monsoon regions where live more than 70 % of the world's population, due to the complex interplay of local and remote influences on a spectrum of space and time scales. This work examines the fidelity to reproduce regional and global monsoons climatological features using the Met Office Unified Model (MetUM) third and fourth generations Global Atmosphere (GA3.0) and (GA4.0), two configurations of the HadGEM3 system developed for seamless use across climate and weather time scales. Results are compared both against multiple observational gridded datasets and outputs from 20 atmospheric-only GCMs simulations from the CMIP5 campaign. Furthermore, we investigate the influence of remote versus local atmospheric circulation errors by constraining realistically HadGEM3 circulations over a prescribed monsoon domain and examining the consequences outside and inside this domain using the grid-point nudging method. The GA3.0 largely captures global monsoon features, including the monsoon precipitation patterns and extent of regional monsoon domains, when integrated using a low (~ 135-km in mid-latitudes), medium (~ 60 km in mid-latitudes) and high (~ 25 km in mid-latitudes) horizontal resolutions. GA4.0 and GA3.0 results display a close similarity, and compares reasonably well against the best available CMIP5 models. The common failure of HadGEM3 configurations is the simulated weak magnitude and extent of the Asian Summer Monsoon (ASM) precipitation pattern, and associated low-level Somali jet. This situation is also apparent within HadGEM2-A, ACCESS1-0, and CSIRO-Mk3-6-0 – three GCMs sharing dynamical and physical components. HadGEM3 performance improves significantly over ASM with atmospheric circulations constrained realistically over the tropics, West African and Asian Summer monsoon domains. Conversely, constraining atmospheric circulations over other remote monsoon domains show little influence on the ASM precipitation. We argue that GA4.0 and GA3.0 poor simulations over the ASM domain are attributable, partly to local atmospheric circulations errors and excessive precipitation over the southwest equatorial Indian Ocean, rather than to remote tropical atmospheric responses of varying forcing fields, such as SST over the Arabian Sea, aerosols, and growing greenhouse gas emissions. The improved understanding of GCM performance in monsoon regions is an important step for credible projections of global monsoon changes.