High-resolution climate models improve simulation of monsoon rainfall changes in the Ganga-Brahmaputra-Meghna basin

This study examines observed and projected changes in monsoon timing (onset/withdrawal/duration) and strength (total and extreme rainfall) in the Ganges-Brahmaputra-Meghna basin. While prior South Asia monsoon research has mainly used coarse-resolution models, we utilised CMIP6 HighResMIP models to assess whether improved horizontal resolution (HR-models: <0.25°) improves the simulation of the monsoon when compared to low resolution (LR-models: 0.35 to 1.2 °), evaluated against reference datasets (MSWEP and ERA5). Our findings indicate that HR-models generally outperform LR-models in capturing monsoon characteristics, with the bias in annual average rainfall higher in LR-models. Between 1979-2014, MSWEP (ERA5) tends towards an earlier (later) onset by around 7 (3) days, a later withdrawal by around 8 (12) days, resulting in a longer monsoon duration of approximately 15 (9) days, respectively; much larger changes than both HR- and LR- MODELS. The trends in duration are highest for MSWEP and lowest for LR-models. We find that HR-models better capture observed trends in total and extreme rainfall over 1979-2014 compared to LR-models. For the future climate (2015-2050), HR-(LR-) model ensembles project a delay of ~4 (3) days in monsoon onset under the SSP585 forcing scenario, with HR-models indicating a later onset compared to LR-models. However, HR-models project a shorter monsoon duration and an earlier withdrawal (~3 days) compared to LR-models. We find that HR-models project a significantly greater increase in rainfall than LR-models: these project increases in extreme monsoon rainfall (up to 5.55%/decade) and total monsoon rainfall (around 1.4%/decade) in the GBM basin, while LR-models project little change. Our findings highlight the large uncertainties in simulating monsoon characteristics from climate models, but show that HR-models can be helpful in studying changing monsoon dynamics over the complex topography of the GBM basin.