Australia-Asian monsoon in two versions of the UK Met Office Unified Model and their impacts on tropical–extratropical teleconnections

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Many climate models have dry biases in tropical monsoon regions, but it is less clear how these errors can affect these model-simulated tropical–extratropical interactions and rainfall teleconnections. In this study, we evaluate the Australia and Asian (A–A) monsoon rainfall in two versions [Global Atmosphere version 6 (GA6), and version 7 (GA7)] of the UK Met Office Unified Model (UM) with uncoupled atmosphere-only simulations using two horizontal resolutions (N96 ~ 135 km and N216 ~ 60 km). Although UM can capture broad features of rainfall seasonal variations in the monsoon region, there are significant model errors in the locations of monsoon rainfall, its magnitudes and time evolutions, especially a climatological rainfall dry bias over the Indian subcontinent. This dry bias is progressively reduced by increased model resolutions from N96 to N216 and improved model physics from GA6 to GA7. Area-averaged peak monsoon rainfall over the Indian subcontinent (70°–90°E, 5°–25°N) increases by ~ 70% from 3 mm day−1 in GA6N216 to 5 mm day−1 in GA7N216, and rainfall dry bias averaged over a large tropical Asian monsoon domain is reduced by 2/3. In the uncoupled atmosphere-only experiment, the model rainfall is excessively sensitive to its underlying SST conditions, with positive local SST-rainfall correlations occurring over whole tropical oceans, while observations show negative correlations in regions dominated by atmospheric influence although such model performance is improved in its fully coupled runs. Most significant improvement of monsoon rainfall and ENSO (El Nino Southern Oscillations) relationship in GA7 occurred in the Australian continent, with GA7N216 better capturing the observed relationship than GA6N216. Although GA7N216 still suffers a dry bias in the Indian monsoon region, the increased monsoon rainfall substantially increases atmospheric diabatic heating over the region in the middle-upper troposphere, leading to more realistic extratropical circulation Gill-type responses to the heating. This results in improved monsoon-desert rainfall teleconnections, more realistic linkage between tropical Asian monsoon with its subtropical East Asian component, and its interhemispheric influence in Southern Hemisphere. Such results are further confirmed by the model nudging experiments in which the atmospheric wind and temperature are nudged towards observations over the Indian monsoon domain. Our study clearly demonstrates that uncertainty associated with monsoon simulations needs to be considered in future climate projections even outside the monsoon domain.