Atmospheric rivers in the Australia-Asian region under current and future climate in CMIP5 models

Atmospheric rivers (ARs), as long and narrow bands of strong water vapour transport in the lower troposphere, have drawn increasing scientific attention in recent years. Results from a collaborative project between the Australian Bureau of Meteorology and China Meteorological Administration have shown some unique AR characteristics embedded within the Australia-Asian monsoon based on observational analyses. As part of the project, this study focused on assessing the skill of global climate models for simulating ARs in the region under current climate and their projected changes due to global warming. Daily data from 17 Coupled Model Intercomparison Project Phase 5 (CMIP5) models in their historical and Representative Concentration Pathway (RCP) 8.5 simulations were analysed for the periods of 1981-2005 and 2081-2100 respectively. Compared with results derived from European Centre for Medium-Range Weather Forecasts ERA-interim reanalysis data, these model ensemble results showed significant seasonal variations of horizontal water vapour transport as observed, but their magnitudes measured by vertically integrated water vapour transport (IVT) were weaker, particularly for the East Asian summer monsoon. Using an objective AR detection algorithm based on 85th percentile IVT magnitude and its geometry, we showed that multi-model-ensemble (MME) averaged AR occurrence agreed well with the results derived from the reanalysis for their spatial distributions and seasonal variations. Under the RCP8.5 global warming scenario, the model ensembles, overall, showed an enhanced water vapour transport, primarily due to increased atmospheric humidity associated with a warmed atmosphere. Consequently, they simulated increased AR frequency and bigger AR size in most of the region, particularly over north and northeast China and southern Australia. However, the MME results showed a reduced AR frequency and size in July/August in southern and eastern part of China and its adjacent waters. We attributed these results to the response of the Western North Pacific Subtropical High (WNPSH) to global warming. Our analysis showed that westward expansion of WNPSH lead to the shift of ARs more inland in East Asia. In this case, eastern China was directly under the control of WNPSH, which did not favour AR development and penetration into the region. Our analyses of ARs in the A-A monsoon system offers new insight in understanding potential climate changes in the monsoon region under warmed climate.