Mechanisms of 20th and 21st Century Asian Monsoon Changes in CMIP5 Models and Observations

We examine Asian monsoon rainfall and circulation changes in 20th century observations, as well as CMIP5 model simulations of the historical period and 21st century projections. By applying the signal to noise maximizing Empirical Orthogonal Function Analysis (S/N EOF) to the Asian monsoon rainfall of the multi-model, multi-ensemble CMIP5 model simulations of the historical period as well as the rcp85 scenario of the 21st century, we can extract the radiatively forced component of the change. The observed and model simulated monsoon rainfall and circulation changes are then regressed onto the forced component to identify the spatial pattern and statistical significance of the forced signal. We find that in model simulations, Asian monsoon change during the historical period is dominated by a drying signal, possibly due to aerosol effect. However in rcp85 future scenario, monsoon precipitation is projected to increase under greenhouse gas forcing. We then examine the past and future changes in moisture, circulation and the atmospheric moisture budget to determine the associated mechanisms. Total moisture transport is divided into monthly mean flow and transient eddy contributions, with the former further broken down into terms related to mass divergence and advection, which introduces a surface term. The moisture convergence associated with this enhanced surface flow over topography plays a significant role in future monsoon change under model simulations. We further contrast the mechanisms of the forced monsoon change with natural variability associated with ENSO on interannual time scale.