The East Asian Atmospheric Water Cycle and Monsoon Circulation in the Met Office Unified Model

In this study the low-level monsoon circulation and observed sources of moisture responsible for the maintenance and seasonal evolution of the East Asian monsoon are examined, studying the detailed water budget components. These observational estimates are contrasted with the Met Office Unified Model (MetUM) climate simulation performance in capturing the circulation and water cycle at a variety of model horizontal resolutions and in fully coupled ocean-atmosphere simulations. We study the role of large-scale circulation in determining the hydrological cycle by analyzing key systematic errors in the model simulations.

MetUM climate simulations exhibit robust circulation errors, also seen in other CMIP models, which include a weakening of the West Pacific Subtropical High, and leads to an underestima- tion of the southwesterly monsoon flow over China and prevents the monsoon precipitation penetrating far enough north. These errors are sensitive to model horizontal resolution, with higher resolution simulations showing improvements over low resolution configurations in simulating the mean state East Asian monsoon. Precipitation and implied diabatic heating biases in the South Asian monsoon and Maritime Continent region are shown, via nudging sensitivity experiments, to have an impact on the East Asian monsoon circulation. By inference, the improvement of these tropical biases with increased model horizontal resolution is hypothesized to be a factor in improvements seen over East Asia with increased resolution. Results from the annual cycle of the hydrological budget components in five domains show a good agreement between MetUM simulations and ERA-Interim reanalysis in northern and Tibetan domains. In simulations, the contribution from moisture convergence to precipitation in the region is larger than in reanalysis, and they display less precipitation recycling over land. The errors are closely linked to monsoon circulation biases.

In addition to the climatological seasonal cycle, we present initial results from a study of the interannual variability of the summer West Pacific Subtropical High in observations and climate simulations, and its impact in the variability of the East Asian hydrological cycle.