Systematic Biases and Future Projections of the East Asian Summer Monsoon in CMIP5

The Meyiu-Baiu large-scale precipitation rainfall band that develops during the East Asian summer monsoon (EASM) season is not well simulated in the state-of-the-art climate models in the CMIP5 archive. Based on twenty years of ECMWF ERA Interim reanalysis data, we study the energetics of the EASM system in the context of the moist static energy (MSE) budget. We find that the stationary meridional eddy velocity plays a significant role in transporting atmospheric enthalpy, and therefore sustaining the well-defined stationary rainfall band, into a region of otherwise negative net energy into the atmospheric column. The stationary meridional eddy velocity is used as a single metric in the evaluation of the CMIP5 decadal2000 scenario simulations and identified as the major culprit in the large spread of rainfall simulations over East Asia and the northwestern Pacific. By using both observational (obs4MIPs) and reanalysis data (ERA Interim), we will explore and attempt to explain the systematic biases in the EASM simulations across CMIP5 models in terms of biases in the stationary meridional eddy velocity as well as in other important environmental parameters, such as the position and strength of the westerly jet, tropospheric temperature, humidity etc.

Moreover, we find that future changes in the EASM in RCP scenarios are strongly coupled to changes in the stationary meridional eddy velocity. The impact of different forcings (such as the greenhouse gases, aerosols etc.) on the EASM circulation and precipitation patterns are analyzed in single changing forcing scenarios, such as historicalGHG, historicalMisc etc., and some idealized scenarios (such as Abrupt4XCO2, sstClim4XCO2, etc.) are also used to insure robustness of our results.