A Comparison of the Madden-Julian Oscillation Simulated by Different Versions of the MIROC Climate Model

Abstract

A pair of 20-year simulations by two different versions of MIROC (Model for Interdisciplinary Research on Climate) was examined by using a standardized set of Madden-Julian Oscillation (MJO) diagnostics. One of the major differences between version 4 (MIROC4) and version 5 (MIROC5) of MIROC is the cumulus parameterization scheme. MIROC4 uses a prognostic Arakawa-Schubert scheme, whereas MIROC5 uses the Chikira scheme. MIROC5 reproduced the MJO better than MIROC4: a stronger signal in the wavenumber-frequency diagram, a slower and more noticeable eastward movement in the lag-correlation plot, and a better phase relationship between outgoing longwave radiation and zonal winds.
To investigate the impact of mid-tropospheric humidity on cumulus development in MIROC5, the atmosphere-only version of MIROC5 was used for a series of sensitivity runs, each with different entrainment parameter values. The entrainment parameter settings significantly influenced the simulated MJO. Large-scale cloud systems tended to move westward with smaller entrainment parameter values, accompanied by a pair of rotations with quasi-symmetry about the equator in the lower troposphere, whereas eastward movements were faster, with larger entrainment parameter values.