AbstractEastward propagation is an essential characteristic of Madden-Julian Oscillation (MJO). Yet, simulation of MJO propagation in global climate models (GCMs) remains a major challenge and understanding the causes of propagation remains controversial. The present study explores why the GCMs have diverse performances in MJO simulation by diagnosis of 24 GCM simulations. We find an intrinsic linkage between MJO propagation and the zonal structural asymmetry with respect to MJO convective center. The observed and realistically simulated MJO eastward propagations are characterized by stronger Kelvin easterlies than Rossby westerlies in the lower troposphere, which is opposite to the Gill pattern where Rossby westerlies are twice stronger than Kelvin easterlies. The GCMs simulating stronger Rossby westerlies tend to show a stationary MJO. MJO propagation performances are robustly correlated with the quality of simulated zonal asymmetries in the 850 hPa equatorial zonal winds, 700 hPa diabatic heating, 1000-700 hPa equivalent potential temperature and convective instability. The models simulating realistic MJO propagation are exemplified by an eastward propagation of boundary layer moisture convergence (BLMC) that leads precipitation propagation by about 5 days. The BLMC stimulates MJO eastward propagation by pre-conditioning and pre-destabilizing the atmosphere, and by generating lower tropospheric heating and available potential energy to the east of precipitation center. The MJO structural asymmetry is generated by the trio-interaction among convective heating, moisture, and equatorial wave and BL dynamics. In GCMs, differing convective heating representation could produce different MJO structural asymmetry, thus different propagations. Diagnosis of structural asymmetry may help revealing the models’ deficiency in representing the complex 47 trio-interaction processes, which involves various parameterized processes.
Journal of Climate – American Meteorological Society
Published: Jul 13, 2017
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