[A-AS03] Large-scale moisture and organized cloud systems

[AAS03-08] Deceleration of MJO Speed Associated with Overly Strong Walker Circulation in a NICAM-AMIP Simulation

*Tamaki Suematsu¹, Hiroaki Miura², Chihiro Kodama³, Daisuke Takasuka³ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Graduate School of Science, The University of Tokyo , 3.Japan Agency for Marine-Earth Science and Technology)

Keywords:Madden-Julian Oscillation , Walker Circulation, NICAM

The eastward moving speeds of Madden-Julian Oscillation (MJO) events simulated in a 30-year simulation on a global cloud resolving model, nonhydrostatic icosahedral atmospheric model (NICAM), following the atmospheric model intercomparison project protocol (AMIP) was analyzed and compared with the observation.
The simulation reproduced the observed tendency of the MJO to slow down when they are embedded within stronger Walker circulation cells intensified by background SST states with larger zonal SST gradients between the warmer western Pacific and the cooler Indian ocean and eastern Pacific. However, compared to the observation, the mean speed of the simulated MJO events displayed a slow bias, and occurred disproportionately during El Niño events. These biases were associated with overestimation of the western Walker circulation cell strength, which was partially counteracted during El Niño events. Our results highlight the importance of adequate representation of the mean atmospheric circulation states for the realistic simulation of the MJO.