Deceleration of Madden–Julian Oscillation Speed in NICAM AMIP-Type Simulation Associated With Biases in the Walker Circulation Strength

Abstract

The eastward movement speed of Madden–Julian Oscillation (MJO) events simulated in a 30-year simulation on a global cloud resolving model, nonhydrostatic icosahedral atmospheric model, following the atmospheric model intercomparison project protocol with a slab ocean was analyzed and compared with the observation. The simulation reproduced the observed tendency of the MJO to decelerate when it is embedded within the stronger Walker circulation, intensified by background sea surface temperature states with larger zonal gradients between the warmer western Pacific and the cooler Indian Ocean and eastern Pacific. However, the simulated MJO events occurred disproportionately during El Niño events and displayed a slow bias. These biases were associated with an overestimation of the western Walker circulation cell strength, which was partially counteracted during El Niño events. Our results highlight the importance of accurately reproducing the mean atmospheric circulation for the realistic reproduction of the MJO in long term simulations.