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 (NICAM), following the
atmospheric model intercomparison project (AMIP) protocol, but with a
slab ocean, was analyzed and compared with the observation. The
simulation reproduced the observed tendency of the MJO to decelerate
when they are embedded within 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 displayed a slow bias
and occurred disproportionately during El Niño events. 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 MJO in long
term simulations.