A mechanism denial study on the Madden-Julian oscillation

A series of Madden-Julian oscillation (MJO) mechanism-denial experiments is performed using an atmospheric general circulation model (AGCM). One of two versions of the AGCM, which has a better ability to represent the MJO than the other, is used to investigate what macroscopic mechanisms are responsible for the difference. Daily climatological seasonal cycles of i) surface latent heat flux, ii) net radiative heating rate, and iii) surface wind stress are obtained from a control simulation and prescribed in the mechanism-denial experiments to turn off the i) wind-induced surface heat exchange (WISHE), ii) cloud-radiation interaction (CRI), and iii) frictional wave-CISK (FWC) mechanisms, respectively. Dual and triple mechanism denial experiments are also conducted by switching off multiple mechanisms together.

The experiments suggest that CRI is the most important of the three processes for the development of MJO events in this model. The simulated MJO tends to weaken and accelerate when CRI is disabled. In the control model, the positive feedback from radiative heating is strong (dimensionless ratio between daily radiative and latent heating >0.2) when anomalous precipitation is small (< 14 mm day-1), consistent with a major role for this process in developing incipient disturbances. When WISHE is turned off, the MJO is strengthened, but only when CRI remains turned on. This is because anomalous surface winds have a phase relationship with precipitation that is different from that observed, such that anomalous surface fluxes negatively feed back to the MJO-related convection anomalies when surface latent heat flux interacts with surface wind in this model. When FWC is switched off, the simulated MJO shows no systematic change compared to the control, although the phase difference between convection and moisture convergence in the boundary layer is reduced as expected.

We propose that it would be informative if similar mechanism-denial experiments were conducted using other models.