A mechanism-denial study on the Madden-Julian Oscillation with reduced interference from mean state changes

Abstract:

Using the Superparameterized Community Atmosphere Model, the authors investigate the importance of (i) the influences of extratropical and circumnavigating waves, (ii) the wind-evaporation feedback and (iii) the radiative-convective feedback to the Madden-Julian Oscillation (MJO). A common issue with mechanism-denial studies is the interference from mean state changes in the simulations when processes are turned off in the model. In the present study, time-invariant forcing and nudging on an effective time scale longer than the intraseasonal time scale are implemented to maintain the mean state. Forced by perpetual February sea surface temperature, the control simulation is integrated for 15 years, and produces satisfactory climatology. The influence of extratropical waves on the MJO is first examined by relaxing the prognostic variables outside the tropics towards the controlled mean state. When the mean state is not maintained, the MJO activity decreases as the Hadley circulation weakens, consistent with a previous study. However, when the mean state is maintained to be the same as that of the control, the MJO activity does not decrease with the suppression of extratropical waves and remains as strong as that in the control. The results are not sensitive to the latitudes beyond which the extratropical waves are suppressed. To further assess the influence of circumnavigating waves on the MJO, the prognostic variables are relaxed towards the controlled climatology all over the globe except within 40°S-30°N, 0°E-90°W, so that the MJO initiation and propagation are confined in the box. In this experiment, strong MJO activity is produced over the tropical Indian Ocean and Pacific Ocean, and the results suggest that the essential processes for the existence of the MJO are internal to these regions. In the last two experiments, the wind-evaporation and radiative-convective feedback are turned off by prescribing surface flux and radiative heating to the climatology of these fields from the control simulation, respectively, and it is found that the wind-evaporation feedback slows down the MJO, while the radiative-convective feedback amplifies the MJO. The present results illustrate the importance of maintaining the mean state in MJO mechanism-denial studies.