| This study has investigated whether the monsoon intraseasonal oscillation (MISO) and Madden Julian Oscillation (MJO) are same or different over the Indo-Pacific tropical region. The results are based on observational analysis with high-resolution diabatic heating as central to all the mechanisms discussed and atmospheric general circulation model experiments using the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 4 (CAM4.0). The MISO and MJO are extracted as nonlinear oscillations during boreal summer and winter, respectively, by applying multi-channel singular spectrum analysis on daily anomalies of diabatic heating over the Indo-Pacific region. The boreal summer leading oscillatory mode in diabatic heating gives the three-dimensional structure of MISO over the tropics, while the boreal winter leading oscillatory mode shows the three-dimensional structure of MJO. The existence of MISO was also found in the control simulation of CAM4.0, but the MJO was found to be less realistic. To understand the mechanisms involved in MISO and MJO, certain experiments with CAM4.0 were conducted by imposing idealized and observed profiles of three-dimensional diabatic heating. Lead and lag relations among the circulation, moisture and other fields in response to the imposed diabatic heating were analyzed to compare the mechanisms of MISO and MJO.;The observational analysis of diabatic heating shows that MISO oscillates with a period of 45 days and MJO with 52 days. Both the oscillations show maximum between 600hPa and 400hPa. Though both the oscillations show eastward propagation, MISO has a strong northward propagation and MJO has a weak southward propagation as well. The zonal extent of the diabatic heating in MISO goes up to 150°--180°E, while MJO extends up to 180°--210°E, and both the oscillations are strongly coupled to the circulation. The meridional propagation of MISO and MJO, i.e., strong northward propagation of MISO and weak southward propagation of MJO, involves two mechanisms. One is the interaction of the vertical shear of the zonal winds with relative vorticity and the other is the moisture preconditioning of the atmosphere. The moisture preconditioning is also seen to be present in the eastward propagation. The analysis of the sea surface temperature (SST) anomalies associated with diabatic heating shows a lag with the convective and suppressed states, which positively feeds back into the atmosphere through the surface fluxes. At the same time, the land-surface temperature is found to be co-located with the convective and suppressed states, driving the winds due to the differential heating between land and ocean, and resulting in MISO propagating strongly northward than its boreal winter counterpart, MJO, propagating southward.;The eastward propagation, common to both MISO and MJO, is better understood by the mechanistic experiments in which a very narrow band of idealized eastward moving heating and cooling signal was added to the model's existing temperature tendency in the equatorial belt. This experiment improved the eastward propagation of MISO and produced a more realistic MJO. However, the addition of a northward moving signal over the Indian Ocean and South Asian region does not improve the spatial structure of MISO, especially the eastward branch and eastward propagation of MISO. The circulation, specific humidity and the diabatic heating of the model were studied along with precipitation. It is found that, by adding heating and cooling signals to the model, the model is able to organize the winds better and transport moisture better. Therefore, by adding specific three-dimensional heating and cooling signal to the model, it was able to simulate the MISO and the MJO closer to observation, without any SST feedback.;The observational analysis and the model experiments have shown more similarities between MISO and MJO than differences. The two oscillations are similar in their period, propagate eastward and can be explained by the same mechanisms. The difference is seen in the strong northward propagation in MISO and a weak southward propagation in MJO and the zonal extent of MISO is found to be slightly shorter than that of MJO. The strong meridional propagation of MISO, which is weak in MJO, is hypothesized to be due to the different differential heating gradient between the land and the ocean in the two seasons. The mechanisms explained in the observational study need to be further addressed through model experiments with SST feedback. The future work will include the study of the coupled model's response to imposed diabatic heating. |
