Research Projects

The Madden-Julian oscillation (MJO) is a slow eastward propagating system of planetary scale tropical disturbances, and it is considered as the dominant source of predictability in the sub-seasonal to seasonal timescale. The persistent diabatic heating and large scale circulation associated with the MJO can be considered stationary (30-90 days) relative to the time scale of synoptic scale (3-10 days) disturbances, and several studies have brought out observational evidence for the MJO modulation of tropical synoptic scale variability. However, a mechanistic framework for the scale interaction between the MJO and the synoptic scale variability and its impact on tropical weather is still missing. We hypothesize that the MJO exerts a significant control on tropical weather by impacting the genesis, strength, and characteristics of equatorial wave modes. The westward propagating Mixed Rossby-Gravity (MRG) waves and eastward propagating Kelvin waves are the major modes of tropical synoptic scale disturbances and they together account for a significant portion of the tropical weather variability. Since the MJO is a main source of diabatic heating in the tropics, it is expected that the MJO might control the frequency of occurrence of these wave events. The excitation of MRG waves over the western and central Pacific region is reported to be linked to the asymmetric diabatic heating associated with the MJO. Our recent study indicates that the lowering of gross moist stability of the atmosphere by MJO might also contribute to an increase in convection-circulation coupling strength associated with these waves. Stronger convection-circulation coupling will make the atmosphere more baroclinic and in turn enhance the vertical propagability of the waves. In addition to the in-situ processes, the MRG and Kelvin waves can also be triggered by lateral forcing from the extratropics. Since the MJO induces upper-level easterly (westerly) wind response to the east (west) of its heating, it is also possible that the location and strength of MJO heating impacts the regions of upper tropospheric westerly winds and in turn modulates the frequency of occurrence of MRG and Kelvin wave events initiated by extratropical intrusions. Quantification of the relative contribution of MJO in modulating the tropical disturbances, which might give an estimate of the predictability of these systems. A mechanistic framework of the scale interactions between the MJO and synoptic scale variability will be useful for addressing many questions related to tropical weather and climate variability. An understanding of the modulation of the vertical propagability of the waves by the MJO might provide a physical explanation for the observed statistical relationship between the MJO and the stratospheric QBO, which has not been explained so far.