Local Signs of MJO Convection Initiation

Abstract:

The Madden-Julian Oscillation (MJO) is an eastward-propagating, planetary-scale envelope of organized convective activity in the tropical atmosphere. Of particular research interest is the dynamic and thermodynamic configurations typically associated with initiation of MJO convection. In light of recent results indicating a build up of lower-tropospheric moisture due to advection over the Western Indian Ocean preceding initiation of MJO convection, here we propose an approach for detecting the typical local change of atmospheric states prior to initiation of the MJO. Permutation entropy (PE), a measure of randomness capable of detecting dynamical changes in a temporal process, is used to characterize the complex time evolution of kinematic and thermodynamic fields in the lower troposphere of selected tropical locations. PE computed within a ~10-day window is compared with phase indices of MJO convective activity to identify the typical dynamical configuration of states and changes which occur throughout the MJO life cycle. In preliminary results, the lower-tropospheric moisture field over the Western Indian Ocean is found to be well organized prior to initiation, transitioning to a disorganized state as MJO convection becomes fully developed. The lower-tropospheric perturbation kinetic energy, on the other hand, is found to exhibit systematic disorganization prior to MJO initiation followed by increased organization during the development of MJO circulation. These results indicate that low-level wind and moisture signals in this region could indeed offer some precursory sign of MJO genesis. It is noted that the complexity measure provided by permutation entropy is independent of signal amplitudes, so that it could be possible to detect these systematic dynamical changes operationally even during a quiescent regime prior to MJO initiation.