Precursor Environmental Conditions Associated with the Termination of Madden-Julian Oscillation Events

Abstract:

Current generations of global climate models continue to struggle with simulating many of the observed features of the Madden-Julian oscillation (MJO) and suffer from low skill regarding initiation forecasts. While recent work has focused on those mechanisms thought to be important for MJO initiation, fewer studies have examined the large-scale conditions associated with quiescent periods of the MJO and the decay of existing events. Understanding these mechanisms may provide a valuable context toward improving simulations of MJO initiation and propagation in climate and operational weather forecast models.

This study presents an analysis of the precursor environmental conditions related to the termination of MJO events. A simple climatology is created using a real-time MJO monitoring index, documenting the locations and frequencies of MJO decay. Lead-lag composites of several atmospheric variables including temperature, moisture, and intraseasonal wind anomalies are generated from three reanalyses. Long-term, lower tropospheric moisture deficits over the local domain best identify terminating events over the Indian Ocean, with a northward shift of the Intertropical Convergence Zone (ITCZ) and corresponding lead times as much as 20 days prior to MJO decay.

Statistically significant differences are also identified more than 10 days in advance of MJO termination events in the west Pacific, though the vertical velocity and moisture anomalies are more symmetric about the equator. We also present results for those MJOs that terminate over the maritime continent. Unlike the Indian Ocean and west Pacific, the likelihood of an MJO to cross the maritime continent appears related to its own intensity, rather than the upstream environmental conditions, with only the strongest MJOs propagating into the warm pool region.

Finally, a budget analysis is performed on the three-dimensional moisture advection equation in order to better elucidate what time-scales and physical mechanisms are most important for MJO termination. The combination of intraseasonal vertical circulation anomalies coupled with the mean state specific humidity best explain the anomalous moisture patterns associated with MJO termination, suggesting that the upstream influence of the MJO circulation can eventually lead to its own demise.