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

Abstract

This study presents an analysis of the precursor environmental conditions related to the termination of Madden-Julian oscillation (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. There is remarkable agreement among the datasets with long-term, lower-tropospheric moisture deficits over the local domain best identifying termination events over the Indian Ocean. MJO termination in the Indian Ocean is also linked to a northward shift of the intertropical convergence zone (ITCZ) with possible lead times as much as 20 days prior to MJO decay. Statistically significant differences in the low-level vertical velocity and specific humidity are also identified more than 10 days in advance of MJO termination events in the western Pacific, though the differences here are more symmetric about the equator. Unlike the Indian Ocean and western Pacific, MJOs that terminate over the Maritime Continent appear to be related to their own intensity rather than the downstream conditions. As such, only the strongest MJOs tend to propagate 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 downstream influence of the MJO circulation can eventually lead to its future demise.

Three different downstream fates of the boreal-summer MJOs on their passages over the Maritime Continent

“…These results suggest that if the MJO convection in Indian Ocean can't create favorable downstream responses over the MC and tropical Pacific, it becomes vulnerable to the hostile environments around the MC and subject to rapid decay during its passage over the MC. In general, this scenario substantiates the finding of Stachnik et al (2015), who also emphasized that the downstream responses of the MJO largely determine its own fate.…”

Evaluating MJO event initiation and decay in the skeleton model using an RMM‐like index

“…In addition, we employ the technique used in Stachnik et al . [], where the daily values of RMM1 and RMM2 must contain nearly continual counterclockwise rotation (i.e., eastward propagation) in the two‐dimensional phase space of Wheeler and Hendon []. We also require that a simulated MJO “event” pass through at least four phases of the SMM index.…”

“…Examples of MJO termination events are shown in RMM phases 7 and 1 for Figures a and b, respectively. Overall, the event histories are much smoother than those in observations [e.g., Stachnik et al ., , Figure 1], even when projecting the model data directly onto the Wheeler and Hendon [] EOFs. This artifact is partially by design, as the skeleton model was specifically developed for intraseasonal and planetary oscillations.…”

“…The MJO represents a convectively coupled mode of cloud and circulation anomalies on the planetary scale (i.e., wave numbers 1–3) with an oscillation frequency of roughly 30–60 days [ Madden and Julian , ; Zhang , ]. Deep convection associated with an MJO event generally appears as a broad envelope of cloud and precipitation anomalies that most commonly begin over the western or central Indian Ocean [e.g., Matthews , ; Straub , ] and propagates eastward at ~5 m s −1 until eventually terminating over the maritime continent or the central Pacific [e.g., Stachnik et al ., ]. The MJO directly impacts local weather and tropical climate and has also been shown to have far‐reaching dynamical effects including modifying the global monsoons [ Lau and Chan , ; Goswami and Ajaya Mohan , ; Matthews , ; Wheeler and Hendon , ], genesis and intensification of tropical cyclones [ Maloney and Hartmann , ; Jiang et al ., ], midlatitude weather anomalies [ Higgins et al ., ; Bond and Vecchi , ; Guan et al ., ], and may impact the onset of El Niño–Southern Oscillation (ENSO) events [e.g., Lau and Waliser , ].…”

A skeleton model for the MJO with refined vertical structure

“…The MJO events are organized into wave trains with growth and demise, i.e. into series of successive MJO events following a primary MJO event, as seen in nature (Matthews 2008;Stachnik et al 2015). In addition, the MJO events are irregular and intermittent, with a great diversity in strength, structure, lifetime and localization.…”