PNNL

The Science                                       

The Madden–Julian oscillation (MJO) is the dominant mode of tropical intraseasonal variability characterized by an eastward propagating band of clouds and rainfall. Occasionally, the MJO moves south while passing over the Maritime Continent (MC) (known as a “detour”), affecting regional climate and weather over the region. This research investigates the physical mechanisms underpinning the impact of cold surge events on the movement of the MJO. The cross-equatorial cold surge (CES) is a large-scale circulation associated with cold air outbreaks at high latitudes. It amplifies the MJO’s southward detour over the MC during the active phase of the Australian monsoon season (December to March). Surges increase northwesterly and westerly winds, which increase the convergence of moisture in a specific area and positively feed back onto the MJO convection. These results highlight the importance of intraseasonal cross-equatorial winds in governing the magnitude of MJO detours during the active Australian monsoon season.

The Impact

A southward detour of the MJO significantly affects extreme weather along the periphery of the southern Indian Ocean and southern Pacific Ocean. Examples of this extreme weather include the catastrophic floodings in Indonesia and Northern Australia. While prior studies have proposed some compelling hypotheses about the underlying mechanisms, the factors that influence the intensity of a detour remain unknown. This study shows for the first time that the magnitude of the MJO’s southward detour over the MC is amplified. This amplification is caused by the interplay between the extratropical and tropical dynamics via the interaction between the CES and MJO. These results provide new insights into understanding a process that can improve subseasonal weather forecasts and have the potential to address model biases when simulating the movement of the MJO.

Summary

During boreal winter, MJO events that reach the west Pacific tend to detour to the south of the MC, moving south of Indonesia and through the Timor Sea. Currently, there is no agreement on which factors govern the magnitude of this southward detour. By using moisture mode theory, researchers find that the CES reinforces the MJO’s southward detour by increasing the horizontal moisture convergence over the southern MC. Further, researchers examined the contributions of the mean state, intraseasonal anomalies, and high-frequency eddy anomalies to the total horizontal moisture flux convergence. The analysis indicates that the zonal convergence is intensified during CES events. This leads to stronger convective activity in the southern MC region. The background moisture during the active monsoon season helps increase the zonal moisture flux convergence during CES events. Overall, the increase in the moisture convergence over the southern MC is consistent with the CES-induced intensification of low-level northwesterly and westerly winds. This increase in wind intensity strengthens the zonal wind convergences and the positive wind-evaporation feedback onto MJO convection.

PNNL Contact

L. Ruby Leung, Pacific Northwest National Laboratory, ruby.leung@pnnl.gov

Funding

This research was supported by the Department of Energy, Office of Science, as part of research in the Regional and Global Model Analysis program area, Earth and Environmental System Modeling Program.