The role of the stratosphere in teleconnections arising from slow and fast MJO episodes

The Madden-Julian Oscillation (MJO) influences the North Atlantic region on timescales of days to weeks. The extratropical response has been found to depend on the propagation speed of the MJO in the region of the Indian to the Pacific ocean: Here, we define slow (fast) MJO events as events that take more (less) than 16 (10) days to propagate from the Indian Ocean (phase 3) to the Pacific Ocean (phase 6), and the MJO amplitude has to be greater than 1 for at least 3 consecutive days in phase 3 and phase 6. Slowly propagating MJO events lead to a stronger North Atlantic Oscillation (NAO) response than fast MJO events, and the positive (negative) NAO response for slow events occurs after a lag of 10 days following phase 4 (phases 7-8). The increased persistence of the tropical thermal forcing in slow events may contribute to a stronger extratropical response.

Furthermore, the MJO can influence the strength of the stratospheric polar vortex, which in turn can impact the NAO via downward coupling. While the tropospheric pathway for teleconnections from MJO events with varying phase speed is well understood, the stratospheric pathways for MJO events of different propagation speeds has not yet been explored.

In this study, we discuss the stratospheric pathways during fast and slow MJO episodes using reanalysis data with respect to the strength of the Northern Hemisphere stratospheric polar vortex and subsequent downward coupling to the troposphere. The results suggest that the stratosphere plays an important role in generating a strong NAO response during slowly propagating MJO episodes.