The Role of the Stratosphere in Teleconnections Arising From Fast and Slow MJO Episodes

The Madden-Julian Oscillation (MJO) can influence the extratropical circulation on timescales up to several weeks, with a dependence on the MJO characteristics: MJO episodes that propagate slowly across the Maritime Continent have a stronger impact on Euro-Atlantic weather than fast MJO episodes. While the tropospheric pathway for MJO teleconnections with varying phase speeds is well understood, in this study, we investigate the contribution of the Northern Hemisphere stratospheric pathway for fast versus slow MJO episodes. During slow MJO episodes, Phases 5-6 lead to increased upward wave propagation in the North Pacific sector, and subsequently enhanced heat flux at 100 hPa, leading to the weakening of the polar vortex. The results suggest a clear role of stratosphere-troposphere coupling for slow MJO episodes, which is proposed as a mechanism for anomalously strong positive polar cap height anomalies in MJO Phases 7-8. The Madden-Julian Oscillation (MJO) consists of an eastward-moving band of clouds, rainfall, and winds in the tropics. The MJO originates over the West Indian Ocean and typically takes 10-20 days to propagate from the Indian Ocean (Phase 3) to the Western Pacific (Phase 6). The convection associated with the MJO influences global weather via wave-propagation. The waves propagate through both a tropospheric and stratospheric pathway. The MJO is, therefore, an important source of mid-latitude predictability on timescales of weeks to months. Here, we explore the stratospheric pathway, differentiating between fast and slowly propagating MJO episodes. Slowly propagating MJO episodes significantly strengthen the polar vortex from day 0-18 following Phases 3-5 and weaken the vortex following Phases 7-8. The weakening of vortex is due to a strengthening of the low-pressure system in the North Pacific sector, allowing upward wave propagation into the stratosphere during Phases 6-1. This weakened vortex signal then propagates into the troposphere causing persistent warm anomalies in the polar cap region. Fast MJO episodes show a weaker response than slow cases. Our study suggests that slowly propagating MJO episodes have the potential to extend the predictability of the stratosphere and its subsequent impact on the North Atlantic region. Madden-Julian Oscillation (MJO) episodes that propagate slowly from Phase 3 to Phase 6 exhibit significantly stronger anomalies in the stratosphere than fast episodesHeat flux anomalies in the lower stratosphere are significantly enhanced and last longer following MJO Phases 5-7 for slow MJO episodesThe stratospheric signal progresses downward in the slow MJO episodes and extends to the lower troposphere and surface during MJO Phases 7/8