Stratospheric Modulation of the Madden-Julian Oscillation: Potential Influence Mechanisms and Implications for the Behavior of the North Pacific Storm Track

Abstract

The Madden-Julian Oscillation (MJO) is an eastward-propagating, convectively-coupled wave packet, and functions as a key driver of subseasonal-to-seasonal (S2S) variability in tropical convection and precipitation. Recent research has shown that both the phase and the amplitude of the MJO may be modulated by the stratosphere, via both an oscillation of the stratospheric zonal wind known as the Quasi-Biennial Oscillation (QBO) and by changes in stratospheric ozone production induced by variations in solar UV activity. This dissertation investigates, in detail, potential mechanisms behind this modulation, its extent in the tropics, and its implications for mid-latitude weather patterns, including aspects of S2S predictability. Published, peer-reviewed papers resulting from this research are presented in Appendices A and B. Results that are in a near-final form but not yet submitted for publication are presented in Appendices C and D. In Appendix A, we investigate a wave forcing mechanism for the existence of the QBO-MJO connection, one which may explain both its exclusivity to the boreal winter and its relatively recent development within the observational record. In Appendix B, we investigate the possible influence of shorter-term 27-day rotational solar UV variations on MJO behavior. We find significant and long-lasting effects on the effectiveness of MJO propagation past the maritime continent barrier, stemming from static stability changes associated with such short-term solar UV variability. Appendices C and D focus on possible implications of stratospheric MJO modulation on the mid-latitude storm track. In Appendix C, we focus on interannual sources of stratospheric variability, such as the QBO and the 11-year sunspot cycle. The MJO-induced changes in the North Pacific Storm Track (NPST) produced by these stratospheric forcings are compared to well-known ENSO-related NPST influences. In Appendix D, we extend on the findings of both Appendix B and Appendix C by linking changes in the MJO induced by 27-day rotational solar UV variations to lagged effects on the NPST. The nature of the statistically-derived effects on the NPST provide insight into the mechanism leading to stratospheric influences on the MJO-NPST relationship, which is centered on the generation and propagation of Rossby waves. The results of these studies have implications for mid-latitude S2S predictability, given the found stratospheric influence on the NPST, while also providing evidence for a specific mechanism for the modulation of the MJO via the stratosphere. This implicated mechanism may be useful in addressing some of the deficiencies in simulating observed stratosphere-troposphere connections in current-generation global climate models.