Arctic Sea Ice Loss, Long-Term Trends in Extratropical Wave Forcing, and the Emergence of the QBO-MJO Connection

A modulation has been identified of the tropical Madden-Julian oscillation (MJO) by the stratospheric quasi-biennial oscillation (QBO) such that the MJO in boreal winter is ~ 40% stronger and persists ~ 10 days longer during the easterly QBO phase (QBOE) than during the westerly phase (Figure 1a). A similar, but weaker, modulation of the MJO by 11-year solar ultraviolet forcing of the stratosphere has also been reported. A possible cause of the QBO-MJO connection is reductions of tropical lower stratospheric static stability during QBOE. These reductions of static stability are caused by increased tropical upwelling rates associated with an acceleration of the effective residual meridional circulation of the stratosphere (the Brewer-Dobson circulation). The accelerations of the Brewer-Dobson circulation are in turn caused by increased absorption and dissipation of planetary-scale waves in the stratospheric polar vortex. Using reconstructed MJO and QBO indices dating back to ~1905, it has been found in several studies that the observed QBO modulation has only emerged since the early 1980s and is accompanied by long-term decreases in near-tropopause static stability (Figure 1b). Here, it is shown that (a) increased tropical upwelling rates resulting from long-term positive trends in early-winter extratropical wave forcing of the stratospheric circulation are sufficient to explain the observed decline in tropical lower stratospheric static stability in boreal winter; and (b) increased sea level pressure anomalies over northern Eurasia produced by Arctic sea ice loss (and associated increases in Eurasian snow cover) can potentially explain the observed positive trends in early winter wave forcing. If future coupled climate model experiments confirm the latter conclusion and if the observed decreases in static stability are essential for operation of the QBO-MJO connection, then it would follow that Arctic climate change has been a major contributor to the emergence of that connection.