Arctic Sea Ice Loss, Long-Term Trends in Extratropical Wave Forcing, and the Observed Strengthening 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.  A proposed mechanism is reductions of tropical lower stratospheric static stability during QBOE caused by (1) the QBO induced meridional circulation; and (2) QBO influences on extratropical wave forcing of the stratospheric residual meridional circulation during early winter.  Here, long-term variability of the QBO-MJO connection and associated variability of near-tropopause tropical static stability and extratropical wave forcing are investigated using European Center reanalysis data for the 1959-2021 period.  During the most reliable (post-satellite) part of the record beginning in 1979, a strengthening of the QBO-MJO modulation has occurred during a time when tropical static stability in the lowermost stratosphere and uppermost troposphere has been decreasing and extratropical wave forcing in early winter has been increasing.  A high inverse correlation (R = -0.87) is obtained during this period between early winter wave forcing anomalies and wintertime tropical lower stratospheric static stability.  Regression relationships are used to show that positive trends in early winter wave forcing during this period have likely contributed to decreases in tropical static stability, favoring a stronger QBO-MJO connection.  As shown in previous work, increased sea level pressure anomalies over northern Eurasia produced by Arctic sea ice loss may have been a significant source of the observed positive trends in early winter wave forcing.