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Abstract 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 (a) the QBO induced meridional circulation; and (b) 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.
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This content will become publicly available on December 28, 2025
Causes of a Lack of QBO/Solar‐MJO Connection in Certain CMIP6 Models
A connection between the quasi‐biennial oscillation (QBO), solar variability, and the short‐term convective climate oscillation, the Madden‐Julian oscillation (MJO), in boreal winter has been found in observational data, yet it is generally lacking in current global climate models (GCMs). A proposed mechanism is changes in tropical lower stratospheric upwelling rates and static stability caused by QBO and solar UV effects on extratropical wave forcing of the stratospheric residual meridional circulation (the Brewer‐Dobson circulation). The extent to which this mechanism, which operates only in boreal winter and enhances similar effects of the QBO‐induced meridional circulation, is simulated in a series of GCMs participating in the Coupled Model Intercomparison Project 6 (CMIP6) is investigated. The models are found to be often lacking complete representation of several elements of the mechanism, with particular issues being QBOs that are westerly biased and weak in the lower stratosphere, insufficient solar or QBO modulation of extratropical wave activity (the Holton‐Tan effect), too weak reductions in equatorial tropopause static stability in response to extratropical wave forcing, and MJOs that in some cases do not respond to these reductions. Through bypassing many of these deficiencies via data selection, it is demonstrated that effects on the MJO that resemble those found in observations (strengthening of the MJO following early winter sudden stratospheric warmings and during easterly QBO winters) can be simulated by a subset of the models. This supports operation of the proposed mechanism, and points to needed model improvements, although caveats exist and further work is needed.
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- Award ID(s):
- 2039384
- PAR ID:
- 10614382
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Edition / Version:
- 1
- Volume:
- 129
- Issue:
- 24
- ISSN:
- 2169-897X
- Page Range / eLocation ID:
- e2024JD041606
- Subject(s) / Keyword(s):
- Madden-Julian oscillation quasi-biennial oscillation Holton-Tan effect CMIP6 models
- Format(s):
- Medium: X Size: 7MB Other: pdf
- Size(s):
- 7MB
- Sponsoring Org:
- National Science Foundation
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