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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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QBO/Solar Influences on the Tropical Madden‐Julian Oscillation: A Mechanism Based on Extratropical Wave Forcing in Late Fall and Early Winter
Possible sources of the observed modulation of the tropical Madden‐Julian oscillation (MJO) by the stratospheric quasi‐biennial oscillation (QBO) and the 11‐year solar cycle are investigated using 41 years of reanalysis data and archived climate model data. Larger upward fluxes of extratropical planetary‐scale waves, leading in some cases to sudden stratospheric warmings (SSWs), are observed in late fall and early winter during the easterly phase of the QBO than during the westerly phase (the “Holton‐Tan effect”). A similar but smaller increase occurs, on average, during solar minima relative to solar maxima. In addition to the warming at high latitudes, extratropical wave forcing events produce cooling and reduced static stability in the tropical lower stratosphere. Here, it is found that if SSWs occur in early winter (before ∼mid‐January), the reduced static stability produces, on average, a statistically significant, lagged strengthening of the MJO. This therefore represents a possible mechanism for producing, or at least enhancing, the observed QBO and solar modulations of the MJO in boreal winter. An initial analysis of archived climate model data shows that at least one model version with realistic QBO and solar forcing and with 4 X CO2 forcings partly simulates both of these characteristics (QBO/solar modulation of early winter wave forcing and lagged strengthening of the MJO following early winter SSWs). However, the modeled MJO is insufficiently sensitive to QBO‐induced static stability reductions, precluding simulation of the QBO‐MJO connection.
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- PAR ID:
- 10474776
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 128
- Issue:
- 6
- ISSN:
- 2169-897X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1029/2022JD037824
