QBO/Solar Modulation of the Boreal Winter MJO: Observational Tests Through the 2018/2019 Winter
Abstract
It is well established that the stratospheric quasi-biennial oscillation (QBO) modulates the occurrence rate of significant Madden-Julian Oscillation (MJO) events during northern winter such that more events occur during the easterly QBO phase (QBOE) than during the westerly phase (QBOW) [e.g., Yoo and Son, GRL, 2016]. It has also been proposed that the stratospheric quasi-decadal oscillation (QDO), which is driven mainly by 11-year solar UV forcing, provides a secondary modulation with more MJO events during solar minima (SMIN) than during solar maxima (SMAX) [e.g., Hood, GRL, 2017]. The largest number of strong MJO events is found under QBOE/SMIN conditions while the smallest number is found under QBOW /SMAX conditions. For both the QBO and QDO, the leading candidate mechanism is the decrease in static stability in the tropical lowermost stratosphere. Here, we report a new analysis of MJO occurrence rates, mean amplitudes, and lower stratospheric static stabilities for the post-1979 satellite era, including the 2018-2019 winter. Mean MJO amplitudes during December, January, and February (DJF) of each winter are calculated and the phase of the QBO is determined by calculating the mean DJF equatorial zonal wind in the 50 to 70 hPa layer (u5070). The phase of the QDO is determined based on the mean DJF solar UV flux at 205 nm estimated according to the NRL model (F205). DJF mean lower stratospheric static stabilities are calculated from ERA-Interim reanalysis temperature data at the 70 and 100 hPa levels and are averaged over the tropical warm pool region.
Consistent with previous analyses, results show that DJF mean MJO amplitudes are relatively large and static stabilities are relatively low for the 12 QBOE winters while the opposite is true for the 18 QBOW winters (Figure 1a). However, the distributions are widely scattered and overlapping. When only combined QBOE/SMIN or QBOW/SMAX conditions are allowed (5 and 7 winters, respectively), the distributions are more separated (Figure 1b). In particular, the most recent 2018-2019 winter had a larger-than-average mean MJO amplitude of 1.57 and a mean static stability of 11.0 K/km, which was the third lowest static stability for any DJF period. 76 of 90 days (83%) had MJO amplitudes > 1 standard deviation. This most recent winter appears to be consistent with the QBO/QDO hypothesis.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A13I3011R
- Keywords:
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- 3305 Climate change and variability;
- ATMOSPHERIC PROCESSES;
- 3337 Global climate models;
- ATMOSPHERIC PROCESSES;
- 3362 Stratosphere/troposphere interactions;
- ATMOSPHERIC PROCESSES;
- 0550 Model verification and validation;
- COMPUTATIONAL GEOPHYSICS