Recently, a number of authors, beginning with Yoo and Son [2016], have shown that the stratospheric quasi-biennial oscillation (QBO) modulates the amplitude of the boreal winter MJO such that MJO amplitudes are larger on average during the easterly phase (QBOE) than during the westerly phase (QBOW). A major possible mechanism is the decrease in static stability in the lowermost stratosphere under QBOE conditions resulting from relative upwelling associated with the QBO induced meridional circulation. Marshall et al. [2016] have further shown that the observed QBO influence on the MJO should result in improved predictability of the MJO and its convective anomalies in the Pacific during QBOE relative to QBOW.
Here, evidence is presented that tropical upwelling changes related to the 11-year solar cycle also modulate the boreal winter MJO. Using outgoing longwave radiation (OLR)-based MJO index (OMI) amplitude data covering the 1979-2016.3 period (37.3 years), it is found that the increase in MJO occurrence rate and mean amplitude during December, January, and February (DJF) under QBOE conditions is especially large under solar minimum (SMIN) conditions while the decrease in MJO amplitude under QBOW conditions is largest under solar maximum (SMAX) conditions. Consistently, the DJF mean static stability calculated from ERA-Interim reanalysis data in the lowermost stratosphere over the warm pool region is especially high under QBOW/SMAX conditions and is lowest under QBOE/SMIN conditions. Specifically, while the mean MJO amplitude in DJF is ~ 42% larger in QBOE than in QBOW, it is ~ 88% larger in QBOE/SMIN than in QBOW/SMAX. Conversely, the mean MJO amplitude in DJF is only ~ 27% larger in QBOE/SMAX than in QBOW/SMIN. Similarly, while the occurrence rate of daily MJO amplitudes in DJF exceeding unity is ~ 34% larger under QBOE conditions than under QBOW conditions, it is ~ 88% larger in QBOE/SMIN than in QBOW/SMAX. On the other hand, it is only ~ 17% larger in QBOE/SMAX than in QBOW/SMIN. This dependence on the solar cycle is consistent with a solar-induced increase in relative tropical upwelling under SMIN conditions and a decrease (relative downwelling) under SMAX conditions (e.g., Matthes et al. [2004]).
However, these results are based on a limited time record. For example, only 4 to 6 winters qualify for the QBOE/SMIN category while 7 to 9 winters qualify for the QBOW/SMAX category, depending on adopted limits for the QBO and solar phases. During the coming solar minimum, at least one additional winter in the QBOE/SMIN category should occur (possibly as early as the 2017/2018 winter), during which a larger-than-average number of higher-amplitude MJO events is to be expected and an initial test of the proposed relationship will be possible.
References:
Hood, L. L. (2017), QBO/solar modulation of the boreal winter Madden-Julian oscillation: A prediction for the coming solar minimum, Geophys. Res. Lett., to appear.
Marshall, A. G., H. H. Hendon, S.-W. Son, and Y. Lim (2016), Impact of the quasi-biennial oscillation on predictability of the Madden-Julian oscillation, Clim. Dyn., doi:10.1007/s00382-016-3392-0, published online, 12 Oct., 2016.
Matthes, K., U. Langematz, L. Gray, K. Kodera, and K. Labitzke (2004), Improved 11-year solar signal in the Freie Universitat Berlin Climate Middle Atmosphere Model (FUB-CMAM), J. Geophys. Res., 109, doi:10.1029/2003JD004012.
Yoo, C., and S.-W. Son (2016), Modulation of the boreal wintertime Madden-Julian oscillation by the stratospheric quasi-biennial oscillation, Geophys. Res. Lett., 43, 1392-1398.
Zhang, C. (2013), Madden-Julian Oscillation: Bridging weather and climate, Bull. Am. Meteorol. Soc., 94, 1849-1870.
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