Summary
It has been shown by Taguchi (2010) that QBO periods were longer and amplitudes larger during periods of La Niña than during periods of El Niño during the period 1953-2008. Garfinkel and Hartmann (2007) have shown that the sign of the correlation between ENSO and QBO indices changed from negative during the first half of this period to positive during the second half of this period. These results should be better understood since it has been shown that there is QBO modulation of water vapor concentrations entering the stratosphere; there are analyses indicating QBO influences on typhoon tracks and on tropical convection; and there appear to be a QBO influence on Atlantic hurricanes. With the Taguchi (2010) and Garfinkel and Hartmann (2007) results as background, it is shown here that during both the first and second halves of the overall period, from the 1950s to the present, QBO periods are longer during La Niña than during El Niño, but that QBO amplitudes were only larger during La Niña in the second half of this period, and, if anything, QBO amplitudes were larger during times of El Niño during the earlier half of the overall period. A proposed explanation for this is given in terms of the convectively forced gravity waves that play a large role in giving rise to the QBO. The low OLRs, characteristic of tropical deep convection, have broader longitudinal extent over the tropical Pacific during times of El Niño during both the first and second parts of this overall period. Given this, we hypothesize that this implies a larger zonally-averaged gravity wave momentum flux during El Niño, and hence a shorter period than is the case during times of La Niña during both the first and second halves of the overall period. Furthermore, we also show that there was more very deep tropical convection during times of La Niña than during El Niño during the period 1992-2010, and this was not the case during the period 1974- 1984. More vigorous tropical convection, with latent heat release of greater depth, should force a broader gravity wave phase velocity spectrum, and this should give a QBO with larger amplitude. Modeling results support these explanations. Garfinkel, C. I., and D. L. Hartmann, 2007: Effects of the El Niño-Southern Oscillation and the quasibienniel oscillation on polar temperatures in the stratosphere. J. Geophys. Res. 112: D19112. doi: 10.1029/2007JD009481. Taguchi, T., 2010: Observed connection of the stratospheric quasi-biennial oscillation with El Niño-Southern Oscillation in radiosonde data. J. Geophys. Res.115: D18120. doi: 10.1029/2010JD014325
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