On the dominance of 28-month cycle in the equatorial-stratospheric Quasi Biennal Oscillation

The Quasi Biennal Oscillation, the downward propagating easterly and westerly wind regimes in the equatorial stratosphere, has been investigated according to robust statistics providing the reliability of the examined harmonics. The amplitude-to-noise ratio is found to be highest in correspondence of the 28- month harmonic for each examined height; the relative amplitude and phase values are found, respectively, to decrease and increase with the height and to take about a year to descend from 15 to 70 hPa with a progressive lag of about 1 month/km. At the top of the stratosphere, easterlies dominate, while at the bottom, westerlies are more likely to be found.