Connections among Quasi-Biennial Oscillation, Stratospheric Sudden Warming and Solar Cycle

Using Empirical Mode Decomposition of 44 years of NCEP reanalyzed stratospheric data, we showed that the global Quasi-Biennial Oscillation is amplitude-modulated by the 11-year Solar Cycle in the polar region. It is an one-sided modulation in the sense that Solar Cycle only modulates the westerly QBO, while its influence during the easterly phase is insignificant. The modulation by the Solar Cycle appears to be through the preconditioning effect during solar maxima for the occurrence of Stratospheric Sudden Warmings. During the easterly years, the winter polar vortex is already preconditioned for SSW through the Holton-Tan effect regardless of the phase of the Solar Cycle. On the other hand, during the westerly years, the warming during solar max may push the polar region in the Northern Hemisphere over the threshold for SSWs. Because SSW creates large additional, dynamical warming of the polar stratosphere, the amplitude modulation is large. This appears to provide a dynamical amplifier for the small Solar Cycle warming. Such a mechanism is absent in the troposphere, because there is no sudden warming, and so the Solar Cycle effect is not amplified. There is no amplitude modulation of the QBO found in the troposphere.