Atmospheric reanalysis reveals easterly vertical wind shear facilitates Kelvin wave propagation into the upper troposphere, highlighting a physical mechanism linking the QBO to the MJO.
Key Points
To determine whether vertical shear of the zonal wind across the equatorial tropopause modulates the Madden–Julian Oscillation by allowing or disrupting Kelvin wave propagation under different phases of the Quasi-Biennial Oscillation.
Applied linear regression to atmospheric reanalysis data against an MJO index to observe Kelvin wave propagation across the equatorial tropopause.
Conducted historical vertical wind shear analyses and wavenumber-frequency spectrum analyses of lower stratospheric zonal winds across easterly and westerly QBO phases.
Linear regression demonstrated coherent downward-propagating Kelvin waves under easterly shear, whereas westerly shear conditions formed critical layers that suppressed Kelvin wave signals near the tropopause.
Tropopause vertical shear served as the primary source of enhanced MJO variance during easterly QBO phases, driving peak activity from December through February and causing year-to-year variability.
Spectral power shifted from high to low frequency between westerly and easterly QBO phases, aligning with Kelvin waves moving at the phase speed range of the MJO.