Linear interference between effects of ENSO and QBO on the northern winter stratospheric polar vortex

The El Nino-Southern Oscillation (ENSO) and Quasi-Biennial Oscillation (QBO) have an individual impact on the northern winter stratospheric polar vortex. This study considered the joint impact of ENSO and QBO using the reanalysis and observational data. The combinations of ENSO and QBO winds are split into four groups, El Nino-easterly QBO, El Nino-westerly QBO, La Nina-easterly QBO, and La Nina-westerly QBO, and the impact of each joint group is compared with the linear superposition of the individual impacts of ENSO and QBO with the interference from other factors removed. On average, the stratospheric polar vortex exhibits weakening in the pure El Nino and easterly QBO composites and exhibits strengthening in the pure La Nina and westerly QBO composites as compared with the climatology. The observed joint ENSO and QBO signals in the Arctic stratosphere are mostly a linearly superposed combination between them. This interference of QBO and ENSO's impacts is also observed in the tropospheric circulation and tropical convections, which are related to teleconnection in the upper troposphere controlling the amount of planetary waves entering stratosphere to further impact Artic stratospheric polar vortex. Although El Nino and easterly QBO cooperate to weaken the stratospheric polar vortex, and La Nina and westerly QBO cooperate to strengthen, but their impact on the tropospheric North Pacific circulation and tropical rainfall are opposite with signs reversed. As a consequence, the joint effect of El Nino-easterly QBO/La Nina-westerly QBO on the stratosphere is stronger than that of El Nino-westerly QBO/La Nina-easterly QBO. However, the joint impact of El Nino-easterly QBO on the North Pacific troposphere and tropical rainfall is weaker than that of El Nino-westerly QBO. Further analysis on the wave activities and Brewer-Dobson circulation also confirms the linear interference between effects of ENSO and QBO in the stratosphere, although this linearity is less evident in the troposphere.