Natural Tropical Oscillations phase impact on stationary and westward travelling planetary waves

Abstract. We performed a series of numerical experiments to study the main patterns of the Quasi-Biennial Oscillation of the zonal wind in the equatorial stratosphere (QBO) and the El Niño-Southern Oscillation (ENSO) influence on stationary and westward travelling atmospheric planetary waves (PWs) with different zonal wave numbers and periods. The simulation was carried out for boreal winter conditions using the model of the middle and upper atmosphere (MUAM). The results showed that the joint effect of the considered tropical oscillations can significantly, up to tens of percent, change the amplitudes of the PW in the areas of their maxima. Under the El Niño, regardless of the QBO phase, the amplitude maxima of the stationary PW with wave number 1 (SPW1) shift toward high latitudes. The amplitude structure of SPW2 is basically opposite to the SPW1 structure. Increases of the upward wave activity fluxes of quasi- 5-, 10-, 7-day PWs, and the amplitudes of 10- and 7-day PWs are modelled when easterly QBO phase is superimposed on El Niño phase. Conversely, attenuations of the individual PW amplitudes and wave activity fluxes are typically observed under the westerly QBO, as well as under La-Niña/westerly QBO conditions combination in special cases, such as SPW1. The PW study is important due to their significant influence on the middle and upper atmosphere circulation, including the configuration of the stratospheric polar vortex whose deformation can influence the occurrence of extreme weather events, in particular, in the Arctic and Asia-Pacific region during the boreal winter.