Effects of the quasi‐biennial oscillation on low‐latitude transport in the stratosphere derived from trajectory calculations

Abstract

The quasi‐biennial oscillation (QBO) of stratospheric zonal winds induces a secondary meridional circulation (SMC) consisting of QBO variations in meridional and vertical winds. In this work, we investigate how these instantaneous meridional circulation anomalies add over time to variations of stratospheric transport. To that end, we compute backward parcel trajectories on the basis of the output of a chemistry‐climate model (CCM). At the equator, the trajectories show the strongest vertical parcel displacement over a seasonal timescale when the QBO progresses toward easterly phase in the middle stratosphere. During the solstitial seasons a large number of parcels come from the summer hemisphere, causing in addition a QBO variation in the spread of the total ascent among equatorial parcels. A QBO effect on meridional transport is diagnosed from PV gradients during summer in the easterly phase of the QBO, which suggests a variation of the tropical‐subtropical barrier strength. Analyses of the parcel trajectories and CCM trace gas distributions confirm this finding. We suggest that this variation is due to the combined effects of QBO and annual variation in meridional advection and in wave‐induced eddy transport.