Understanding the Mechanisms for Tropical Surface Impacts of the Quasi-Biennial Oscillation (QBO)

The impact of the quasi-biennial oscillation (QBO) on tropical convection and precipitation is investigated through nudging experiments using the UK Met Office Hadley Center Unified Model. The model control simulations show robust links between the internally generated QBO and tropical precipitation and circulation. The model zonal wind in the tropical stratosphere was nudged above 90 hPa in atmosphere-only and coupled ocean-atmosphere configurations. The convection and precipitation in the atmosphere-only simulations do not differ between the experiments with and without nudging, which may indicate that SST-convection coupling is needed for any QBO influence on the tropical lower troposphere and surface. In the coupled experiments, the precipitation and sea-surface temperature relationships with the QBO phase disappear when nudging is applied. Imposing a realistic QBO-driven static stability anomaly in the upper-troposphere lower-stratosphere is not sufficient to simulate tropical surface impacts. The nudging reduced the influence of the lower troposphere on the upper branch of the Walker circulation, irrespective of the QBO, indicating that the upper tropospheric zonal circulation has been decoupled from the surface by the nudging. These results suggest that grid-point nudging mutes relevant feedback processes occurring at the tropopause level, including high cloud radiative effects and wave mean flow interactions, which may play a key role in stratospheric-tropospheric coupling. The interaction between the stratosphere and the troposphere is well known to produce surface impacts in the extratropics. However, whether stratosphere-troposphere interactions affect the surface in the tropics associated with the variability of the stratospheric quasi-biennial oscillation (QBO) is yet to be determined because the observational record is too short and tropical tropospheric variability masks any potential signal of the stratosphere. In this paper, we examine hypotheses that explain how the QBO could affect tropical deep convection and tropical precipitation variability through targeted model experiments which prescribe the equatorial stratosphere. Our results indicate that prescribing the zonal wind in the stratosphere remove links between surface precipitation and the QBO. Therefore, nudging the stratosphere decouples the stratospheric mean flow and anomalies from the tropospheric processes, which emphasizes the role of feedbacks.