Comment on wcd-2022-14

Abstract

The influence of the quasi-biennial oscillation (QBO) on tropical climate is demonstrated using a 500-yr pre-industrial control simulation of the Met Office Hadley Centre model. Robust precipitation responses to the phase of the QBO are diagnosed in the model which show zonally asymmetric features, consistent with observational studies. The response in precipitation resembles the El Niño-Southern Oscillation (ENSO) impacts, however, regression analysis shows that there is a QBO signal in precipitation that is independent from ENSO. Moreover, the observed uneven frequency of ENSO events for each QBO phase is also found in these simulations, with more El Niño events found under the westerly phase of the QBO (QBOW) and more La Niña events for the easterly phase (QBOE). No evidence is found to suggest that these QBO-ENSO relationships are caused by ENSO modulating the QBO in the simulations. A previously unknown relationship between the QBO and a dipole of precipitation in the Indian Ocean is found in models and observations in boreal fall, characterized by a wetter western Indian Ocean and drier conditions in the eastern part for QBOW and the opposite under QBOE conditions. QBO W-E differences show a stronger East Pacific Inter-tropical Convergence Zone (ITCZ) in boreal winter and a northward shift of the Atlantic ITCZ in boreal spring and summer. The Walker circulation is found to be significantly weaker during QBOW compared to QBOE, explaining the observed and simulated zonally asymmetric responses at equatorial latitudes. Further work, including targeted model experiments, is required to betters understand the mechanisms causing these relationships between the QBO and tropical convection.