The Influence of the Quasi-Biennial Oscillation on the North Pacific and El-Nino teleconnections

Reanalysis and model data are used to study El-Nino teleconnections in the North Pacific during boreal winter. El-Nino events appear to have a significantly stronger teleconnection when the equatorial stratospheric Quasi-Biennial Oscillation (QBO) is in its westerly phase relative to its easterly phase at 70hPa in both the reanalysis record and in the Whole Atmosphere Community Climate Model (WACCM). The difference is much larger in the reanalysis, however. Composites of easterly QBO at 70hPa show an anomalous ridge over the North Pacific, which explains part of the weakening of El-Nino teleconnections. Associated with this ridge are easterly anomalies at the climatological subtropical jet position and westerly anomalies in the deep tropics. A shallow water model linearized about such zonal wind anomalies gives a weaker extratropical response to a deep tropical vorticity anomaly, suggesting a dynamical explanation of how the QBO can influence the extratropical response to anomalous ENSO convection. These results suggest that modification of wave propagation is likely an important cause of the observed differences in El-Nino teleconnections between easterly and westerly QBO phases, but sampling variability and differences in underlying tropical convection are also likely contributors.