The Disruption of QBO Westerlies by Shallow Easterly Jets (Invited Presentation)

In early 2016, the quasi-biennial oscillation was disrupted by a shallow easterly jet that emerged within a deep westerly phase near 40 hPa. This disruption is unprecedented in more than 50 years of observations. We report here on a set of integrations with a dry dynamical core that produces quasi-periodic disruptions of an equatorial westerly jet by shallow easterly winds. Like the observed disruption, the events in the dry dynamical core are driven by extratropical eddy momentum fluxes impinging on the core of the equatorial westerly winds. Further controlled numerical experiments identify a strong positive feedback by which the potential vorticity mixed across the tropics by the eddy forcing produces a shallow region of enhanced potential vorticity gradients in the subtropic, acting as a kind of 'lightning rod' for focusing further wave activity onto developing easterly anomaly. Similar structures are found in the ERA Interim reanalysis, suggesting this same feedback was in part responsible for the observed disruption. This perspective points towards the initial trigger for the event occurring in late November through December of 2015. The implications for seasonal forecasting of this event will be discussed.