A Predictability Study on the Role of Extratropical Transition of Tropical Cyclone Events on the Weak Stratospheric Vortex of November 1996

This predictability study examines the role of the extratropical transition (ET) of tropical cyclones (TCs) Lili in the North Atlantic and Dale in the North Pacific on stratospheric forecast skill in the period leading to a November 1996 weak stratospheric vortex event. The weak vortex event began with a short-lived deceleration of the 10-hPa zonal mean wind initiated in the period after the ET of TC Lili. Approximately two weeks later, a period of upward wave activity flux into the stratosphere followed the ET of TC Dale and resulted in the reversal of the 10-hPa zonal mean winds at 65˚N from westerly to easterly on 21 November 1996. This investigation will examine the hypothesis that predictive skill of the ensemble forecast of the stratosphere is coupled to a model’s ability to resolve or skillfully forecast the extratropical reintensification and the tropospheric blocking associated with the ET events that were tied the tropospere-stratosphere wave coupling.

            Using operational ensemble reforecast data from the Subseasonal-to-Seasonal (S2S) Prediction Project dataset, this investigation focuses on the thermodynamic and dynamic processes associated with both the extratropical reintensification of Lili and Dale after ET as well as the tropospheric blocking that developed in association with the ET respective processes. The analysis will discuss the underlying sources of predictability that influence forecasts of the stratosphere and the role of the stratosphere in S2S forecasts. When anomalous stratospheric flow is skillfully predicted it can provide a window-of-opportunity for increased skill in S2S forecasts, the window-of-opportunity associated with this event will be discussed.