The 2016-2017 Early Season Weak Stratospheric Vortex Event: Precursors and Predictability

The Northern Hemisphere stratosphere experienced a deceleration of the high latitude zonal mean zonal wind in late October 2016. The 10-hPa zonal mean zonal wind remained at or near satellite era minima for approximately 1 month and while the winds did not reverse to easterly at 60˚N, it did reverse at 65˚N in late November 2016. Using data from the ERA-Interim Reanalysis, this analysis first examines two mid-October tropopause-level ridge amplification events that occurred in succession in the North Atlantic and North Pacific basins. Both of these ridging events occurred downstream of tropical cyclones that entered the midlatitude flow and were associated with tropopause-level heat flux anomalies with respect to climatology that exceeded 2.5-sigma level. These tropospheric events were associated with a period of troposphere-stratosphere wave coupling and the amplification of wavenumber two in the stratosphere that led to an early season weak vortex event.

The second part of this analysis considers the predictability aspects of this early season weak vortex event. First, we examine the medium-range forecast skill of the stratospheric zonal mean wind prior to the event in the National Centers for Environmental Prediction (NCEP)-Global Forecast System (GFS) forecasts. The forecast uncertainty at medium-range lead-times was large and tied to the ability to resolve the tropospheric events in the North Atlantic and North Pacific regions. The forecast probability of the first October major sudden stratospheric warming greatly exceeded zero. Finally, using operational ensemble reforecast data from the sub-seasonal–to–seasonal (S2S) Prediction Project dataset, we examine the whether the accepted window-of-opportunity for increased skill in S2S forecasts applied to the early season weak vortex events that occurred in October 2016.