Predictability of the stratospheric polar vortex breakdown: An ensemble reforecast experiment for the splitting event in January 2009

Abstract

A series of ensemble reforecast experiments is conducted to investigate the predictability and the occurrence mechanism of a stratospheric sudden warming occurred in late January 2009, which is a typical polar vortex splitting event. To fully examine the rapid vortex splitting evolution and predictability variation, ensemble forecasts are carried out every day during January 2009. The vortex splitting event is reliably predicted by forecasts initialized after 6 days prior to the vortex breakup. It is also found that the propagating property of planetary waves within the stratosphere is a key to the successful prediction for the vortex splitting event. Planetary waves incoming from the troposphere are reflected back into the troposphere for failed forecasts, whereas they are absorbed within the stratosphere for succeeded forecasts. Composite analysis reveals the following reflection process of planetary waves for the failed forecast: Upward propagation of planetary wave activity from a tropospheric blocking over Alaska is weaker during initial prediction periods; then, the deceleration of the zonal wind in the upper stratosphere becomes weaker over Europe, which produces a preferable condition for the wave reflection; hence, subsequently incoming wave activity from the troposphere over Europe is reflected back over the Siberia inducing the eastward phase tilt of planetary waves, which shuts down the further upward propagation of planetary waves leading to the vortex splitting. Thus, this study shows that the stratospheric condition would be another important control factor for the occurrence of the vortex splitting event, besides anomalous tropospheric circulations enforcing upward propagation of planetary waves.It is also well known that SSWs are classified into the following two types based on the synoptic structure of the stratospheric circulation: "vortex displacement" type characterized by a shift of the polar vortex off the pole and "vortex splitting" type in which the polar vortex breaks up into two pieces [Charlton and Polvani, 2007]. It has been also revealed by several studies based on the composite of reanalysis data sets that the NOGUCHI ET AL.PREDICTABILITY OF VORTEX SPLITTING EVENT 3388

Comparison of Subseasonal‐to‐Seasonal Model Forecasts for Major Stratospheric Sudden Warmings

“…Marshall and Scaife (2010) suggested the importance of the downward propagating SSW signals in the upper stratosphere when they explained improved SSW predictability in a high-top version of a general circulation model compared to its low-top version. The possible importance of the upper stratosphere was also pointed out by Noguchi et al (2016). Conducting ensemble HC experiments for the 2009 MSSW, Noguchi et al (2016) argued that the upper stratospheric condition just before the event plays a key role in controlling the propagating property of wave activity from the troposphere.…”

“…The possible importance of the upper stratosphere was also pointed out by Noguchi et al (2016). Conducting ensemble HC experiments for the 2009 MSSW, Noguchi et al (2016) argued that the upper stratospheric condition just before the event plays a key role in controlling the propagating property of wave activity from the troposphere.…”

The Role of the Stratosphere in Subseasonal to Seasonal Prediction: 1. Predictability of the Stratosphere

“…In a common classification, there are two major types of midwinter SSW events: (1) "split" events, for which the polar vortex splits into two separate vortices, and (2) "displacement" events, for which the polar vortex is distorted and displaced off the pole (e.g., Charlton & Polvani, 2007). Taguchi (2018) provides an analysis of the predictability in the S2S hindcasts of five SSW events (December 1998, December 2001, January 2009, January 2013in the NH, and September 2002, showing that the vortex split SSWs (i.e., 2002SSWs (i.e., , 2009SSWs (i.e., , and 2013 were more difficult to forecast than the displacements (1998 and 2001). Here, we extend that analysis by considering the predictability of 11 NH midwinter SSW events in ERA-Interim during the 1996-2010 period.…”

Does Extreme El Niño Have a Different Effect on the Stratosphere in Boreal Winter Than Its Moderate Counterpart?

“…This would imply that vortex variability on monthly and seasonal time scales is extremely difficult to predict, at least in a deterministic sense. Evidence for such bifurcations in forecast models has been found (Noguchi et al, 2016). Although El Niño can help enhance stratospheric predictability in a statistical sense (Domeisen et al, 2015;Sigmond et al, 2013), anomalies of stratospheric vortex cannot be assumed to be forced by that winter El Niño (Garfinkel, Hurwitz, Oman, & Waugh, 2013).…”