Predicting Sudden Stratospheric Warming 2018 and Its Climate Impacts With a Multimodel Ensemble

Abstract

Sudden stratospheric warmings (SSWs) are significant source of enhanced subseasonal predictability, but whether this source is untapped in operational models remains an open question. Here we report on the prediction of the SSW on 12 February 2018, its dynamical precursors, and surface climate impacts by an ensemble of dynamical forecast models. The ensemble forecast from 1 February predicted 3 times increased odds of an SSW compared to climatology, although the lead time for SSW prediction varied among individual models. Errors in the forecast location of a Ural high and underestimated magnitude of upward wave activity flux reduced SSW forecast skill. Although the SSW's downward influence was not well forecasted, the observed northern Eurasia cold anomaly following SSW was predicted, albeit with a weaker magnitude, due to persistent tropospheric anomalies. The ensemble forecast from 8 February predicted the SSW, its subsequent downward influence, and a long‐lasting cold anomaly at the surface.

Enhanced extended‐range predictability of the 2018 late‐winter Eurasian cold spell due to the stratosphere

“…We select members that did accurately predict the central date of the SSW (within +/ 3 days) to form an “SSW” cluster, and members that failed altogether to predict an SSW event up until March 6 to form a “no SSW” cluster. In forecasts initialized after February 1, all members capture the SSW (Karpechko et al ., ). Therefore, forecasts initialized on February 1 are used to investigate uncertainties in the stratospheric state and their impact on the troposphere.…”

“…However, it should be kept in mind that neither the presence of enhanced extended‐range predictability (Karpechko et al ., ; Ferranti et al ., ) nor the fact that the event followed the composite mean evolution after SSWs (Ayarzagüena et al ., ) allows for unambiguous conclusions to be drawn on a causal link between the SSW and the cold spell for individual cases such as that of February/March 2018. Arguably, an unambiguous quantification of the role of the stratosphere in extending the range of predictability of a single event is not possible with the use of ensemble forecasts alone.…”

“…The SSW was predicted about 10 days in advance, with some dependence on the modeling system (Karpechko et al ., ), in agreement with the typical predictability limit of SSWs (Taguchi, ; Tripathi et al ., ; Karpechko, ). Ferranti et al .…”

“…Also, Karpechko et al . () showed that forecasts initialized on February 8 (whose ensemble members all predicted the SSW) predicted an enhanced likelihood of low temperatures over northern Eurasia in late February/early March. The evolution of the NAO phase and the cold spell following the SSW are remarkably similar to the composite mean flow evolution following SSW events.…”

Increased Arctic influence on the midlatitude flow during Scandinavian Blocking episodes

“…This period contains the Year of Polar Prediction (YOPP) First Special Observing Period (SOP1), February–March 2018, during which many of the Arctic radiosonde stations doubled or tripled the frequency of radiosonde launches (Goessling et al ., ). Apart from the Sudden Stratospheric Warming event which occurred around mid‐February (Karpechko et al ., ) this season does not stand out from a synoptic point of view. Indeed, the distribution of Euro‐Atlantic regimes is quite close to climatology (not shown).…”