Stratosphere–troposphere coupling enhances subseasonal predictability of Northern Eurasian cold spells

Abstract

AbstractHere we explore the stratospheric influence on the predictability of Eurasian cold‐spell events using the European Centre for Medium‐Range Weather Forecasts (ECMWF) ensemble hindcasts obtained from the Subseasonal‐to‐Seasonal (S2S) archive. To isolate the stratospheric influence, we subsampled two groups of hindcasts according to the strength of the stratospheric polar vortex preceding the cold spells at the surface. The predicted probability of cold spells, defined as the lowest 10th percentile of weekly mean temperature anomalies over northern Eurasia (10° W–130° E and 50° N–65° N), is systematically higher, by 0.05–0.2, at lags 7–24 days in the forecasts initialized during the weak stratospheric vortex compared to the strong stratospheric vortex group, extending the predictability of cold spells by 3–5 days. Our results suggest that, in the case of the weak polar vortex, stratosphere–troposphere coupling favors the negative Northern Annular Mode (NAM) regime and the cold‐air outbreaks in Eurasia. As a consequence, the long stratospheric predictability extends the predictability of the cold spells. On the other hand, when the polar vortex is strong, the stratospheric anomalies do not favor the observed negative NAM regime, which thus results from the internal tropospheric processes only. In this case the predictability of cold‐air spells is limited. Furthermore, we show that the extended predictability of cold spells arising from the stratosphere–troposphere coupling is captured by a simple statistical model, suggesting that governing large‐scale dynamics behave effectively linearly over some limited periods. Quantified contribution of the stratosphere–troposphere coupling to the enhanced skill of the extended‐range cold‐spell forecasts documented in our paper may prove useful in the development of forecasting tools.