Stratospheric Connection to the Abrupt End of the 2016/2017 Iberian Drought

Abstract

Southwestern Europe experienced extraordinary rainy and windy conditions in March 2018, leading to the end of the most severe drought since 1970 at continental scale. This anomalous weather was linked to a persistent negative North Atlantic Oscillation pattern. Two weeks earlier a sudden stratospheric warming (SSW) took place, preceded by the strongest planetary wave activity on record. In this study, we explore the connection between the SSW and the weather shift by employing a weather regime approach and flow analogues. The timing of the downward propagation of the stratospheric anomalies, the transition to and persistence of the negative North Atlantic Oscillation weather regime, and the sudden precipitation increase are all consistent with the typical tropospheric state after SSWs. Our results evidence a significant role of the 2018 SSW in the record‐breaking precipitation event.

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

“…Ayarzagüena et al . () showed that there was a detectable downward Northern Annular Mode signal of this SSW to the troposphere in the reanalysis data.…”

“…While Ayarzagüena et al . () did not specifically focus on the cold spell itself, but rather on the extreme precipitation event in the Iberian Peninsula, they concluded that the cold spell was likely triggered by the SSW. Due to the similarity to the composite mean behaviour, the cold spell was also indicated purely based on statistical models (Cohen et al ., ).…”

“…Ayarzagüena et al . () described the event as being close to the “canonical” behaviour expected after SSWs. While Ayarzagüena et al .…”

“…The cold spell was associated with a shift of the NAO towards its negative phase at the end of February (see Figure ). Preceding the negative NAO (henceforth NAO) phase, persistent Scandinavian blocking was observed during the second half of February (Figure a; see also Ayarzagüena et al ., ; Ferranti et al ., ). Moreover, in mid‐February 2018, a major SSW was observed (as shown by the reversal of the 10‐hPa zonal wind at 60°N in Figure a), as the polar vortex split into two separate sub‐vortices (known as a splitting event, in contrast to a displacement event).…”

Observed Relationships Between Sudden Stratospheric Warmings and European Climate Extremes

“…In some cases these SSW events and associated blocking have been linked with extreme cold temperatures and heavy snowfall in parts of Europe, such as the February 2018 cold spell dubbed “The Beast from the East” in the United Kingdom (e.g., BBC News, ). Additionally, drought‐breaking precipitation in the Iberian Peninsula was also linked with the February 2018 SSW event (Ayarzagüena, Barriopedro, et al, ).…”