Extreme weather anomalies and surface signatures associated with merged Atlantic-African jets during winter

Abstract. The winter-long merging of the African and Atlantic jets is associated with extreme winter weather across the Northern hemisphere. Past studies have shown that merging of the Atlantic and African jets  is linked to weaker Atlantic eddy activity and stronger tropical heating, and is strongly correlated with a negative NAO state. In this study we examine the relation between jet merging and extreme weather, taking care to separate out the effects of the NAO and El Nino, in order to be left with the added influence of Atlantic-African jet merging. Our analysis, considering percentile exceedance and anomaly composites of surface temperature, surface wind and precipitation, identifies distinct weather signatures of merged jet winters, notably affecting the Iberian Peninsula, North Africa, southern Mediterranean, southwest Greenland, and Northern Europe.  Additionally, we analyse the relationship between merged jets and shifts in cyclone track orientation contributing to the observed extreme weather patterns over these regions. Furthermore, once we remove the NAO effect from the merged-jet surface temperature anomaly signal, we find that winter-long jet merging coincides with anomalous warm Arctic, cold Eurasia, and strong El Niño conditions. This weakens the high latitude temperature gradient which affects the midlatitude baroclinicity resulting in the weakening of eddy activity and ultimately leading to persistent jet merging. Thus the African and Atlantic jet merging during winter appears to further align with the theoretical concept that includes a dynamical regime transition of the jet from eddy driven to mixed eddy–thermally driven, consistent with the corresponding weakened mid-latitude eddies and intensified tropical heating.