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

Abstract. The winter-long merging of the African and Atlantic jets during 2009–2010 was associated with extreme winter weather across the Northern Hemisphere. Past studies have shown that merging of these two jets is linked to weaker Atlantic eddy activity and stronger tropical heating, and is strongly correlated with a negative North Atlantic Oscillation (NAO) state. Here, we examine the relationship between jet merging and extreme weather, taking care to separate out the effects of the NAO and El Niño, in order to be left with the added influence of Atlantic–African jet merging. Our analysis, which examines 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, the southern Mediterranean, southwest Greenland, and Northern Europe. Additionally, we analyze 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 anomalously warm Arctic, cold Eurasian, and strong El Niño conditions. The resulting weaker high-latitude temperature gradient is consistent with a weakening of eddy activity and, alongside stronger tropical heating, is thought to ultimately lead to persistent jet merging. This is consistent with previous theoretical work suggesting that reduced midlatitude baroclinicity and stronger tropical heating result in a transition from coexisting thermally driven subtropical and eddy-driven midlatitude jets to one with a single mixed eddy–thermally driven jet. This provides further evidence that Atlantic–African jet merging constitutes a dynamical regime transition of the Atlantic jet.