Winter-long Atlantic-African jet merging and the effect on extreme weather

Variability in the jet stream can have a significant influence on the distribution of extreme weather, with winter-long anomalies leading to extreme seasons. During winter of 2009/2010, the Atlantic and African jets were anomalously merged for most of the winter, resulting in a persistent zonally oriented single jet. At the same time, intense and prolonged negative phase of the North Atlantic Oscillation (NAO) and unusually cold and extreme weather conditions were reported over the Northern Hemisphere. Such a merging was only observed to occur for a whole winter during the winters of 1968-69 and 1969-70. Preliminary results indicate that such persistent winter merged jets could be more frequent in a future global warming scenario and thus it is important to understand this rare dynamical jet state and its effects on the weather patterns. In this study, we explore the associated distributions of extreme weather during merged-jet winter months, in comparison to the distribution of extremes during negative NAO months. We show that merged jet winter months have a signature weather pattern distribution that is different from the negative NAO phase, affecting Northern Africa, Europe and south-west Greenland, which is associated with synoptic storms tending to either propagate zonally towards the Mediterranean, with a secondary northward then westward branch propagating to Greenland. We further find a complex relation to Arctic-Eurasian temperature anomalies. Once we remove the NAO-related high-latitude cold surface temperature anomalies, we find that Atlantic-African jet merging, especially the winter-long persistent ones, occur during winters with unusually warm Arctic and cold Eurasia. This is consistent with the theory that winter-long jet merging represents a dynamical regime transition to a merged jet, which is more likely to occur when mid-latitude baroclinicity is weaker.