Séminaire
El Niño-Southern Oscillation (ENSO) is arguably one of the strongest internal drivers of global weather variability.
While the North Atlantic was traditionally assumed to be one of the regions which are only marginally affected by ENSO, there is a renewed interested in how ENSO affects the North Atlantic circulation on the scale of individual weather systems. I present an overview of our recent research on variability in extratropical cyclogenesis downstream of the Rocky Mountains, over the Gulf Stream and at Greenland and how ENSO-related large-scale circulation anomalies affect cyclone formation in these regions. Surprisingly, there is a significant relationship between extratropical cyclogenesis over the Gulf Stream and leeward of the Rocky Mountains. During La Niña winters, the more active polar jet creates conditions amenable for cyclogenesis leeward of the Rocky Mountains chain. The more active polar jet is a consequence of stationary wave propagation from tropical sources and the stronger La Niña ridge in the northeast Pacific. The downstream propagation of equatorward oriented upper-level eddies, which are reinforced by the polar jet, cause cyclogenesis-friendly conditions at Greenland by pushing the jet poleward. The eddies cause an overall positive NAO response to La Niña as described in the previous literature. During El Niño winters, cyclogenesis occurs more frequently over the Gulf Stream, as a consequence of a more extended Pacific jet. There is an interesting difference between central Pacific and eastern Pacific El Niño-variantes. During the former, Gulf Stream cyclogenesis occurs below the jet exit region, during the latter it preferentially occurs below the jet’s entrance region. Accordingly, during central Pacific El Niño winters, the storm track anchor over the Gulf Stream shifts poleward.