The positive North Atlantic oscillation with downstream blocking and middle east snowstorms: The large-scale environment

In this study, the atmospheric conditions for the December 2013 Middle East snowstorm are examined from a case study perspective and by performing a composite analysis of extreme winter events from 1950 to 2013 using reanalysis data. It is revealed that this snowstorm arises from the occurrence of an omega (Ω)-type European blocking (EB) with a strong downstream trough that is associated with a southward-displaced positive-phase North Atlantic Oscillation (NAO ⁺ ) event. In the anomaly field, the EB exhibits a northeast-southwest (NE-SW)-tilted dipole structure. The ω-type EB transports cold air into the Middle East and produces snowfall within the trough over the Middle East. The composite analysis shows that the location of cold temperatures depends strongly on the tilting direction and strength of the EB dipole anomaly. The NE-SW[northwest-southeast (NW-SE)]-tilted EB dipole occurs with a southward (northward)-displaced NAO ⁺ event. TheNE-SW-tilted EBdipole anomaly is associated with an arching-type low-frequency wave train that spans the North Atlantic, Europe, and the Middle East. This tilting has the most favorable structure for cold air outbreaks over the Middle East and southeastern Europe because this tilting leads to an intense downstream trough over this region. In contrast, a NW-SE-tilted EB dipole anomaly leads to cold temperatures over northwestern Africa and southwestern Europe. The analyses herein also suggest that a strong jet over the North Atlantic may be a precursor for a southward-displaced NAO ⁺ event that is usually associated with an Ω-type EB with a NE-SW-tilted dipole in the anomaly height field that favors a cold air outbreak over the Middle East.