The Positive North Atlantic Oscillation with downstream blocking and Middle East snowstorms: Impacts of the North Atlantic Jet

• Published in: Journal of Climate
• Other Authors: Yao, Yao (author), Luo, Dehai (author), Dai, Aiguo (author), Feldstein, Steven (author)
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Meteorological Society 2016
• Subjects: Tilting; North Atlantic; North Atlantic oscillation
• Online Access: http://nldr.library.ucar.edu/repository/collections/OSGC-000-000-022-698; https://doi.org/10.1175/JCLI-D-15-0350.1

A recent study revealed that cold winter outbreaks over the Middle East and southeastern Europe are caused mainly by the northeast-southwest (NE-SW) tilting of European blocking (EB) associated with the positive-phase North Atlantic Oscillation (NAO+). Here, the North Atlantic conditions are examined that determine the EB tilting direction, defined as being perpendicular to the dipole anomaly orientation. Using daily reanalysis data, the NAO+ events are classified into strong (SJN) and weak (WJN) North Atlantic jet types. A composite analysis shows that the EB is generally stronger and located more westward and southward during SJN events than during WJN events. During SJN events, the NAO+ and EB dipoles exhibit NE-SW tilting, which leads to strong cold advection and large negative temperature anomalies over the Middle East and southeastern Europe. In contrast, northwest-southeast (NW-SE) tilting without strong negative temperature anomalies over the Middle East is seen during WJN events. A nonlinear multiscale interaction model is modified to investigate the physical mechanism through which the North Atlantic jet (NAJ) affects EB with the NAO+ event. It is shown that, when the NAJ is stronger, an amplified EB event forms because of enhanced NAO+ energy dispersion. For a strong (weak) NAJ, the EB tends to occur in a relatively low-latitude (high latitude) region because of the suppressive (favorable) role of intensified (reduced) zonal wind in high latitudes. It exhibits NE–SW (NW-SE) tilting because the blocking region corresponds to negative-over-positive (opposite) zonal wind anomalies. The results suggest that the NAJ can modulate the tilting direction of EB, leading to different effects over the Middle East.