Identification

Title

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

Abstract

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.

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document

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https://n2t.org/ark:/85065/d7348mzg

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Dataset language

eng

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geoscientificInformation

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publication

effective date

2016-01-01T00:00:00Z

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publication

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2016-03-01T00:00:00Z

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Copyright 2016 American Meteorological Society (AMS). Permission to use figures, tables, and brief excerpts from this work in scientific and educational works is hereby granted provided that the source is acknowledged. Any use of material in this work that is determined to be "fair use" under Section 107 or that satisfies the conditions specified in Section 108 of the U.S. Copyright Law (17 USC, as revised by P.L. 94-553) does not require the Society's permission. Republication, systematic reproduction, posting in electronic form on servers, or other uses of this material, except as exempted by the above statements, requires written permission or license from the AMS. Additional details are provided in the AMS Copyright Policies, available from the AMS at 617-227-2425 or amspubs@ametsoc.org. Permission to place a copy of this work on this server has been provided by the AMS. The AMS does not guarantee that the copy provided here is an accurate copy of the published work.

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OpenSky Support

organisation name

UCAR/NCAR - Library

full postal address

PO Box 3000

Boulder

80307-3000

email address

opensky@ucar.edu

web address

http://opensky.ucar.edu/

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pointOfContact

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