Abstract
The dipole structure of the North Atlantic Oscillation (NAO) is examined in this study by defining the tilt of the NAO dipole centers on synoptic time scales. All the positive NAO phase (NAO+) and negative NAO phase (NAO−) events are divided into three tilting types according to their definition; namely, northeast–southwest (NE–SW), north–south symmetric (N–S, not tilted), and northwest–southeast (NW–SE) tilting NAO events. Then, the associated surface air temperature (SAT), geopotential height, zonal wind, and SST (surface sea temperature) anomalies of each type are examined. It is found that, for different asymmetric NAO tilt types, the local SATs exhibit significantly different distributions. The zonal wind has a good match with the NAO dipole tilt, which also includes the positive feedback of the NAO circulation. The basic zonal flow that removes the NAO days also exhibits a clear tilt structure that favors the tilt of the NAO dipole. Moreover, it is found that the Atlantic Multidecadal Oscillation (AMO) may be an important factor affecting the tilt of the NAO dipole. The AMO index has a significant 15-year lead for the NAO index and basic zonal flow index, with a high correlation coefficient, which might be seen as a precondition that indicates the tilt of the NAO events, especially on decadal or multidecadal time scales. However, the physical mechanisms and processes are still not fully understood.
Highlights
The North Atlantic Oscillation (NAO) is the most dominant low-frequency mode in the NorthernHemisphere in winter [1,2,3] and has a significant impact on regional or even hemispheric-scale weather and climate [4,5,6,7,8]
In this study, according to the climatic characteristics of the NAO dipole mode and the tilt characteristics of the dipole centers, NAO events are divided into NE–SW, N–S and NW–SE tilting
The spatial asymmetric quadrupole structure of the surface air temperature (SAT) anomaly can be observed during the NAO life cycle
Summary
The North Atlantic Oscillation (NAO) is the most dominant low-frequency mode in the Northern. Some studies have found that the dipole structure of the NAO and blocking is very important for midlatitude extreme cold weather, storms and rainfall, especially on synoptic time scales [5,18,19,20]. A specific tilt direction can open up a critical path for the transport of water vapor and cold advection, as indicated by Yao et al [5] In their studies, the blocking events are classified into several categories. Atmosphere 2019, 10, 781 based on the definition of the Europe blocking dipole tilting direction They did not examine the titling characteristic of the NAO dipole pattern. All NAO events during the winters of 1950–2011 are examined and defined based on their tilting direction and weather impact, and their physical mechanisms are investigated.
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