Synoptic View of the North Atlantic Oscillation

Abstract

This article investigates the synoptic characteristics of individual North Atlantic Oscillation (NAO) events by examining the daily evolution of the potential temperature field on the nominal tropopause (the 2-PVU surface). This quantity is obtained from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis dataset for the winter season. For both phases, the NAO is found to originate from synoptic-scale waves. As these waves evolve into the low-frequency NAO pattern, they break anticyclonically for the positive phase and cyclonically for the negative phase. The results of this analysis suggest that it is the remnants of these breaking waves that form the physical entity of the NAO. Throughout the NAO events, for both phases, the NAO is maintained by the successive breaking of upstream synoptic-scale waves. When synoptic-scale disturbances are no longer present, mixing processes play an important role in the NAO decay. As in other recent studies of the NAO, it is found that these individual NAO events complete their life cycle in a time period of about two weeks. Additional differences between the wave breaking characteristics of the two NAO phases are found. For the positive NAO phase, anticyclonic wave breaking takes place in two regions: one over the North Atlantic and the other near the North American west coast. For the negative NAO phase, on the other hand, there is a single breaking wave confined to the North Atlantic. An explanation based on kinematics is given to account for this difference.

Dynamics of Eddy-Driven Low-Frequency Dipole Modes. Part II: Free Mode Characteristics of NAO and Diagnostic Study

“…3c). This implicates that negative-phase NAO events observed in the real atmosphere looks like a meandering blocking flow (Berggren et al 1949;Benedict et al 2004;LLW).…”

“…In a previous paper (Luo et al 2007, hereafter LLW), a weakly nonlinear model is proposed to account for the interaction between preexisting planetary-scale and synoptic-scale waves leads to an NAO event, a zonally localized dipole mode (Hurrell 1995b;Vallis et al 2004;Cash et al 2005). Results from this theoretical model are found to bear a striking resemblance to observed NAO events for the negative and positive phases (Benedict et al 2004). It is shown that the phase of the NAO event depends strongly upon whether the preexisting planetary waves and preexisting eddy forcing (PEF) from synoptic-scale waves have a matching spatial structure.…”

“…But for the NAO ϩ phase, the q T ( T ) scatter diagrams do not show such a characteristic. To further confirm the above ideas, a diagnostic composite study of observed NAO events and its diagnostic study are performed based upon the daily NAO index by Benedict et al (2004).…”

“…A comparison between observed NAO and blocking events indicated that the negative-phase NAO event is actually identical to blocking events in the Atlantic sector because there is a large similarity between the negative-phase NAO and blocking events in many aspects (period, pattern, and driving mechanism) (Berggren et al 1949;Benedict et al 2004;LLW). Thus, it was concluded that the negative-phase of the NAO event (NAO Ϫ ) is a free mode and could be described by the weakly nonlinear model in LLW in that the blocking events can exhibit local free-mode dynamics (Butchart et al 1989;Ek and Swaters 1994).…”

“…In section 3, the scatter diagrams of the total streamfunction against its corresponding PV are presented to theoretically verify that the steepening (shoaling) and shoaling (steepening) of the scatter diagrams during the life cycle of the NAO Ϫ (NAO ϩ ) events are strongly associated with upstream synopticscale eddies. In section 4, we present a diagnostic study of the scatter diagrams of streamfunction versus PV using two composite fields of NAO Ϫ and NAO ϩ events using the daily NAO index of Benedict et al (2004). In section 5, the scatter diagrams of the streamfunction versus PV for unfiltered fields of two NAO cases are computed.…”