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

Abstract

Through calculating the scatter diagrams of the streamfunction ( P or T ) versus potential vorticity (PV) (q P or q T ), where P and T are the planetary-scale streamfunction and total streamfunction, respectively, and using a weakly nonlinear NAO model proposed in Part I of this paper, it is suggested that negative-and positive-phase NAO events may approximately correspond to free modes even though driven by synopticscale eddies. In a planetary-scale field, the q P ( P ) scatter diagram of an NAO event exhibits a linear multivalued functional relationship in a narrow region for the negative phase, but exhibits a linear singlevalued functional relationship during the positive phase. It was also found that there is no steepening of the slope of the main straight line in the q P ( P ) scatter diagrams for two phases of the NAO event. Instead, the slope of the straight line in the scatterplots is time independent throughout the life cycle of the NAO event.However, when synoptic-scale eddies are included in the streamfunction field, the q T ( T ) scatter diagram of the negative-phase NAO event shows a trend toward steepening during the intensification phase, and this tendency reverses during the decay phase. During the positive NAO phase the slope of the q t ( T ) scatter diagram shoals during the intensification phase and then steepens during the decay phase. Thus, it appears that the steepening and shoaling of the scatter diagrams of the streamfunction versus PV for the negativeand positive-phase NAO events are attributed to the effect of synoptic-scale eddies that force NAO events to form. Diagnostic studies using both composite and unfiltered fields of observed NAO events are presented to confirm these conclusions.

Dynamics of Eddy-Driven Low-Frequency Dipole Modes. Part III: Meridional Displacement of Westerly Jet Anomalies during Two Phases of NAO

“…In this paper, to theoretically explain the north-south shift of the zonal mean wind anomalies associated with the NAO, we will still use an equivalent barotropic vorticity equation with bottom topography as in Luo (2005) in that the NAO has generally an equivalent barotropic structure in the troposphere (Wallace 2000;Vallis et al 2004;Gerber and Vallis 2005;Wittman et al 2005).…”

“…In a recent theoretical study by Luo et al (2007a), the phase of the eddy-driven NAO is found to depend upon the spatial structures of preexisting planetary-and synoptic-scale waves, implying that the NAO is indeed a nonlinear initial-value problem (Benedict et al 2004). However, the analytical solutions in this model cannot be directly used to interpret the north-south shift of the westerly jet associated with the phase of NAO although synoptic-scale eddies are found to maintain the zonal mean wind anomalies through exciting the NAO anomaly.…”

“…This is because the climatological stationary waves induced by the LSC topography are excluded in this theoretical model. In this paper, the theoretical model established by Luo et al (2007a) is directly extended to include climatological stationary waves induced by the LSC topography in order to clarify why the meridional shift of the westerly jet anomaly takes place when the NAO is in either the positive or negative phase.…”

“…In section 2, the composite of zonal mean westerly wind at 300 mb and the corresponding wintertime mean barotropic streamfunction anomalies for two phases of the NAO are presented. In section 3, the theoretical NAO model by Luo et al (2007a) is directly extended to include the climatological stationary wave induced by the LSC topography and analytical solutions are presented. The relationship between the NAO-like events and storm track is described in section 4.…”

“…Since the action centers of the NAO anomalies have been recognized to be situated in the region from the western to central Atlantic upstream of the European continent (Wallace 2000), it is inevitable to conclude that the NAO anomalies are located upstream of the climatological stationary waves. Based upon this observational evidence, in the next section we will attempt to establish a weakly nonlinear model to provide a likely explanation as to why the westerly jet anomalies undergo a north-south shift during two phases of the NAO by extending the studies by Luo (2005) and Luo and Chen (2006).…”