Dynamics of eddy-driven low-frequency dipole modes. Part IV: Planetary and synoptic wave-breaking processes during the NAO life cycle

Based on a highly idealized, analytical solution of the North Atlantic Oscillation (NAO) derived in Part III of this series, it is shown that wave breaking is not a necessary condition for the occurrence of NAO events. The breaking of synoptic waves can arise from the interaction between planetary and synoptic waves that gives rise to NAO events, and the type of wave breaking is dominated by the initial conditions of the two waves that determine the phase of the NAO. The planetary wave breaking (PWB) seems to be attributed to an amplification of the NAO amplitude. It is further found that both the planetary wave breaking and the cyclonic (anticyclonic) breaking of synoptic waves undergo an in-phase (out phase) evolution during the life cycles of negative (positive) phase NAO, or NAO^- (NAO⁺), events. An interesting result found is that for NAO^- (NAO⁺) events the breaking of synoptic waves is enhanced (weakened) during the growing phase, but is weakened (enhanced) during the decaying phase. In the absence of a topographic planetary wave (TPW), PWB occurs mainly in the midlatitude regions of the Atlantic basin for NAO^- events, but is concentrated in subtropical and subpolar regions for NAO⁺ events. However, once the TPW is involved, the reversed planetary-scale potential vorticity (PV) gradient that characterizes the PWB exhibits a southwest-northeast (southeast-northwest) tilted tripole for NAO^- (NAO⁺) events, in agreement with the diagnostic results presented herein. The PWB in the subtropical Atlantic is found to occur more frequently for NAO⁺ events than for NAO^- events because the weaker subtropical mean flow is more likely to emerge during the NAO⁺ life cycle. In conclusion, the results of the highly idealized model used here appear to show that the PWB, synoptic wave breaking, and meridional shift of the westerly jet may be different descriptions of the NAO phenomenon.