Synoptic View of the North Atlantic Oscillation

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.