A New Rossby Wave–Breaking Interpretation of the North Atlantic Oscillation

TLDR

The North Atlantic Oscillation’s low‑frequency variability is linked to upper‑level Rossby wave‑breaking events that produce high‑latitude blocking episodes. The study proposes that the NAO’s low‑frequency variability results from changes in the frequency of upper‑level Rossby wave‑breaking over the North Atlantic. Using a two‑dimensional potential‑vorticity index applied to 40‑year ECMWF ERA‑40 winter data, the authors identify wave‑breaking events and link them to NAO phases, also noting dynamical precursors that raise breaking likelihood. The results indicate that variability in the tropical Pacific and the stratosphere can influence NAO through their impact on wave‑breaking activity.

Abstract

Abstract This paper proposes the hypothesis that the low-frequency variability of the North Atlantic Oscillation (NAO) arises as a result of variations in the occurrence of upper-level Rossby wave–breaking events over the North Atlantic. These events lead to synoptic situations similar to midlatitude blocking that are referred to as high-latitude blocking episodes. A positive NAO is envisaged as being a description of periods in which these episodes are infrequent and can be considered as a basic, unblocked situation. A negative NAO is a description of periods in which episodes occur frequently. A similar, but weaker, relationship exists between wave breaking over the Pacific and the west Pacific pattern. Evidence is given to support this hypothesis by using a two-dimensional potential-vorticity-based index to identify wave breaking at various latitudes. This is applied to Northern Hemisphere winter data from the 40-yr ECMWF Re-Analysis (ERA-40), and the events identified are then related to the NAO. Certain dynamical precursors are identified that appear to increase the likelihood of wave breaking. These suggest mechanisms by which variability in the tropical Pacific, and in the stratosphere, could affect the NAO.