Quasi-stationary Rossby wave breaking (PWB) over the subtropical North Atlantic is observed over 45 winters (Dec 1958 - Mar 2003) using NCEP/NCAR reanalysis data. PWB is manifested in the rapid, large-scale and irreversible overturning of potential vorticity (PV) contours on isentropic surfaces in the subtropical upper troposphere. As breaking occurs over the subtropical North Atlantic, an upper-tropospheric PV tripole anomaly forms with nodes over the subtropical, midlatitude and subpolar North Atlantic. The northern two nodes of this tripole are very similar to the spatial structure of the North Atlantic Oscillation (NAO), with positive polarity.

Nonlinear reflection is identified in approximately a quarter of all PWB events. Following breaking, two distinct circulation regimes arise, one in response to reflective events and the other in response to non-reflective events.

For reflective events, anomalies over the North Atlantic rapidly propagate away from the breaking region along a poleward arching wave train over the Eurasian continent. The quasi-stationary wave activity flux indicates that wave activity is exported out of the Atlantic basin. At the same time, the regional poleward eddy momentum flux goes through a sign reversal, as does the polarity of the NAO, from positive to a negative NAO.

For non-reflective events, the dipole anomaly over the North Atlantic amplifies. Diagnostics for non-reflective events suggest that wave activity over the Azores gets absorbed, allowing continued enhancement of both the regional poleward eddy momentum flux and the positive NAO.

While the flow field over the North Atlantic is crucial for allowing wave breaking, the non-reflective (reflective) outcome of PWB has a positive (negative) feedback on the flow field and the NAO. Thus, a dynamical mechanism that is internal to the atmosphere has been identified as contributing to interannual NAO variability. The importance of this mechanism will be further discussed.