Opposing effects of reflective and non–reflective planetary wave breaking on the NAO

Upper tropospheric planetary wave breaking (PWB) over the North Atlantic sector is observed over 45 winters (Dec 1958 - Mar 2003) using NCEP-NCAR reanalysis data. PWB is manifested in the large-scale and rapid irreversible overturning of potential vorticity (PV) contours on isentropic surfaces in the upper troposphere. PWB occurs as wave trains propagate across the North American continent and equatorward over the subtropical Atlantic where the background zonal flow is weak. While linear theory predicts that the equatorward directed wave activity flux will be absorbed, nonlinear critical layer dynamics predicts the wave activity will be reflected poleward away from the critical layer.

We find that while the occurrence of anticyclonic PWB is associated with the emergence of the positive state of the NAO, significant differences arise in the ensuing days between reflective and absorptive (non-reflective) PWB events. In all cases of PWB low-PV air is advected poleward toward the Azores, while high-PV air is advected equatorward into the subtropics. Unique to PWB over the Atlantic, anomalously high PV is concurrently observed near the Icelandic low. This allows for the formation of a meridional tripole stretching from the subtropics to the subpolar region, of which the northern two nodes characterize the NAO. Although the subtropical origin of wave breaking does not directly impact the subpolar node of the NAO, PWB appears to excite this mode of variability.

Following the break two distinct circulation patterns arise in response to reflective and non-reflective events. For reflective events (20% of all PWB) anomalies over the North Atlantic are rapidly translated into poleward propagating wave-like perturbations over the Eurasian continent in the ensuing days. Eddy geopotential height fields closely follow the wave activity flux vectors over the eastern North Atlantic and western Europe, indicative of an export of wave activity out of the Atlantic basin. Wave activity diagnostics imply a weakening of both the subpolar (wave sink) and mid-latitude (wave source) anomalies over the North Atlantic which lead to the rapid decay of the NAO. Wave-mean flow diagnostics provide further support as they suggest reflective PWB shifts the jet equatorward over the Atlantic.

Non-reflective events do not show evidence of wave propagation outside of the Atlantic basin. In the days following breaking, anomalies in the dynamical fields amplify and remain fixed as a meridional dipole, akin to the NAO. Diagnostic fields exhibit an accumulation of wave activity over the eastern subtropical Atlantic (wave sink) and a divergence over the subpolar North Atlantic (wave source). Consequently, the NAO strengthens following non-reflective PWB and remains in a positive state up to eight days later. Wave-mean flow diagnostics suggest that non-reflective events induce a poleward shift in the jet, in agreement with the enhancement of the NAO. We additionally note that the continued equatorward flux following non-reflective PWB predisposes the Atlantic basin to further PWB. Observations hint that this interaction between the NAO and non-reflective PWB may form a positive feedback enabling the maintenance of the NAO over the course of a winter season.