Observations of planetary wave breaking and nonlinear reflection

Large-scale atmospheric circulation regimes across much of the globe are determined by quasi-stationary Rossby waves. Rossby waves propagate linearly in the presence of a westerly jet and a strong latitudinal gradient of potential vorticity (PV). However, as a Rossby wave train propagates toward lower latitudes and encounters weak ambient winds, the flow becomes highly nonlinear and wave breaking may occur. Planetary wave breaking (PWB) is defined as the rapid, large-scale and irreversible quasi-horizontal overturning of PV contours on isentropic surfaces. Among the implications of PWB is the possibility of nonlinear reflection following the breaking event. While identified in analytical and modeling studies, the first observational evidence of nonlinear reflection is presented.; PWB is found to be a ubiquitous feature year-round in the upper troposphere and in the stratospheric winter hemisphere. Daily reanalysis datasets are used to identify PWB in two main regions: the subtropical upper troposphere, and along the periphery of the stratospheric vortex. The subtropical jet and wave forcing are found to play key roles in determining both the annual cycle as well as the interannual/intraseasonal variability of tropospheric PWB.; Nonlinear reflection is identified in a subset of all PWB events. In the troposphere, reflection is characterized by a poleward arching wave train downstream of the breaking region and back into midlatitudes. The absorptive-reflective outcome of wintertime PWB over the North Atlantic basin strongly impacts the intraseasonal North Atlantic Oscillation (NAO). For non-reflective events, the absorption of wave activity amplifies the regional poleward eddy momentum flux and the positive phase of the NAO. Conversely, the reflection of wave activity out of the North Atlantic basin leads to a reversal of both the regional poleward eddy momentum flux and the phase of the NAO. In the stratosphere, the reflection of wave activity toward the pole leads to a strong deceleration at high latitudes and forces the stratospheric Northern Annular Mode (NAM) into a negative phase. These anomalies propagate downward in the following weeks and induce the negative phase of the tropospheric NAM.