Abstract
Abstract Objective analysis of several hundred thousand anticyclonic and cyclonic breaking Rossby waves is performed for the Northern Hemisphere (NH) winters of 1958–2006. A winter climatology of both anticyclonic and cyclonic Rossby wave breaking (RWB) frequency and size (zonal extent) is presented for the 350-K isentropic surface over the NH, and the spatial distribution of RWB is shown to agree with theoretical ideas of RWB in shear flow. Composites of the two types of RWB reveal their characteristic sea level pressure anomalies, upper- and lower-tropospheric velocity fields, and forcing of the upper-tropospheric zonal flow. It is shown how these signatures project onto the centers of action and force the velocity patterns associated with the North Atlantic Oscillation (NAO) and Northern Hemisphere annular mode (NAM). Previous studies have presented evidence that anticyclonic (cyclonic) breaking leads to the positive (negative) polarity of the NAO, and this relationship is confirmed for RWB over the midlatitudes centered near 50°N. However, an opposite and statistically significant relationship, in which cyclonic RWB forces the positive NAO and anticyclonic RWB forces the negative NAO, is shown over regions 20° to the north and south, centered at 70° and 30°N, respectively. On a winter mean basis, the frequency of RWB over objectively defined regions covering 12% of the area of the NH accounts for 95% of the NAO index and 92% of the NAM index. A 6-hourly analysis of all the winters indicates that RWB over the objectively defined regions affects the NAO/NAM without a time lag. Details of the objective wave-breaking analysis method are provided in the appendix.
Highlights
Rossby waves depend upon a nonzero background potential vorticity (PV) gradient for their restoring mechanism
We detected nonlinear reflection following breaking in approximately a third of all events (Abatzoglou and Magnusdottir 2006a), and we found that winter breaking over the North Atlantic has an opposite effect on the North Atlantic Oscillation (NAO) depending on whether or not the Rossby wave breaking (RWB) results in reflection (Abatzoglou and Magnusdottir 2006b)
The longitudinal offset between the ␥c and ␥a maxima was shown to be consistent with nonlinear Rossby wave critical layer theory of small-amplitude Rossby waves on a basic-state shear flow where breaking occurs at the critical line, which is where the phase speed of the waves matches the background flow speed
Summary
Rossby waves depend upon a nonzero background potential vorticity (PV) gradient for their restoring mechanism. We have closely examined large-scale events that break anticyclonically in a hierarchy of model simulations (Walker and Magnusdottir 2002; Walker and Magnusdottir 2003, and references therein) and in reanalysis We extend this line of work in two ways to include the indirect effects of breaking on the atmospheric general circulation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
