The occurrence of boreal winter Rossby wave breaking (RWB) along with the quantitative role of synoptic transient eddy momentum and heat fluxes directly associated with RWB are examined during the development of Euro-Atlantic circulation regimes using the ERA Interim Reanalysis. Results are compared to those from seasonal re-forecasts made using the Integrated Forecast System model of ECWMF coupled to the NEMO ocean model. The development of both Scandinavian Blocking and the Atlantic Ridge is directly coincident with anticyclonic wave breaking (AWB); however, the associated transient eddy fluxes do not contribute to (in fact, oppose) ridge growth, as indicated by the local Eliasson-Palm (EP) flux divergence. Evidently other factors drive development, and it appears that wave breaking assists more with ridge decay. The growth of the North Atlantic Oscillation (NAO) in its positive phase is independent of RWB in the western Atlantic, but strongly linked to AWB further downstream. During AWB, the Equator-ward flux of cold air at upper levels contributes to a westerly tendency just as much the pole-ward flux of momentum. The growth of the negative phase of the NAO is almost entirely related to cyclonic wave breaking (CWB), during which Equator-ward momentum flux dominates at jet level, yet low-level heat fluxes dominate below. The re-forecasts yield realistic frequencies of CWB and AWB during different regimes, as well as realistic estimates of their roles during development. However, a slightly weaker role of RWB is simulated, generally consistent with a weaker anomalous circulation.