The link between Rossby wave breakings and weather regimes transitions

The link between Rossby wave breakings (RWB) and the four wintertime weather regimes over the North Atlantic domain is studied. Using the 40-yr ECMWF re-analyzed data (ERA-40), frequencies of occurrence of anticyclonic (AWB) and cyclonic (CWB) wave-breaking events are computed. Each weather regime has its own characteristic pattern of RWB frequencies. CWB events are found to be the most frequent for the Greenland anticyclone weather regime while AWB events for the Atlantic ridge and the zonal regimes. Time-lag composites show that the RWB events characterizing each weather regime occur more often during the formation of the regime rather than during its decay. This suggests a reinforcement of the weather regime by RWB. An exception is the blocking weather regime which is destroyed by an increase of CWB events south of Greenland.

Weather regimes transitions are then studied using the low-frequency streamfunction tendency budget. Two types of precursors for the transitions have been identified. One is related to linear propagation of low-frequency transient eddies and the other to non-linear interactions between the low- and high-frequency transient eddies. The latter has been related to the anomalous frequencies of occurrence of RWB. Two transitions are more precisely analyzed. The blocking to Greenland anticyclone transition is triggered by a decrease of AWB events over Europe as well as a strong CWB event south of Greenland. The zonal to blocking transition presents evidences of two distinct precursors: one is a low-frequency wave train driven by tropical convection over the tropical Atlantic followed by a decrease of AWB and CWB events over western Europe that cannot maintain anymore the westerlies in that region.