The Role Of Synoptic-Scale Rossby Wave Breaking In The North Pacific Eddy-Mean Flow Interaction

Mid-latitude synoptic-scale Rossby waves propapate eastward over the ocean and breaks at the end of their lifecycles. The breaking of these high-frequency waves act to transfer their energy into the low-frequency flow, which may accelerate or shift the jet in a few days. The synoptic-scale Rossby wave breaking is the most crucial stage in the process of eddy feedback on jet. The study of such key stage would shed light on the dynamics of wave-mean flow interaction midlatitude.Using NCEP-NCAR reanalysis dataset, this study identifies Rossby wave breaking (RWB) events according to the overturning of potential temperature contours at2PVU surfaces. The composites with respect to RWB events are analyzed to reveal the high and low-frequency circulation patterns that might favor the occurrence of two different types of RWB events. Besides, a jet index is defined based on the EOF1of the intraseasonal variability of the low-level zonal wind. The composites with respect to the jet index are made so that the role RWB plays in the wobbling of the jet could be analyzed. Finally, HadISST dataset is used to identify two different types of El-Nino events (Eastern Pacific El-Nino, and Central Pacific El-Nino), and composites of mean flow, storm track and RWB anomalies are made with respect to these two types of events. Such efforts are made to study the wave-mean flow interaction under the El-Nino conditions, and to the role RWB plays in such interplay.This study finds that the synoptic-scale transient eddies associated with Anticyclonic Wave Breaking (AWB) exhibit an equivalent barotropic vertical structure and move eastward fast. The large-amplitude disturbances are confined at the upper levels. The transient eddies associated with Cyclonic Wave Breaking (CWB) move notheastward, and the disturbances are developed at low-levels first and then propagate upward. The low-level disturbances are the successive anticylonic and cyclonic low-level wave breaking events before the occurrence of high-level CWB. Meridional shear of zonal wind dominates the low-frequency flow pattern related to CWB, while both zonal shear of meridinoal wind and meridional shear of zonal wind in the low-frequency criculation are important for the occurrence of AWB. Hence, the AWB not only occurs at the flanks of jets, but also at troughs or ridges related to planetary-scale stationary waves. The quasi-stationary ridge over the northeastern Pacific only decides the climatological distribution of AWB, but significantly affects its interannual variability. The two types of RWB all take place at critical layer. The width of the critical layer associated with CWB is much larger than that associated with AWB.The first mode of the intraseasonal variability of the Pacific jet is a meridional wobbling of the eddy-driven jet. The latter corresponds to a state that the eddy-driven jet shifts poleward and is well separated from the subtropical jet to its equatorward side. AWB (CWB) drives the poleward (equatorward) shift of the eddy-driven jet through eddy momentum flux. The jet is stronger and more persistent when shifting equatorward than poleward. This is due to the fact that CWB occurring dominantly in the equatorward shift of the jet last longer than its AWB counterpart. The longer duration allow more accumulative forcing on the jet and hence the stronger westerly. When the eddy-driven jet locates poleward (equatorward) persistently, AWB (CWB) occurs more frequently, which means the location of the jet and the occurrence of RWB influence each other. In Eastern Pacific (EP) El-Nino years, the Pacific jet is strengthened and extends downstream, corresponding to a PNA-like anomaly pattern. In Central Pacific (CP) El-Nino years, the Pacific jet shift equatorward. The anomalies barocilinicity and storm track overlaps with that of upper-level zonal wind with the same sign, indicating the impact of jet on the storm track anomaly. Over the North Pacific, AWB occurs more frequently during EPEl-Nino and CWB occurs less frequently during CP El-Nino. Where AWB occurs less frequently, the zonal wind is weakened (strengthened) to the north (south). Where CWB occurs more frequently, the zonal wind is weakened at the center and strengthened to the south. The composites with respect to individual RWB events reveal that AWB is associated with the strengthened (weakened) zonal wind to the north (south) and CWB is associated with the weakened zonal wind at the center and strengthened zonal wind to the south, consistent with the relationship between the anomalous RWB occurrence and the related mean flow anomalies. The lead-lag composites of eddy momentum flux with respect to RWB events indicate their feedback on mean flow. The composites of mean flow with respect to the two types of El-Nino which removes the RWB events demonstrate that RWB acts to strengthen or shift the anomalous centers in the El-Nino conditions.