Mean FlowStorm Track Relationship and RossbyWave Breaking in Two Types of El-Nio

The features of large-scale circulation, storm tracks and the dynamical relationship between them were examined by investigating Rossby wave breaking (RWB) processes associated with Eastern Pacific (EP) and Central Pacific (CP) El-Nio. During EP El-Nio, the geopotential height anomaly at 500 hPa (Z500) exhibits a Pacific-North America (PNA) pattern. During CP El-Nio, the Z500 anomaly shows a north positive-south negative pattern over the North Pacific. The anomalous distributions of baroclinicity and storm track are consistent with those of upper-level zonal wind for both EP and CP El-Niňo, suggesting impacts of mean flow on storm track variability. Anticyclonic wave breaking (AWB) occurs less frequently in EP El-Nio years, while cyclonic wave breaking (CWB) occurs more frequently in CP El-Nio years over the North Pacific sector. Outside the North Pacific, more CWB events occur over North America during EP El-Nio. When AWB events occur less frequently over the North Pacific during EP El-Nio, Z500 decreases locally and the zonal wind is strengthened (weakened) to the south (north). This is because AWB events reflect a monopole high anomaly at the centroid of breaking events. When CWB events occur more frequently over the North Pacific under CP El-Nio conditions, and over North America under EP El-Niňo condition, Z500 increases (decreases) to the northeast (southwest), since CWB events are related to a northeast-southwest dipole Z500 anomaly. The anomalous RWB events act to invigorate and reinforce the circulation anomalies over the North Pacific-North America region linked with the two types of El-Nio.