Modulation of the PDO to the relationship between moderate ENSO events and the winter climate over North America

The modulation of the Pacific Decadal Oscillation (PDO) to the relationship between the El Nino-Southern Oscillation (ENSO) events and the winter climate over North America (NA) was investigated using both observational data and a simple general circulation model. ENSO events are divided into strong and moderate events according to their amplitudes. These ENSO events are further categorized into two groups according to the phases of the PDO. The present study focuses on the impact of moderate ENSO events on the wintertime climate over NA. The results show that a moderate El Nino signal is strong and stable and can significantly influence the wintertime climate variability over NA during a cold PDO phase, whereas climate anomalies are weak and are not significantly associated with moderate El Nino events during a positive PDO phase. Possible mechanisms accounting for the modulation of the impacts of moderate El Nino events on the wintertime climate over NA are analysed. Two factors may contribute to the different impacts of moderate El Nino events during warm and cold PDO phases. First, the different characteristics of the tropical Pacific sea surface temperature anomalies (SSTAs) associated with moderate El Nino events. Second, the difference in the climatological mean state between the warm and cold phases of PDO years. Our numerical experiments suggest that the cold PDO mean state is more favourable to a wavetrain-like atmospheric response to an El Nino-like tropical Pacific forcing over the North American sector than the warm PDO mean state is.