Dynamic Synoptic Eddy Feedback Contributes to the ENSO Annual Cycle Combination Mode-Associated Variation in the PNA Between Early and Late Winter

While prior studies have revealed that the El Niño-Southern Oscillation (ENSO) annual cycle combination mode (C-mode) can excite distinct wave activity that peaks in late winter, making the spatial pattern of the Pacific-North American (PNA) teleconnection in response to strong El Niño events quite different from those in response to other ENSO events. Based on this, we can easily understand that the C-mode can determine the corresponding processes of dynamic synoptic eddy (SE) feedback by influencing the intensity and structure of the PNA flow. However, little attention has been paid to the other route, that is, the C-mode can also modulate the SE feedback by influencing the atmospheric environment favorable to SE activity through air-sea interactions. This study found that the PNA is characterized by clear enhancement from early winter to late winter exclusively during strong El Niño events, which can be explained by the dominant role of the dynamic SE feedback modulated by the C-mode. Specifically, the C-mode-associated air-sea interactions in late winter can contribute to the reduction of the north-to-south air temperature gradient over the northeast Pacific, which weakens the SE activity over the northeast Pacific. It is beneficial to the conversion of the barotropic energy from the SE to the mean flow, which enhances the PNA.