Interannual Variations of the North Equatorial Current Across the Pacific Ocean

The interannual variation of the North Equatorial Current (NEC) across the North Pacific Ocean and the modulation of the Pacific Decadal Oscillation (PDO) are investigated based on both observations and reduced gravity model experiments. Results reveal that the interannual variation of the NEC is highly related to El Nino-Southern Oscillation (ENSO) with unsynchronized variations both in zonal and meridional structure. The tropical and subtropical gyre branches of the NEC (NEC-S and NEC-N) behave antiphase variation during El Nino events with the transport of the former being enhanced and the latter being weakened across the Pacific. The NEC axis in the western/eastern Pacific shifts northward/southward during El Nino events, consistent with the meridional shift of the zero-line of wind stress curl. Model experiments indicate that Rossby waves mainly modulate the NEC near the western boundary and the NEC-N, while Ekman pumping modulates NEC-S in the western basin. Meanwhile, the relationship between the interannual variation of the NEC and ENSO events is significantly modulated by different PDO phases. The PDO cold phase favors a much closer connection between the NEC and the Nino 3.4 index. The dynamics lie in that PDO-related wind stress curl and sea level anomalies are consistent with that of El Nino, favoring the enhanced correlation between NEC and El Nino. The reverse is true for the warm phase. These results will contribute to a better understanding of the behavior of the tropical Pacific Ocean circulation and its role in climate change. Serving as the boundary of tropical and subtropical gyres, the North Equatorial Current plays an important role in the exchanging of mass, heat, and salt between the mid- and low-latitude oceans. It is a very broad current and behaves quite differently in both zonal and meridional directions across the Pacific due to the phase lag of propagated Rossby waves. This study reveals that the interannual variation of the North Equatorial Current is highly related to El Nino-Southern Oscillation (ENSO) with unsynchronized variations in zonal and meridional structure. The tropical and subtropical gyre branches of the North Equatorial Current behave antiphase variation during El Ninos with the transport of the former is enhanced and the latter weakened across the Pacific. Rossby waves excited west of the dateline modulate the North Equatorial Current near the western boundary, while those excited in the eastern Pacific modulate the subtropical branches. Local Ekman pumping plays a dominant role in tropical branches in the western Pacific. Further analysis indicates that the ENSO-related air-sea state in the tropical Pacific Ocean is modulated by Pacific Decadal Oscillation phases. The cold phase favors a much closer connection between the North Equatorial Current and ENSO events. Interannual variation of the North Equatorial Current (NEC) is highly related to El Nino-Southern Oscillation (ENSO) with unsynchronized variations both in zonal and meridional structure Rossby waves mainly modulate the NEC near the western boundary and subtropical gyre branch, while Ekman pumping modulates the tropical gyre branch The relationship between the NEC and ENSO is nonstationary and significantly modulated by different phases of the Pacific Decadal Oscillation