The Observed Impacts of the Two Types of El Nino on the North Equatorial Countercurrent in the Pacific Ocean

This study investigates the interannual variations in the North Equatorial Counter Current (NECC) associated with the eastern Pacific and the central Pacific types of El Nino. Using observational analysis and ocean simulations, we show that the wind stress anomalies during the two El Nino types are of comparable amplitude but have different spatial structures, which results in significant and distinct variations in the NECC. The NECC shifts southward and intensifies during the developing phase of El Nino, but the variations are confined in the central eastern Pacific for the eastern Pacific type and the western central Pacific for the central Pacific type. These differences can be attributed to modulations in equatorial Kelvin wave and tropical Rossby wave propagation as well as Ekman pumping. Plain Language Summary The North Equatorial Counter Current (NECC) is an eastward flowing surface current that transports a significant amount of water from the warm pool in the western Pacific to the east. Variations in the NECC exert profound effects on the tropical Pacific climate and need to be better understood. This study conducts statistical analyses and ocean model experiments to show that different flavors of El Nino can displace the NECC location and alter its intensity in distinct ways. The cause and the physical mechanisms behind the distinct impacts are identified and verified using specific El Nino events of each flavor. The findings reported in this study contribute to advance our understanding of El Nino diversity and its impacts on the circulation and variability of the Pacific Ocean.