On the Physics of the Warm Water Volume and El Nino/La Nina Predictability

Previous work has shown that warm water volume (WWV), usually defined as the volume of equatorial Pacific warm water above the 20 degrees C isotherm between 5 degrees S and 5 degrees N, leads El Nino. In contrast to previous discharge-recharge oscillator theory, here it is shown that anomalous zonal flow acceleration right at the equator and the movement of the equatorial warm pool are crucial to understanding WWV-El Nino dynamics and the ability of WWV to predict ENSO. Specifically, after westerly equatorial wind anomalies in a coupled ocean-atmosphere instability push the warm pool eastward during El Nino, the westerly anomalies follow the warmest water south of the equator in the Southern Hemisphere summer in December-February. With the wind forcing that causes El Nino in the eastern Pacific removed, the eastern equatorial Pacific sea level and thermocline anomalies decrease. Through long Rossby wave dynamics this decrease results in an anomalous westward equatorial flow that tends to push the warm pool westward and often results in the generation of a La Nina during March-June. The anomalously negative eastern equatorial Pacific sea level typically does not change as much during La Nina, the negative feedback is not as strong, and El Ninos tend to not follow La Ninas the next year. This El Nino/La Nina asymmetry is seen in the WWV/El Nino phase diagram and decreased predictability during "La Nina-like" decades.