Time‐Dependent Role of Multiyear La Niña in Impacting the Pacific Tropical Instability Waves

Abstract

AbstractPrevious studies have shown that the tropical instability waves (TIWs) in the Equatorial Pacific are enhanced during La Niña and suppressed during El Niño. Unlike El Niño which tends to decay quickly, La Niña often persists through the subsequent one or 2 years, becoming a multiyear La Niña. Using a time‐dependent and space‐localized energetics formalism and a suite of observational and reanalysis data, we found that the eddy kinetic energy (EKE) associated with TIWs only peaks during the first‐year La Niña and decreases significantly during the second‐year La Niña. This is caused, to the first order, by the weaker equatorial cold tongue near the equator and anomalous cooling in the off‐equator regions during the second‐year La Niña, as compared to the first‐year La Niña, which leads to a significant decrease in meridional density shear, and thus a reduction in baroclinic instability. Meanwhile, the weakened South Equatorial Current during the second‐year La Niña also contribute to EKE decrease by transferring less kinetic energy to the TIWs via barotropic instability. The meridionally broad spatial pattern of negative temperature anomalies during the second‐year La Niña is the major reason for the weakened TIW activity.