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

Previous studies have shown that the tropical instability waves (TIWs) in the Equatorial Pacific are enhanced during La Nina and suppressed during El Nino. Unlike El Nino which tends to decay quickly, La Nina often persists through the subsequent one or 2 years, becoming a multiyear La Nina. 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 Nina and decreases significantly during the second-year La Nina. 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 Nina, as compared to the first-year La Nina, 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 Nina 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 Nina is the major reason for the weakened TIW activity. Tropical Instability Waves (TIWs) are mesoscale waves propagating westward on edges of the equatorial Pacific cold tongue. Their interannual variability is strongly modulated by El Nino-Southern Oscillation, enhanced during La Nina and suppressed during El Nino years. In contrast to El Nino which tends to decay quickly, La Nina often persists through the subsequent one or two years, becoming a multiyear La Nina. For the first time, this study unveils the time-dependent feature of the TIW activity in the multiyear La Nina events. That is, the TIW activity, as measured by eddy kinetic energy, is largely weakened during the second-year La Nina as compared to the first-year La Nina. An energetics analysis reveals that baroclinic instability, a dominant energy source of the TIWs, is significantly weakened during the second-year La Nina. Meanwhile, the strength of barotropic instability (another major energy source of the TIWs) is also reduced. Our study indicates the importance of both baroclinic and barotropic instabilities in shaping the interannual evolution of TIWs during the multiyear La Nina. While the tropical instability wave (TIW) activity is believed to be enhanced by La Nina, in a multiyear La Nina, it is greatly decreased after the first yearThe decrease in TIW eddy kinetic energy (EKE) during the second-year La Nina is mainly due to a weakened baroclinic instability of the equatorial currentsThe spatial difference in the meridional structure of temperature pattern causes the difference in TIW EKE during multi-year events