Modulation Of Different ENSO Modes On Tropical Pacific Instability Wave

Tropical Instability Waves(TIWs)are westward propagating wave trains on the edge of the equatorial Pacific cold tongue,which interact strongly with El Ni?o-Southern Oscillation(ENSO).Using a recently developed multiscale decomposition tool,multiscale window transform(MWT),and the MWT-based theory of canonical transfer and localized multiscale energy analysis,the generation mechanisms of the two different meridional TIW modes in the equatorial Pacific Ocean,i.e.,the Yanai-mode at the equator and the Rossby-mode around 5°N,and their associated spatial coupling are investigated in this study.In the subsurface layer(30-200 m),barotropic instability is the primary energy source for the Yanai-mode TIWs,while baroclinic instability dominates the eddy kinetic energy(EKE)generation of the Rossby-mode TIWs.In contrast,the two TIW modes in the surface layer(0-30 m)are largely modulated by nonlocal mechanisms through pressure work,which functions to transport EKE upward from the subsurface to the surface layer,and southward from the Rossby-mode region to the Yanaimode region.The pressure fluxes substantially couple the two TIW modes in both the meridional and vertical directions,leading to significant coherence between the EKE in different vertical layers and regions of the TIWs.This nonlocal energy pathway,as well as the well-known instability processes,undergoes strong seasonal variations that determine the seasonal cycles of the TIWs.Using a quantitive causality analysis rigorously established from first principles,we identified distinct modulation processes of the Eastern-Pacific(EP)and the Central-Pacific(CP)ENSO on the interannual variations of TIWs.It is found that during EP La Ni?a phase,from June to the following February,the strengthened equatorial currents and equatorial cold tongue lead to the strengthened barotropic and baroclinic instability(baroclinic instability is primary)in the Rossby-mode region,which increase the TIW EKE,and vice versa for EP El Nino phase.During CP La Ni?a phase,from January to February,the accompanying subsurface cold water leads to strengthened baroclinic instability in the Rossby-mode region,which is responsible for the anomalously high TIW EKE in the phase,and vice versa for CP El Nino phase.The spatial energy transport fulfilled by pressure work works to transport the TIW EKE upward from the subsurface to the surface layer,and southward from the Rossby-mode region to the Yanai-mode region,causing the interannual TIW EKE anomaly in the surface layer of the Yanai-mode region.