Parameterization of Tropical Instability Waves and Examination of Their Impact on ENSO Characteristics

Abstract

he impact of tropical instability waves (TIWs) on El Niño?Southern Oscillation (ENSO) characteristics is investigated by introducing a new parameterization of TIWs into an atmosphere?ocean general circulation model (AOGCM), the Model for Interdisciplinary Research on Climate (MIROC), with a medium-resolution (~1.4°) ocean model (known as MIROCmedres). Because this resolution is not sufficient to reproduce eddies at the spatial scale of TIWs, this approach isolates TIW effects from other factors that can affect ENSO characteristics. The parameterization scheme represents the effect of baroclinic eddy heat transport by TIWs. A 100-yr integration reveals a significant role of TIWs in observed ENSO asymmetry. Asymmetric heat transport associated with TIWs that are active (inactive) during La Niña (El Niño) generates a significant asymmetric negative feedback to ENSO and explains the observed asymmetric feature of a stronger-amplitude El Niño and weaker-amplitude La Niña. Furthermore, the parameterized eddy heat flux also affects the mean subsurface heat balance via the shallowing and steepening thermocline. This change in subsurface stratification induces a stronger thermocline feedback and a longer ENSO period.