Roles of Tropical Instability Waves (TIWs) in Modulating El Nino-Southern Oscillation (ENSO)

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Almost all global climate models are deficient in their representations of the El Nino - Southern Oscillation (ENSO) phenomenon, in regard to its structure, intensity, and temporal evolution. Zhang and Busalacchi will test their hypothesis that these deficiencies stem from the inability of present generation of GCMs to simulate tropical instability waves (TIWs) in the ocean. TIWs are mesoscale phenomena arising from fluid dynamical instabilities of the equatorial zonal currents. The Principal Investigators (PIs) hypothesize that the sea surface temperature (SST) anomalies associated with TIWs give rise to fluctuating winds, that these winds contribute to the irregularity of ENSO, and that including a depiction of these fluctuating winds in GCMs will improve their representation of ENSO.The PIs will develop an empirical model for the atmospheric wind stress induced by TIWs. These TIW-induced winds will be applied, together with climatological winds, to an ocean model, in order to determine the rectified effect of this additional atmospheric variability upon the climate of the ocean. The TIW-induced winds will similarly be applied to a coupled atmosphere-ocean model in which ENSO variability is resolved, but not TIWs. These experiments will reveal the effect of TIWs, through the atmosphere, on the interannual variability in the tropical Pacific, as well as on the predictability of ENSO. Analyses of these experiments should reveal the mechanisms through which TIWs influence both the mean climate and ENSO variability. The PIs also will develop a high-resolution coupled atmosphere-ocean model that explicitly resolves TIWs. Finally, they will analyze output from two versions of the National Center for Environmental Prediction Climate Forecast System model (NCEP CFS), one of which has sufficient resolution to resolve TIWs. They will explore the role of TIW induced variability in causing differences in ENSO between these two models, especially its temporal irregularity. The broader impacts of this work are in the potential for applying the TIW parameterization that is developed to climate models used for climate simulation and for ENSO prediction.