Modelling El Niño-southern oscillation : the utility of contemporary diagnostic frameworks

The development and application of process-based diagnostics is fundamental in improving our understanding of the simulation of El Nino-Southern Oscillation (ENSO) behaviours by coupled general circulation models (CGCMs). Several theoretical frameworks have potential utility as diagnostics; however, their accuracy in describing the dynamics governing ENSO evolution has not yet been systematically evaluated. This thesis presents a comprehensive analysis of a number of theoretical frameworks - the unified oscillator, Bjerknes stability index, and heat budget equation - to assess their representation of ENSO dynamics and capacity to diagnose ENSO behaviours in CGCMs. The unified oscillator description of ENSO was first investigated for its accuracy in replicating the ENSO cycle simulated by flux-forced output from the Australian Community Climate and Earth System Simulator ocean model (ACCESS-OM). The unified oscillator equations for anomalous sea surface temperature, thermocline depth, and zonal wind stress tendencies were unable to capture the structure, amplitude, and period of the corresponding tendencies in ACCESS-OM. Furthermore, the unified oscillator was considerably less effective than the simple delayed oscillator model in replicating the simulated ENSO cycle. It was concluded that the overly simplified formulation of the unified oscillator limits its power as a diagnostic of ENSO behaviours. This thesis also evaluated the effectiveness of the Bjerknes stability index in representing ENSO ocean feedbacks. Output from ACCESS-OM was used to calculate the Bjerknes stability index feedbacks, which were compared with the corresponding heat budget equation feedbacks. Due to the assumption of linearity in its derivation and an oversimplification of the relationship between ENSO variables, the Bjerknes stability index feedbacks correlated poorly with the original heat budget feedbacks. Therefore, the Bjerknes stability index could not necessarily be relied upon to accurately quantify ENSO dynamics. It follows that in its current form, and compared with the heat budget equation, the Bjerknes stability index has limited utility as a diagnostic tool, particularly in the context of intercomparisons between models that are inherently nonlinear. Finally, the heat budget equation was employed to diagnose the realism of El Nino evolution simulated by an ACCESS coupled model (ACCESS-CM1.3). The heat budget equation for the tropical Pacific was calculated in ACCESS-CM1.3 and the ocean feedbacks benchmarked against the corresponding terms calculated in ACCESS-OM. Instead of simulating the central Pacific El Nino type, the ACCESS-CM1.3 captured a double peaked event with warming centres evolving concurrently in the eastern and western Pacific. The separation between the two peaks arose due to a westward bias in the dynamic warm pool edge. Coupling biases in ACCESS-CM1.3 also modified the magnitude, location, and timing of the heat budget dynamics during El Nino evolution relative to ACCESS-OM. Finally, some double peaked events in ACCESS-CM1.3 were erroneously categorised as central Pacific El Ninos by standard SST metrics. This thesis highlights the importance of considering multiple diagnostics, including those that inherently account for spatial and temporal variability such as the tropical Pacific heat budget equation, when evaluating ENSO behaviours in CGCMs.