Analysis Of ENSO Cycle In FIO-ESM

As the most significant phenomenon of coupled ocean-atmosphere system,the El Ni?o and Southern Oscillation(ENSO)has crucial influences on global weather and climate.Numerical model has become an important tool to simulate and project ENSO,while multi-model ensemble analysis on CMIP5 shows that obvious differences still exist among different models.In this paper,based on the CMIP5 historical experiment outputs,the simulation ability of FIO-ESM is evaluated through the comparison of both ENSO behavior and background climate state with observations.The upper-ocean heat budgets of El Ni?o and La Ni?a are analyzed for understanding the dynamic,thermodynamic and air-sea coupling processes related to ENSO evolution.Generally,the variability of sea surface temperature(SST),wind field and precipitation in the tropical Pacific Ocean during the ENSO period is reasonably simulated by FIO-ESM.However,we also find some deficiencies in the modelling results compared with observations.Slightly positive biases of climatological SST exist in the eastern Pacific Ocean;the strength of the trade wind is relatively weaker;the variability of model ENSO presents quasi-regular periods and stronger amplitudes;the maximum of some ENSO events does not happen in winter;and most of the El Ni?o events are eastward-propagating.Our analysis indicates that all three components of ocean advection contribute to the development and decay of ENSO.In accordance with previous studies,temperature advection associated with anomalous zonal current and mean vertical upwelling drives the development and decay of ENSO,manifesting important impacts on the entire evolution of ENSO.Temperature advection associated with anomalous vertical upwelling and mean meridional current makes positive contributions to SST anomaly.Surface heat flux has a damping effect on ENSO due to its opposite variation tendency with SST anomaly.Decomposition of upper-ocean heat budget shows that the deviation of temperature advection component in FIO-ESM is controlled by the distribution of temperature and current biases in the upper ocean during El Ni?o and La Ni?a,which can be attributed to the errors in simulating background tropical climate.Stronger westerly wind during El Ni?o can be induced by the positive deviation of SST in the eastern Pacific,thus enhancing the strength of thermocline feedback.The domination of thermocline feedback amplifies the amplitude of ENSO,weakens its irregularity and causes the eastward-propagating El Ni?o.Biases of the background seasonal cycle of tropical Pacific cause sudden reduction of thermocline feedback and zonal advective feedback in winter,resulting in errors in seasonal phase locking.