Simulation of El Niño-Southern Oscillation (ENSO) using a HYbrid Coordinate Ocean Model (HYCOM)

El Nino and Southern Oscillation (ENSO) is an atmosphere-ocean coupled phenomenon that exhibits interannual variability in tropical Pacific Ocean and plays an important role in modulating the interannual variability of regional climate globally. In this study, the ENSO events are simulated using the Hybrid Coordinate Ocean Model (HYCOM) with daily atmospheric forcing derived from the NCEP/NCAR reanalysis dataset for the period of 1951-2006. The result indicates that with prescribed atmospheric forcing, the HYCOM model can accurately simulate the major characteristics of ENSO events. The correlation coefficient between the simulated Sea Surface Temperature (SST) anomalies and observed SST anomalies in the Niño3.4 region is 0.73. However, the simulated SST anomalies (SSTA) have an anomalous rising trend compared with the observed SSTA. SST can be influenced by horizontal advection, radiation flux, sensible and latent heat flux and entrainment/detrainment process. Two choices of bulk parameterization schemes of surface fluxes due to different latent and sensible heat exchange coefficients are used in HYCOM. Sensitivity analysis shows the unrealistic rising trend produced by the model was more due to weak model mixing than to trends in the surface forcing. However, extracting the interannual band of the simulated tropical Pacific SST from the model data shows a near-perfect match between the model data and the observations at the interannual band. This suggests that even with the presence of erroneous trends in the simulated data, the model was able to accurately simulate the internnual variability in the tropical ocean.