Simulation of the Leeuwin Current System by Eddy-resolving Ocean General Circulation Model

The Leeuwin Current (LC) is a narrow eastern boundary current in the southeast Indian Ocean, flowing poleward along the western Australian coast. The heat advection by the anomalous LC is a major contributing factor for the development of the Ningaloo Niño. However, the simulation of LC is still a major challenge in ocean modeling, since it requires a very high horizontal resolution. Consequently, the sea surface temperature (SST) anomalies caused by the LC variability during the Ningaloo Niño cannot be adequately resolved by most models. In this study, we investigate the LC variability associated with the Ningaloo Niño by using a 1/12° Indo-Pacific basin Hybrid Coordinate Ocean Model (HYCOM) with atmospheric forcing fields from ERA5. The results are compared with satellite observations and historical hydrostatic datasets. To evaluate the influence of horizontal resolution on the LC simulations, 1/4° HYCOM experiments with the same atmospheric forcing are also conducted. Compared with the low-resolution HYCOM, the LC in the 1/12° HYCOM has a better 3D structure and more realistic annual mean speed (~ 69% stronger than 1/4° HYCOM). On interannual time scales, the LC is influenced by the ENSO related thermocline changes that propagate from the equatorial Pacific Ocean to the southeast Indian Ocean through the Indonesian Seas, where the 1/12° HYCOM is able to resolve narrow straits and small islands. The thermocline variation and its influences on the time evolution of the LC and SST anomalies in the southeast Indian Ocean during the Ningaloo Niño is further examined based on the composite analysis.