Importance of Resolving Mesoscale Eddies in the Model Simulation of Ningaloo Nino

Satellite observational data and a regional ocean model are used to understand the evolution of Ningaloo Nino (NN) anomalies near Western Australian coast. In observation and high-resolution (similar to 3 km) simulations, coastally trapped positive sea level anomalies (SLAs), originated largely from the Indonesian Throughflow, are transmitted westward by mesoscale eddies. Few eddies propagate long distances offshore due to dissipation, and as a result NN signatures are predominantly confined near the coast. In coarse-resolution (similar to 100 km) simulations that cannot resolve eddies, oceanic anomalies propagate swiftly as long Rossby waves with much weaker dissipation and more anomalies spread to the ocean interior. Eddy-enhanced surface warming promotes surface latent heat release and mesoscale air-sea interactions, which acts to damp NN surface warming. These processes are not resolved by coarse-resolution models. This study highlights the importance of resolving mesoscale oceanic processes in the simulation and prediction of NN. Plain Language Summary Ningaloo Nino (NN) is a surface warming surge phenomenon of the southeast Indian Ocean. Here we describe how NN anomalies develop in high- (similar to 3 km) and low-resolution (similar to 100 km) ocean model simulations. We found that in high-resolution simulations and observational data, the sea level rise and warming of the NN spread offshore as eddies with wavelengths of 10-500 km. Because eddies move slowly and are subjected to strong dissipation, NN anomalies cannot spread long distance offshore and tend to concentrate near the west coast of Australia. However, these eddies are not resolved in low-resolution simulations. Instead, NN anomalies travel westward quickly as long free Rossby waves without obvious dissipation. Eddies also enhance local surface warming anomalies and cause strong latent heat loss, leaving impacts on the local climate. These processes are absent in low-resolution simulations. Therefore, it seems necessary to take into account effects of oceanic mesoscale eddies (OMEs) in the simulation and prediction of NN.