Ocean general circulation models (OGCMs) become increasingly important for understanding oceanic dynamic processes and ocean environment forecasting. In recent decades, OGCMs have been developed with finer resolution (10km for eddy-resolving OGCMs) given the large computational resources.

"However, the state-of-the-art eddy-resolving OGCMs tend to simulate a less energetic surface ocean on the global scale. In addition, there is so far no comprehensive and systemic evaluation of the model performance in simulating global mesoscale eddies. Many issues have therefore not been fully discussed," said LIU Hailong, one of the corresponding authors of a study recently published in Geophysical Research Letters. 

To address these issues, a research group from the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS), investigated the performances of eddy-resolving OGCMs in simulating mesoscale eddies using four eddy-resolving OGCMs forced by different atmospheric reanalysis products, including the self-developed LASG/IAP Climate system Ocean Model version 3 (IAP-LICOM3).

Results show that the eddy-resolving OGCMs tend to simulate more (less) energetic eddy-rich (eddy-poor) regions with a smaller (larger) spatial extent. Quantitatively, there is an approximately 27-60% overestimation of eddy kinetic energy intensity (EI) in the eddy-rich regions, which are mainly located in the Kuroshio-Oyashio Extension, the Gulf Stream, and the Antarctic Circumpolar Currents regions, although the global mean EI is underestimated by 25-45%. Apparently, EI in the eddy-poor region is underestimated.

Further analyses after eddies are identified and tracked show that the overestimation in the eddy-rich regions is mainly contributed by coherent mesoscale eddies' intensity rather than frequency and is more prominent when mesoscale eddies are in their growth stage.

"It's an interesting story, as the eddy-rich regions are simulated more energetic while eddy-poor regions are simulated less energetic," said LIU. "It points out some of the shared problems model developers face before taking further efforts to improve the eddy-resolving ocean models."