Representation of ocean meso-scale eddies in ocean reanalyses

Ocean eddies, which range in spatial scale from 10 to 250 km and can last from a few days to several months,  play a critical role in regulating ocean heat balance, transporting energy and nutrients, and influencing global ocean circulation. Recent studies also show an increase in the number of eddies in the ocean. Therefore, it is important to understand the statistics, movement, and variability of ocean eddies. Altimetry satellite missions launched in 1993 provided the community with a great opportunity to understand ocean eddies. The Surface Water and Ocean Topography (SWOT) mission launched in 2022, measures the ocean surface at spatial scales of 15–25 km allowing us to understand even finer ocean structures. Meanwhile, the spatial resolution of ocean numerical models has been increasing to capture multi-scales of ocean features and has finer resolution than AVISO. The representation of ocean meso-scales eddies in numerical models is important for the ocean models to provide accurate ocean state estimates and the SWOT mission data allows the model community to understand the capability of representation of ocean eddies in ocean models.  
The aim of this study  is to analyse mesoscale eddies characteristics in satellite altimetry data (AVISO and SWOT) and ocean reanalyses at two different spatial resolutions to understand the impact of spatial resolution on ocean meso-scales in ocean models.   These datasets were analyzed to identify eddies based on physical criteria, such as radius, amplitude, and contour area. Eddies are identified based on closed contours in absolute dynamic topography (ADT) fields. This helps distinguish eddies from oceanic features such as meanders. Preliminary results include a detailed statistical validation of eddy distributions by radius and spatial location, with datasets revealing consistent patterns for mesoscale eddies with radii between 25 and 250 km. The results show that the number of eddies between 25-250km is higher in the finer resolution of both ocean reanalyse data and altimetry data (1/4 vs 1/12°) as expected. However, the number of eddies in 1/12 ocean reanalyses is higher than AVISO satellite observations but lower than SWOT altimetry data. It indicates the importance of spatial resolution in numerical models to represent the finer scale of ocean features. The ongoing activities will explore in detail eddy characterise in different datasets and the outcome of this study will provide useful information for the community to use ocean models to investigate ocean eddies.