Hydrologic regionalisation from Crozet to Kerguelen and subtropical southern Indian Ocean

The TAAF are located at the junction of the South Indian Ocean and the Southern Ocean, an area of great environmental contrasts due to the meeting of subtropical warm and salty waters and polar cold waters. Frontal structures generated by the Antarctic Circumpolar Current divide these oceans into several frontal zones. Several regionalisations have been carried out in recent decades, but only on the epipelagic or mesopelagic zones, without considering both. As these frontal zones have a strong influence on environmental conditions and the distribution of biodiversity, it is important to be able to identify their spatial distribution and boundaries. The aim of this study was to identify the different hydrological zones in the Indian sector of the Southern Ocean and the Southern Indian Ocean from open access data. Temperature and salinity profiles from the surface to 1000 m depth were retrieved from Copernicus reanalyses between 2010 and 2020. A functional PCA was applied to these data to account for the entire water column, followed by k-means clustering to identify regions with a common hydrological profile. We also calculated the mean and standard deviation of the cluster value for each geographical cell to identify the most stable hydrological regions. Each identified hydrological region was then linked to environmental data from the whole study area to characterise it. We were able to identify 7 hydrological regions, 4 in the Southern Indian Ocean and 3 in the Southern Ocean. The regions dividing the Southern Ocean coincide strongly with the frontal zones identified in previous regionalisation work over the Southern Ocean and are strongly associated with the subantarctic and polar fronts. Conversely, the regions dividing the Southern Indian Ocean are explained by differences in salinity, particularly at depth, associated with the action of the South Equatorial Current.