Studying Southern Ocean fronts is key to understanding global circulation and climate. Fronts separate water masses and influence heat and carbon transport. Traditionally, fronts were identified by contours of sea surface height, but this approach yields conflicting results about frontal migration. This work aligns with studies using satellite altimetry and local gradient methods, but offers a more rigorous identification approach.
The WHOSE (Wavelet/Higher Order Statistics Enhancement) method is applied to daily absolute dynamic topography maps spanning 1993–2015. It uses wavelet decomposition and higher-order statistics to extract step-like signals characteristic of fronts from noise. Then, frontal occurrence frequency maps are constructed and fitted with sums of skewed normal functions. The number of functions is chosen using the Akaike information criterion, allowing the number of fronts at each longitude to be estimated.
The number of fronts varies greatly along longitude, from 2 to 15. Increases in frontal count (splitting) occur downstream of major topographic obstacles, such as the Kerguelen Plateau, while decreases (merging) occur where bottom topography steers the current. Frontal meandering remains relatively constant, with a standard deviation of about 0.5° of latitude. Over 1993–2014, no significant southward shift of fronts was detected, and sensitivity to the Southern Annular Mode and El Niño is weak and local.
This work shows that local detection methods yield a more realistic picture of frontal variability than contour-based methods. Unlike neighboring studies that use sea surface height contours, here fronts are treated as transient structures that can split and merge. This changes our understanding of Antarctic Circumpolar Current dynamics and calls for revisiting conclusions about frontal migration.
Future research could focus on the consequences of frontal reorganization for large-scale circulation. It is also important to better characterize frontal regimes and their connection to potential vorticity. The method could be applied to other regions and data types, such as sea surface temperature.
The results will influence oceanography, climatology, and modeling of Southern Ocean circulation. They will help improve interpretation of satellite data and predictions of climate change.
A next step could be analyzing the link between frontal splitting and tracer transport. It would also be worthwhile to compare WHOSE with other local methods on independent data.
This work addresses the challenge of defining ocean fronts and their role in water mass mixing. It raises the question of how frontal reorganization affects deep water ventilation and the exchange of properties across latitudes.
Southern Ocean fronts can split into multiple branches, much like a river dividing into channels in a delta.