New Perspectives on Frontal Variability in the Southern Ocean Using a Local Identification Scheme

TL;DR

This study uses a novel wavelet-based method to analyze 21 years of sea-surface height data, revealing high variability in Southern Ocean fronts, limited southward migration, and questioning previous contour-based findings.

Contribution

Introduces the WHOSE method for frontal detection, providing new insights into frontal variability and challenging prior assumptions based on contour methods.

Findings

Number of fronts varies with bathymetry

Limited southward front migration observed

Frontal meandering remains relatively constant

Abstract

The frontal structure of the Southern Ocean is investigated using a sophisticated frontal detection methodology, the Wavelet/Higher Order Statistics Enhancement (WHOSE) method, introduced in \cite{Chapman2014}. This methodology is applied to 21 years of daily gridded sea-surface height (SSH) data to obtain daily maps of the locations of the fronts. By forming `heat-maps' of the frontal occurrence frequency and then approximating these heat-maps by a superposition of simple functions, the time-mean locations of the fronts, as well as a measure of their capacity to meander, are obtained and related to the frontal locations found by previous studies. The spatial and temporal variability of the frontal structure is then considered. The number of fronts is found to be highly variable throughout the Southern Ocean, increasing (`splitting') downstream of large bathymetric features and decreasing (`merging') in regions where the fronts are tightly controlled by the underlying topography. In contrast, frontal meandering remains relatively constant. Contrary to many previous studies, little no southward migration of the fronts over the 1993-2014 time period is found, and there is only weak sensitivity to atmospheric forcing related to SAM or ENSO. The reasons for the discrepancy between this study and previous studies using contour methods are investigated and it is shown that the spatial variability of the frontal structure is not tied to the underlying sea-surface height. It is argued that the results of studies using sea-surface height contours to define front must be interpreted with care.