Southern Ocean jets and how to find them: Improving and comparing common jet detection methods

Abstract

This study undertakes a detailed comparison of different methods used for detecting and tracking oceanic jets in the Southern Ocean. The methods under consideration are the gradient thresholding method, the probability density function (PDF) method, and the contour method. Some weaknesses of the gradient thresholding method are discussed and an enhancement (the WHOSE method), based on techniques from signal processing, is proposed. The WHOSE method is then compared to the other three methods. Quantitative comparison is undertaken using synthetic sea-surface height fields. The WHOSE method and the contour method are found to perform well even in the presence of a strong eddy field. In contrast, the standard gradient thresholding and PDF methods only perform well in high signal-to-noise ratio situations. The WHOSE, PDF, and the contour methods are then applied to data from the eddy-resolving Ocean General Circulation Model for the Earth Simulator. While the three methods are in broad agreement on the location of the main ACC jets, the nature of the jet fields they produce differ. In particular, the WHOSE method reveals a fine-scale jet field with complex braiding behavior. It is argued that this fine-scale jet field may affect the calculation of eddy diffusivities. Finally, recommendations based on this study are made. The WHOSE and gradient thresholding methods are more suitable for the study of jets as localized strong currents, useful for studies of tracer fluxes. The contour and PDF methods are recommended for studies linking jets to hydrographic fronts.

Using kinetic energy measurements from altimetry to detect shifts in the positions of fronts in the Southern Ocean

“…These include changes in the ocean heat content in the upper ocean between the 1930s-1950s and the 1990s (e.g., Böning et al, 2008;Gille, 2008), increases in the heat content of deep water between the 1990s and 2005 (e.g., Purkey and Johnson, 2010), and increases in eddy kinetic energy in the Indian and Pacific oceans since 1993 (Hogg et al, 2015). Observational evidence of shifts in the winds, however, indicates that while there may be a slight southward shift in winds during the Southern Hemisphere summer, the overall yearly average shift is not significant (Swart and Fyfe, 2012).…”

Role of Mesoscale Eddies in Cross-Frontal Transport of Heat and Biogeochemical Tracers in the Southern Ocean

“…procedure (e.g., Sallée et al 2008a;Dufour et al 2011;Langlais et al 2011;Dufour et al 2013) that reconciles the hydrographic and dynamical views of the fronts (Langlais et al 2011;Chapman 2014). More precisely, for each front the method identifies a single SSH contour based on hydrographic criteria (e.g., temperature and salinity) and dynamic criteria (e.g., transport and SSH gradients).…”

“…More precisely, for each front the method identifies a single SSH contour based on hydrographic criteria (e.g., temperature and salinity) and dynamic criteria (e.g., transport and SSH gradients). For a review and comparison of the strength and weaknesses of the various jet detection methods, the reader is referred to Chapman (2014).…”

A Linear Decomposition of the Southern Ocean Thermohaline Structure

“…These fronts are prone to merging and separations as part of the intense eddy activity observed along the ACC. A number of frontal definitions were later proposed based on this same idea (Boning et al 2008;Sallée et al 2008;Langlais et al 2011;Chapman 2014;Kim and Orsi 2014), and it was found that SSH satellite observations were very useful to track fronts and eddies.…”