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1. Introduction
The Antarctic Circumpolar Current (ACC) plays a crucial role in the Southern Ocean, and a knowledge of its structure and variability is essential for understanding global climate changes (Sloyan and Rintoul 2001; Toggweiler and Russell 2008; Morrison and Hogg 2013; Wang et al. 2014). Its path between subtropical and subpolar circulations is delineated by the Subtropical Front (STF) and the southern boundary of the ACC (SBdy); the bulk of its transport is carried out by multiple deep-reaching jets, of which three correspond to the circumpolar Subantarctic Front (SAF), the Polar Front (PF), and the southern ACC Front (sACCf) from north to south (see Fig. 1a; Orsi et al. 1995). This robust frontal system acts as a thermohaline boundary between subtropical and subpolar regimes (Naveira Garabato et al. 2011). Therefore, even a minor change in their meridional position could give a nontrivial impact, not only on volume transport (Morrison and Hogg 2013), but also on a hydrological cycle associated with sea ice extent (Liu and Curry 2010).
ACC fronts, defined as water mass boundaries, have been traced around Antarctica by means of specific indicators, that is, particular subsurface water property values from in situ hydrographic data (Nowlin and Clifford 1982; Orsi et al. 1995; Belkin and Gordon 1996). To overcome the spatial and temporal restrictions inherent in these observations, remote sensing data have been used to track sea surface signals associated with the fronts (e.g., Gille 1994; Dong et al. 2006). Sokolov and Rintoul (2002) suggested that a selection of contours in altimetry sea surface height (SSH) could effectively trace frontal locations in a region. Those particular streamlines appeared to be tightly aligned with narrow current jets observed between Tasmania and Antarctica (Sokolov and Rintoul 2007) and also throughout the Southern Ocean with no substantial changes during the altimetry era (Sokolov and Rintoul 2009a,b). Recently, the applicability of this contour-based approach has been a subject of debate, especially on detecting a regional current jet (Graham et al. 2012; Thompson and Sallée 2012; de Boer et al. 2013; Gille 2014). A comparison study between common jet detection methods (Chapman 2014), however, demonstrated that this contour method has advantages for studying dynamic jets linked with hydrographic fronts with superior accuracies at all signal-to-noise ratios (SNRs). Additional...