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In the last couple of years, the SIO eastward current (i.e., the tropical EGC and the subtropical SICC), have been found to influence the biology and biogeochemistry of the South Indian Ocean. For instance, Feng et al. (2011) find both currents to be important for the recruitment of the Australian western rock lobsters, the most valuable single-species fishery in Australia. Huhn et al. (2012) show that the boundaries of the Madagascar plankton bloom are controlled by the SICC. The EGC is also a key contributor to the biogeochemistry of the eastern Indian Ocean (Waite et al., 2013) and to the transport of larva from southern bluefin tuna (Matsuura et al., 1997).

Despite their importance and the recent growing number of studies, and some successful numerical simulations (e.g., McCreary et al., 2007; Divakaran and Brassington, 2011; Schott et al., 2009), our knowledge about the these currents are still poor, especially in relation to the SICC. For instance, little is known about the SICC spatial distribution, vertical structure and temporal variability as we will describe below. Even the basic dynamics controlling these eastward currents are still to be determined.

The SICC originates at the southern tip of Madagascar around 25°S, possibly fed by a partial retroflection of the East Madagascar Current (EMC) (Palastanga et al., 2007; Siedler et al., 2006, 2009). According to numerical experiments

conducted by Siedler et al. (2009) about 40% of the SICC transport originates from the EMC region. However, because the SICC is shallower than the EMC, Palastanga et al. (2007) suggest that only the near-surface part of the EMC might be involved in the retroflection. No specific studies have been done so far to evaluate the vertical structure of the EMC retroflection, to determine the EMC retroflection temporal variability and other possible source regions to the SICC. At the eastern end of the SICC, off Western Australia, little is known about how the SICC interacts with the Leeuwin Current system, the only poleward-flowing eastern boundary current of the global ocean, and with the tropical EGC (e.g., Schott et al., 2009; Divakaran and Brassington, 2011).

Sharma (1976) showed that his shallow Tropical Countercurrent (SICC) was associated with a thermal front around 25°S and suggested that this front would be a permanent feature. In spite of this, Palastanga et al. (2007) and Siedler et al. (2006) speculate that the SICC, instead, might be associated with a salinity front at the sea surface. Palastanga et al. (2007) did not find a thermal front at the surface in the SICC region using the WOA1 (World Ocean Atlas 2001) and satellite-derived sea surface temperature data. However, they found strong meridional salinity gradients at the sea surface between 20°S-26°S east of 75°E. This salinity front lies between the fresh tropical/Indonesian Throughflow waters carried westward by the SEC, and the saltier subtropical waters (Figure 1.13)(Rochford, 1962; Warren, 1981; Gordon et al., 1997; Wijffels et al., 2002; Katsumata and Fukasawa, 2011).

Both Palastanga et al. (2007) and Siedler et al. (2006) conjecture that the surface salinity front causes a secondary density front to exist in the band of 20°S-30°S, with the SICC being the associated current in terms of thermal wind balance. They stress, however, that whether and how this salinity frontal zone is linked to the existence of the SICC remains to be clarified.

Figure 1.13.: Mean sea surface salinity in the South Indian Ocean and surface geostrophic circulation relative to 1950 dbar for the 2004-2014 period based on the Roemmich-Gilson Argo Climatology 2015 (Roemmich and Gilson, 2009). Colours are used for salinity and and vectors for the direction and intensity of the surface geostrophic currents. Only vectors with intensities greater than 2 cm/s are plotted.

basin), and Huhn et al. (2012) (western basin), suggest that the SICC is organized into several jets, embedded in a broad and weaker eastward flow regime. The SICC geostrophic velocities have been described from 2-3 cm/s (Jia et al., 2011b) up to 50 cm/s (Siedler et al., 2006), depending on the region and time in which the velocities were estimated. This large range in the SICC strength indicates that it may experience strong temporal and spatial variability. To the best of our knowledge, no studies have been done to determine the dominating scales of these variabilities. For the tropical EGC, Meyers (1996) finds its transport is enhanced (reduced) during La Niña (El Niño) events.

Both Palastanga et al. (2007) and Jia et al. (2011b) describe the SICC being stronger in the austral summer and weaker in the winter. Sharma (1976)

already suggested that the SICC exhibits seasonal variability influenced by the monsoonal system, shifting poleward during the northeast monsoon (Dec- Feb/austral summer) and equatorward during the southwest monsoon (June- Aug/winter).

The SICC region is characterized by relatively high eddy kinetic energy (EKE) and large sea surface height variability (Siedler et al., 2006; Palastanga et al., 2007; Jia et al., 2011b,a). Palastanga et al. (2007), Jia et al. (2011a), and Jia et al. (2011b) attribute the relatively high EKE in the SICC region to baroclinic instability of the eastward SICC and the subsurface westward-flowing current. They suggest that the large EKE variations along 25°S may be related with changes in the strength and/or position of the SICC. The subtropical SIO is also characterized by westward propagating planetary waves from sources at the eastern boundary in several time scales, from intraseasonal to interannual (e.g., Birol and Morrow, 2001, 2003; Perigaud and Delecluse, 1993; Siedler et al., 2006; White, 2000; White et al., 2004, and references therein). How the westward propagating waves and eddies interact with the SICC is considered still unclear (Palastanga et al., 2009).

Several studies describe the SICC as a shallow eastward current in the upper 300 m depths (Sharma, 1976; Sharma et al., 1978; Palastanga et al., 2007; Schott et al., 2009; Jia et al., 2011b). Siedler et al. (2006), however, suggest that the SICC can reach 800 m or deeper, although its core is largely trapped in the top 200 m depth. The latter authors estimated the SICC transport is about 10 Sv based on three WOCE sections. Palastanga et al. (2007), however, consider that the deep-reaching eastward flows in two of these sections are due to anticyclonic eddies and do not represent the SICC. In the southeast Indian Ocean, Divakaran and Brassington (2011) describe the SICC vertical structure to be similar to that found by Siedler et al. (2006), such that the eastward flows can reach mid and abyssal depths, but with weaker intensities. Thus, even the mean vertical structure and volume transport of the SICC are, in fact, still uncertain.