The connection of the Indonesian Throughflow, South Indian Ocean Countercurrent and the Leeuwin Current

Abstract

Abstract. East of Madagascar, the shallow "South Indian Ocean Counter Current (SICC)" flows from west to east across the Indian Ocean against the direction of the winddriven circulation. The SICC impinges on west Australia and enhances the sea level slope, strengthening the alongshore coastal jet: the Leeuwin Current (LC), which flows poleward along Australia. An observed transport maximum of the LC around 22 • S can likely be attributed to this impingement of the SICC. The LC is often described as a regional coastal current that is forced by an offshore meridional density gradient or sea surface slope. However, little is known about the controls of these open-ocean gradients. The regional circulation system is embedded in the subtropical "super gyre" that connects the Indo-Pacific via the Tasman Gateway and the Indonesian passages. The Indonesian Throughflow (ITF) circulates through the Indian Ocean back into the Pacific south of Australia. This return pathway appears to be partly trapped in the upper layer north of an outcrop line. It is redirected along this outcrop line and joins the eastward flow of the SICC. To study the connection of the basin-scale and the inter-oceanscale dynamics, we apply both an ocean general circulation model and a conceptual two-layer model. Shutdown of the ITF in the models leads to a large decrease in Leeuwin Current transport. Most of the SICC was found to then reconnect to the internal gyre circulation in the Indian Ocean. ITF, SICC and LC thus appear to be dynamically connected.

Estimates of Surface and Subsurface Boundary Current Transport Around Australia

“…Increase in LC transport between transects W-1 and W-2 (Table 1) can be attributed to the inflow from the cSICC that feeds the LC between 228S and 248S (see also Lambert et al, 2016). The southern SICC (sSICC) flows almost zonally from Madagascar to Australia along 268S, with a slight poleward orientation (Lambert et al, 2016;Menezes et al, 2014). There was an increase of 1.2 Sv in the southward transport in the LC between transects W-3 and W-5 where the sSICC input to the LC (Table 1).…”

“…The ITF contributes to the South Equatorial Current (SEC), the major westward flow across the Indian Ocean to the eastern Indian Ocean. A portion of the SEC returns eastward as the South Indian Counter Current (SICC) (Lambert et al, 2016;Menezes et al, 2013Menezes et al, , 2014Menezes et al, , 2016. Part of the SEC recirculates eastward as the Eastern Gyre (EG).…”

Role of Indian Ocean Dynamics on Accumulation of Buoyant Debris

“…The SICC flows eastward between 22 • S and 30 • S (Figure 5b; Wijeratne et al, 2018;Menezes et al, 2014), through the northern part of the drogued SIO accumulation region. When the SICC reaches the coast of Western Australia, it feeds into the Leeuwin Current (LC; Lambert et al, 2016). The LC flows poleward, wrapping around the southern Australian coastline (geostrophic current velocities in light blue in Figure 5b, also seen in yellow EKE g in Figure 5d; Pattiaratchi & Woo, 2009;Yit Sen Bull & van Sebille, 2016).…”

“…The South Indian Counter Current (SICC; see Figure 1b in Lambert et al, 2016) flows eastward through the center of the subtropical gyre, opposite to the direction expected from Sverdrup theory (Palastanga et al, 2007;Wijeratne et al, 2018). Subtropical countercurrents exist in the other oceans as well, but they dissolve halfway through the basin.…”

On the Leeuwin Current System and Its Linkage to Zonal Flows in the South Indian Ocean as Inferred from a Gridded Hydrography

“…The first is the Indonesian Throughflow (ITF), which raises sea level north of Australia; the higher sea level extends southward along the WA coast through the propagation of coastally trapped waves, thereby establishing a zonal-pressure-gradient field that drives the LC (e.g., Godfrey and Ridgway 1985;Kundu and McCreary 1986). The second is near-surface, eastward flow across the interior of the SIO [section 1a(3)], which bends southward at the WA coast to form the LC Thompson 1987;Weaver andMiddleton 1989, 1990;Furue et al 2013;Benthuysen et al 2014;Lambert et al 2016). A general result from the second mechanism is that a southward-flowing LC can be produced, provided the onshore interior transport is large enough to overwhelm offshore Ekman drift driven by the prevailing southerly winds.…”