Estimates of Surface and Subsurface Boundary Current Transport Around Australia

Abstract

A 15 year (2000–2014) simulation of the oceans around Australia, with the shelf‐scale model ozROMS, was used to estimate the mean, seasonal, and interannual variability of the surface and subsurface boundary currents and associated inflows. The simulation clarified some previous points of uncertainty and provided new information previously unknown and this is listed here. In the Indian Ocean, flow through the Timor Passage was linked to southeast Australia through the Holloway (HLC), Leeuwin (LC), South Australian (SAC), and Zeehan (ZC) Currents. The main inflows were from the Indonesian Throughflow and Eastern Gyral Current in the north whilst the central and southern branches of the South Indian Counter Current (SICC) provided major (>60%) inflows to the LC in the west. The HLC at North‐west Cape was at a maximum in April–May and its annual cycle accounted for 70% of the seasonal variance of LC, SAC, and ZC. In the Pacific Ocean, the northern branches of the South Equatorial Current were the main inputs to initiate the Hiri and East Australian (EAC) Currents flowing north and south, respectively, at ∼15°S. Inflow from the South Caledonia Jet to the EAC was ∼35%. The Flinders Current (FC) contributed to the Leeuwin Undercurrent (LU) directly as a northward flow and LU was enhanced from inflow from the subsurface southern SICC in the west (∼32–33°S). The majority of LU flowed westward offshore between 24 and 29°S while ∼25% continued northward to the northwest shelf. All Australian surface boundary currents systems were enhanced during the 2011–2013 La Niña.

Timing and Pacing of Indonesian Throughflow Restriction and Its Connection to Late Pliocene Climate Shifts

“…sacculifer data show that the Pliocene sea level lowstand culminating in MIS M2 (Miller et al, ) likely established an early form of the Sahul‐Indian Ocean Bjerknes mechanism leading to upwelling along the NWS (Di Nezio et al, ). This mechanism also enhanced seasonality and aridification on the Australian Continent after 3.3 Ma, by establishing a more seasonally variable LC (Godfrey & Mansbridge, ; Ridgway & Godfrey, ; Wijeratne et al, ). The permanently tilted thermocline by 3.3 Ma and heightened glacial/interglacial LC variability after ~3.2 Ma resulted in oceanographic conditions along the NWS that were much closer to their Pleistocene configuration characterized by a weaker (stronger) LC during glacials (interglacials) (Gallagher et al, ; Spooner et al, ). These insights also illustrate the significant role eastern Indian Ocean surface water conditions played during the inception of the Australian Transitional Interval (Christensen et al, ).…”

“…Their study suggests that sea level drops around 30 m are enough to significantly alter the hydroclimatic conditions along northwest Australia, leading to stronger upwelling and reduced humidity via the Sahul‐Indian Ocean Bjerknes mechanism with an increased east‐west tilt of the Indian Ocean thermocline. In summary, it appears that the heightened glacial/interglacial variability after 3.3 Ma resulted in oceanographic conditions along the NWS that were already comparable to its proposed Pleistocene configuration implying a stronger ITF and Leeuwin current during interglacials and a weaker ITF during glacials leading to increased seasonal upwelling of cool nutrient‐rich water masses of the Leeuwin Undercurrent (Gallagher et al, ; Godfrey & Mansbridge, ; Ridgway & Godfrey, ; Spooner et al, ; Wijeratne et al, ).…”

“…This reinvigorated ITF also strengthened the LC enough for LC eddy to again reach Site 763 by 3.2 Ma (De Vleeschouwer et al, ; Karas et al, ). This strong but seasonally variable LC is likely the effect of a stronger steric height gradient forcing the LC along the NWS after temperature and salinity gradients increased between the Indonesian Seaways and the south eastern Indian Ocean (D'Adamo et al, ; Furue et al, ; Gordon et al, ; Wijeratne et al, ), resulting in a LC much closer to its later Pleistocene configuration (Gallagher et al, ; Spooner et al, ). We thus propose that the reduced equatorial Pacific intermediate water component flowing into the Indian Ocean after ITF reconfiguration ~3.54 Ma was unable to warm the eastern Indian Ocean enough to reverse the east‐west tilt of the Indian Ocean thermocline that formed between 3.4 and 3.3 Ma (Figures , , and ).…”

“…Color gradients denote temperature (red‐blue) and salinity (green) gradients within the currents. Current paths along Western Australia were adapted from Furue et al (), Wijeratne et al (), and B. Wilson ()). The dominant throughflow paths and major equatorial Pacific and Indonesian currents are based on observations and models in Gordon et al () and Schiller et al (, ).…”

Role of Indian Ocean Dynamics on Accumulation of Buoyant Debris

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