First full-depth monitoring


RV Solander in Darwin preparing for research expedition. Image: Jed Garland.

The Australian continent sits between the South Pacific and Indian Oceans, and its marine climate is influenced by the major current patterns in these two different ocean basins. As a result, Australia is draped by poleward currents on both sides:  the East Australian Current, which forms the western boundary current of the South Pacific subtropical gyre, and the Leeuwin Current, which forms the eastern boundary current of the Indian Ocean subtropical gyre.

 

The boundary currents impinge on Australia's continental shelf south of Broome in the west and Cooktown in the east. Above these latitudes, the two ocean basins are connected by large flows of warm equatorial surface water that pass from the western Pacific Ocean along the north side of Papua New Guinea, turn south at the western tip of Irian Jaya, flow as swift currents through deep water passages in the Indonesian Archipelago, and turn west into the Indian Ocean to form a strong current along the Indonesian continental margin.  This inter-basin exchange is known as the Indonesian Through Flow and it has a major influence on the climate of the Indian Ocean as well moderating the flow of the Leeuwin Current.

In 2010-11, CSIRO oceanographers used the AIMS vessel, RV Solander, to deploy moorings in the deep trenches on either side of Timor Leste - in the Timor Passage and the Ombai Strait - which are ‘chokepoints' in the global system of ocean currents. These moorings, which are up to three kilometres tall and anchored with four tonnes of weight, cost over a million dollars provided by Australia's Integrated Marine Observing System (IMOS). Carrying an array of oceanographic sensors, these moorings will form part of an international collaboration (the INSTANT project) to monitor interannual variations in the full depth transport of ITF. This is the only location in the global ocean where warm tropical water flows from one ocean basin to another, with a counter balancing flow of cold water at higher latitudes south of the Australian continent. These exchanges have to be well-calibrated to provide accurate models of global ocean circulations.

The largest signal of variation in the ITF is associated with the El Nino Southern Oscillation (ENSO) phenomenon, which also influences the strength of the Leeuwin Current flowing down the coast of Western Australia. To monitor the latter, AIMS' oceanographers in 2010-11 deployed an additional four moorings across the Sahul Shelf to extend the IMOS line from Timor to the coast near the border between WA and the Northern Territory.

La Niña events in the Pacific Ocean result in a stronger Leeuwin Current in the Indian Ocean. The strong La Niña of 2010-11 (responsible for Queensland's summer of extreme weather) also caused a marine heatwave along much of the coastline of Western Australia. In February 2011, sea surface temperatures from Exmouth to Geraldton were 3°C above average for the time of year, while a larger area extending south of Perth and 500km offshore was warmer by more than 2°C. For some locations, 2010-11 produced the hottest water ever recorded. The impact was coral bleaching from the Sahul Shelf to Rottnest Island as well as significant kills of fish and marine invertebrates.