Oceanography

As the earth spins the water moves. It radiates out from the equator in eddies that bump against land masses and slowly head towards the poles. While they may be rich with life in the tropics they are quickly depleted of nutrients as they travel south. They bring clear and warm water to our coastlines. There are four major currents in Australian waters: the East Australian Current (EAC), the Leeuwin, the Antarctic Circumpolar Current and the Indonesian Throughflow. These currents have a big impact on marine life, especially in determining where eggs and larvae of animals will go. If they are taken to unsuitable areas they die, affecting the productivity of reefs and commercial fisheries many years later. These currents are changing as the globe warms and are altering the pattern of our marine life. Warmer currents allow tropical species to ‘invade’ new territory further south, to the detriment of many species.

EAC

East Australian Current (EAC) Water from the tropics moves southwards down the Australian coast. This current is quite noticeable if you travel well offshore. Divers visiting offshore wrecks, or boaters with motor trouble, will be swept away by a 4 knot current in any spot away from the calming influence of the shallow seabed near the shore. The EAC brings water that is warm, clear and free of nutrients. That’s great for water activities but a poor source of food for marine animals. However, just like any stream the EAC has eddies, caused by factors like the shape of the seabed. These eddies suck up nutrients from the seafloor and cause localised blooms of plankton that feed fish, seabirds and sea mammals. When the winds blow onshore, the end result is often mass strandings of jellyfish and other planktonic animals on NSW beaches.

Off northern NSW the current stream splits (the Tasman Front), with half of it bathing Lord Howe Island in a mix of warm and colder water. The rest of the EAC moves southward towards eastern Tasmania, sometimes only reaching the north eastern tip, at other times almost to the southern tip of Tasmania. As the globe warms, a strengthening current is making Tasmania’s cold water species compete with new species. It is now warm enough for some of them to breed there. It gives us an insight into what might happen if global warming were to uniformly heat up all of the world’s ocean by the same amount.

Leeuwin and Zeehan Current

If the EAC is all about summer fun at warm beaches, the Leeuwin Current (LC) is strongest in winter. The LC flows down the Western Australian Coast to Cape Leeuwin before travelling east across the Great Australian Bight. In stronger years it can reach the west coast of Tasmania when it is known as the Zeehan Current. It can even round the southern coast and travel as far north as Freycinet Peninsula in eastern Tasmania. It is the longest current system in the world. The LC causes massive eddies that are often 100km across. These are often created by wind-driven counter currents flowing in the opposite direction including the Ningaloo, Capes and Flinders Currents. It has a big influence on stimulating some fisheries. The number of tropical species seen at Rottnest Island, or at the Busselton jetty observation station will vary depending on the strength of the LC and its countercurrents. When heat waves hit the Indian Ocean, the LC brought this warm water south, causing damage to corals, seaweed beds and fisheries in Western Australia.

Antarctic Circumpolar Current

The 20 000-kilometre-long Antarctic Circumpolar Current is considered to be the powerhouse for global climate. It connects the Atlantic, Pacific and Indian Oceans with an eastward flow equivalent to 150 times the combined flow of all the world’s rivers. This is an ocean current created by density and temperature variations in the water and flows well to the south of Australia. As the Antarctic water begins to freeze, very cold and salty water is left behind. The density of this water becomes quite high and it sinks below the surface layer. It moves deeper into the ocean, and also moves toward the equator. Eventually this water will warm, and as it does, it rises to the surface bringing with it nutrients from the deep seabed. It feeds huge quantities of krill, migratory fish, birds and marine mammals.

The Indonesian Throughflow

This system of currents brings warm water from the Pacific to the Indian Ocean via Indonesia. This is unique because it is the only area in which warm water from the equator flows from one ocean to another. It is an important source of heat transport to the Indian Ocean and affects regional sea surface temperatures and rainfall, including the Asian and Australian monsoons.

Are the oceans stable or changing? – ENSO

Australia’s weather is influenced by many climate drivers. A natural cycle known as the El Niño–Southern Oscillation (ENSO) has a very strong influence on year-to-year climate variability in Australia. It causes many months of warming (El Niño) or cooling (La Niña) weather in the central and eastern tropical Pacific, which in turn changes Australia’s weather. The ENSO cycle loosely operates over timescales from one to eight years. The warmth of recent El Niño events has been amplified by background warming trends which means that El Niño years have been tending to get warmer since the 1950s.

El Nino has a strong effect on the intensity of the southward flowing East Australian Current and Leeuwin Current. During La Niña (cool) years, Leeuwin Current is stronger in the winter, with higher sea surface temperature. Storms drive more rock lobster larvae inshore where they can settle, especially in the areas south of Geraldton. It also makes nutrient lie deeper in the water so that the wind-driven upwelling is less effective. This seems to reduce larvae settlement north of Houtman Abrolhos Islands. It also seems to disrupt seabird breeding and the seasonal migration of whale sharks. Unlike our farmers, scallops in Shark Bay love the low rainfall during an El Niño year. During the spawning season there is much less mortality caused by flushing of fresh water after rain.

Off the South West of Western Australia, stronger current patterns sweep away eggs and larvae of pilchards, Australian salmon and herring but strangely the climate variations seem to help whitebait. Stronger currents take eggs and larvae further, so the survivors can settle in more distant areas. Although Australian salmon can suffer off Western Australia, more settle along the distant sections of the southern coastline of Australia. On Australia’s east coast it is an El Niño event that strengthens the Eastern Australian Current in the summer. On the plus side it causes a greater abundance of black marlin off northeast Australia. Two years after an El Niño event, damselfish, strombus shells and green turtles seem to breed well. On the down side, there are significant decreases in seabird populations of the southern GBR as a result of reduced availability of food for hatchlings. Banana prawns are harder to catch in the Gulf of Carpentaria as they are less likely to be flushed out of the drier rivers. The environment is often able to recover from these intermittent impacts.

More serious are extreme events that cause damage to marine habitats and may be increasing in frequency as the world warms. El Niño events lead to an increased likelihood of unusually warm waters which leads to coral bleaching during the late summer-autumn of the second year of El Niño events in the Great Barrier Reef. This has contributed to the loss of over 50% of the coral cover in inshore reefs since the end of WWII. There has also been the virtually total loss of giant kelp ((Macrocystis pyrifera) off the east coasts of Tasmania associated with major El Niño events. The animals that depend on these habitats then also suffer.

Warmer waters also allows tropical species to settle in new areas, where they fail to establish if the water is too cold, but begin to breed if change (such as climate change) has made the area more suitable. These are new areas where they often face little competition. Some key species are able to alter the habitat, especially when they breed in very large numbers. They can devastate large areas. This has occurred to large parts of Tasmania’s east coast due to the invasive NSW black urchin. Tropical fish can also eat out seaweed beds off the Western Australian coast.

Upwellings

Hotspots for marine life Upwellings and downwelling describe mass movements of the ocean, which affect both surface and deep currents. These movements are essential in stirring the ocean, delivering oxygen to deep water, distributing heat, and bringing nutrients to the surface. Upwellings are common close to the coast when winds blowing along the coast displace water causing eddies that replace the displaced water with water from the depths. The nutrients that reach sunlight at the surface can be eaten by photosynthesising plankton. These phytoplankton (tiny alga) form the base of the ocean food web. Upwelling regions are less than 1 per cent of the world’s ocean by area, but account for greater than 20 percent of the global fish catch. Upwelling also occurs when surface waters diverge, such as where a major current splits. Currents can also cause localised small upwellings where the seabed topography disrupts current flow. You can see the effect on a micro-scale when diving or fishing around a rock, the upward movement of water will draw up nutrients. This then attracts school fish and invertebrate filter feeders wanting to feed on the denser marine life in the water column. The rock will be swarming with life like schools of baitfish, while the seabed around it is relatively bare. Australia has relatively small upwelling areas due to the dampening effects of its major currents. Regional-scale upwellings occur in WA, where the East Australia Current leaves the east coast off Sugar Loaf Point NSW, and in SE Australia, including the Victorian Bonney Coast. These upwellings are important food sources for plankton, fish, whales, seabirds, dolphins and seals. They support an active whale watching industry.