El Nino - Southern Oscillation

El Niño–Southern Oscillation (ENSO) is the dominant global mode of year-to-year (interannual) climate variability. It is a major contributor to Australia’s climate, and affects Australia’s Exclusive Economic Zone (EEZ) marine waters to differing degrees around the coast. ENSO has a strong and very significant effect on the intensity of the southward flowing Leeuwin Current, and Australia’s west coast waters, being transmitted by ENSO-generated planetary waves that propagate from the western Pacific Ocean through the Indonesian Throughflow which become trapped along Australia’s west coast. The ENSO signal in the Leeuwin Current is further transmitted along the south coast of Australia. ENSO is observed as a weaker signal in the southward flowing East Australian Current along Australia’s east coast. There have been several published studies of the effects of ENSO on marine biota and seabirds. Along Australia’s west coast, for example, La Niña events have been found to assist the transport of western rock lobster (Panulirus cygnus) larvae, while El Niño events are better for scallops. Further, ENSO is also known to disrupt seabird spawning and the seasonal migration of whale sharks (Rhincodon typus) in this region. The strength of the Leeuwin Current also influences recruitment of pilchard, whitebait, Australian salmon and herring along Australia’s south coast. To the east, there is evidence for a greater abundance of black marlin (Makaira indica) off northeast Australia during El Niño years, greater likelihood of unusually warm waters leading to coral bleaching during the late summer-autumn of the second year of El Niño events in the Great Barrier Reef, and loss of giant kelp ((Macrocystis pyrifera) off the east and south coasts of Tasmania associated with major El Niño events. Along Australia’s north coast, enhanced catches of banana prawns (Panaeus merguiensis) in the Gulf of Carpentaria during high rainfall/river flow La Niña years have also been reported. Based on the suite of IPCC AR4 model simulations forced with projected increases in greenhouse gases, the multi-model mean represents an overall weak shift in the background state towards future climate conditions which have been described as ‘El Niño-like’. While there is 80% consensus across the AR4 models for an ‘El Niño-like’ future climate (which could be interpreted as a shift in the mean), there is no consistent indication of future changes in ENSO amplitude or frequency. The pragmatic approach is to assume that ENSO events will continue as a source of significant interannual climate anomalies affecting the marine environment. Future ENSO events will be superimposed on 1) warmer sea surface temperatures than present – making El Niño impacts associated with unusually warm waters (e.g., coral bleaching) more severe, 2) more intense (though maybe fewer) tropical cyclones causing increased physical destruction of, for example, coral reef structures during La Niña events, c) more extreme rainfall (though changes in average rainfall are unclear), with more intense drought periods (exacerbated by warmer air temperatures) during El Niño events and more intense high rainfall events with increased freshwater/sediment flow to coastal environments during La Niña events, and d) higher sea levels which, as well as reducing land areas of island and cays (important nesting grounds for marine reptiles and seabirds), will increase impacts of tropical and extra-tropical cyclones. We expect a reduction in the overall intensity of the Leeuwin Current due to the projected change in background state towards a shallower thermocline in the Warm Pool region to the north of Australia – unless this is offset by changes in the alongshore winds and global hydrological cycle. Moving anti-clockwise around Australia’s coast from the northeast, we have high confidence that ENSO affects Australia’s northeast and north coast waters, medium/high confidence that ENSO affects Australia’s west coast wa