Australia: The Land Where Time Began
El Niño-Southern Oscillation (ENSO)
The ENSO system dominates the Australian climate at present. It is the swings of ocean surface temperatures and atmospheric circulation over the Pacific. The system moves warm water pools from west to east and back again, bringing drought and heavy rainfall to Australia, on a cycle that has been about 5 years long. Between 1788 and 1795, just as the first European settlement at Port Jackson, New South Wales, was being established there was an El Niņo event that resulted in drought in Australia. It has also been connected to crop failures in France with a general worsening of conditions some believe may have contributed to triggering the French Revolution (Grove, 2005). The 1997-1998 El Niņo led to devastating forest fires in Indonesia.
Fossil corals have shown that the climatic variations on the same scale as ENSO have been operating for at least 150,000 years (Tudhope et al., 2001). It is believed there are 2 main controls of the strength of the ENSO system. One is that the variability of ENSO seems to be greatest in interglacial phases, and less so during glacials. The other controlling factor that is believed to affect the variability of the strength of ENSO is predicted by climate models linking ENSO activity to the 22-year cycle of precession, the wobbles of orientation of the earth in relation to the sun, influencing the distribution of warmth from solar radiation over the equator (Clement et al., 1999). It is the interaction of these 2 cycles that control the variability of ENSO strength (Tudhope et al., 2001). Over the last 3,000-4,000 years these 2 cycles have coincided, leading to the most severe ENSO cycling of any period in the last 150,000 years. About 120,000 BP, in the last interglacial, ENSO was strong. For much of the last glacial phase it was of moderate to low strength, becoming very weak 6,500 years ago.
It has been suggested that about 400,000 BP there was an increase in ocean temperatures in the western Pacific that marked the beginning of ENSO (Kershaw et al., 2000,2003). It has been suggested that the northward movement of the Australian continent restricted the westward flow of water from the Pacific to the Indian Ocean as the northern margin of the continent ploughed into the island chain that stretched across its path, trapping a pool of warm water in the western Pacific. This West Pacific Warm Pool is the origin of the temperature gradient across the Pacific. It is believed possible that it was the accumulation of this warm pool that started up the ENSO system (Johnson, 2006).
The Southern Oscillation, represents variations in the Walker circulation (Walker cell), a conceptual model of the air flow of the tropics in the troposphere (the lower atmosphere), parcels of air that follow a closed circulation in the zonal and vertical direction. This is caused by heat distribution differences between the ocean and the land. The tropical atmosphere also has considerable meridional motion as part of the Hadley circulation, e.g. The variation of sea surface temperatures across the Pacific are also involved. El Niņo is the presence of unusually warm waters in the eastern pacific, especially along the tropical coast of South America. El Niņo and the Southern Oscillation are often represented by the term ENSO. (Enfield, 1989; Philander, 1990).
It has been found that pressures tend to below average over Australia and the Indian Ocean when they are above average over the central and eastern Pacific and vice versa. The term Southern Oscillation refers to the alternating pressure regimes between the western and eastern Pacific. When the pressure at Darwin are below average they tend to also be low at some other locations, such as the area from Australia to India and eastern Africa. The same 3 regions also experience above average pressures at the same time.
The Southern Oscillation Index (SOI) is designed to be a measure of the Southern Oscillation that can be displayed for each month. It is based on the difference between normalised pressure variations between Darwin and Tahiti, in the central Pacific Ocean. When pressures are low over the Australia-Indian Ocean region the SOI is positive and the pressures are high over the area of the central to eastern pacific, the opposite pressure patterns prevailing when the SOI is negative. The oscillation is not actually regular, as implied by the name, but strong extremes of the system can occur over a period of an irregular number of years, rather than a yearly 'oscillation'. The extreme events, that occur when the pattern of pressure remains unchanged across the Pacific, can last for up to 10 years. Areas affected by high pressure usually have reduced rain, rain being above average in years of low pressure, as occurred at the time of the La Nina of 2011, associated with widespread severe flooding over large parts of Australia and very destructive cyclones. At these times severe flooding can also be expected in parts of eastern Africa, Indonesia and India, as well as possibly southern China. During La Niņa events in Australia, islands of the central pacific and parts of the west coast of South America can be expected to experience below average rainfall. Locations widely separated around the world can be seen to experience similar conditions in times of positive or negative periods of the SOI.
When there are no extreme events occurring, La Niņa or El Niņo, surface ocean currents flow from the south and off the western tropical coast of South America, that are produced by south or southeast trade winds, are deflected to the west by the Coriolis Effect. As this surface water moves away from the South American coast it causes an upwelling of deep ocean water that is rich in nutrients which allows fish populations to flourish. The temperature gradient across the Pacific, between these cold waters from the depths, that cause the sea temperatures to be lower than would normally be the case in their tropical location, and the warm waters around Indonesia, is the driving force for the Walker circulation, an atmospheric circulation cell.
The warm air over Indonesia, being lighter, rises to produce the thick clouds and heavy rain of the tropics, resulting in low atmospheric pressure over the area. The atmospheric pressure over the eastern Pacific is high, the denser cold dry air sinking, returning to the west as easterly winds along the surface. This circulation cell, of westerly winds in the upper atmosphere and easterlies in the lower atmosphere, pull the surface water from east to west by the friction between the air and the water surface, piling the water higher in the western Pacific, as along the eastern coast of Australia, than would be the case if there were no easterly winds.
At times when the SOI is more positive the Walker cell is stronger, so the pull of the water across the Pacific is stronger, resulting in more cold water being dragged up in the east, resulting in a high temperature gradient of the ocean surface between the Australian-Indonesian area and the west coast of South America. When the SOI is high it is often called a La Niņa event, or in reference to the cold water along the South American coast, a 'cold event'. At these times the pressure over the Australian-Indonesian area drops further and that over the east Pacific increases. The increased pressure gradient leads to stronger easterlies, which in turn allows anomalies in the atmosphere and the ocean to reinforce each other, resulting in longer periods of such flows.
When the SOI is increasingly negative the easterlies weaken across the tropical Pacific, allowing the warm water to flow to the east, eventually displacing the cold water off the coast of South America, the presence of warm water in the central and eastern Pacific lowers the temperature gradient across the Pacific. This further weakens the Walker circulation, sometimes to the point where it breaks down, being replaced by a more complex circulation cell structure. Under these conditions cloud formation over Australia-Indonesia is reduced, so rainfall is less, the opposite occurring in the central and eastern Pacific, the pressure dropping over central and eastern areas of the ocean, which causes the SOI to become even more negative. The atmospheric and ocean anomalies again reinforce each other. The term El Niņo was used for the times when the ocean surface off the South American coast were unusually warm. At such times the effects on the marine ecosystems can be severe, as well as having severe effects on the human economy, with the anchovy fisheries of Peru crashing. According to Whetton (1997), evidence has also been found that a series of atmospheric waves that propagate into the Northern hemisphere can be induced by these warm waters.
Severe winter weather over North America has been associated with such atmospheric waves induced by some El Niņo events. Whetton says the knowledge of the events triggering El Nino events is poor (Whetton, 1997).
When the southeast trades weaken in the western pacific in about late summer-early autumn it is commonly believed that there is a good chance of an approaching El Niņo. There is also some evidence that markedly lower sea and air temperatures around northern Australia in early summer often indicates major drought in the winter to spring period (Nicholls, 1985). The triggering event is not known at the time of writing, and it has been suggested that the trigger may lie elsewhere (Enfield, 1989; Philander, 1989, 1990). El Niņo events have also been called 'warm events'.
SOI and Australian climate
Major floods and droughts have been associated with the SOI, though there are a few exceptions to this rule. Among the droughts those occurring in 1944 and 1967 did not occur when the SOI was strongly negative, and the only major flood that didn't occur at times of very positive SOI conditions was that of 1920-1921. Statistical analysis of the relationship between the SOI and rainfall in Australia found that the strength of this relationship varies at different places on the continent (Pittock, 1975; McBride and Nichols, 1983). It has been found that the relationship is at its highest along the eastern half of Australia, as seen in the 1982-1983 drought. Further work has shown that floods and droughts associated with the SOI can be forecast, but not with complete certainty. In some years predictions in early winter can be made for the winter and spring conditions in southeast Australia when the SOI becomes strongly positive or negative in the autumn, as they usually continue until the following summer. These predictions are not always accurate, as the atmosphere-ocean Southern Oscillation system does not always evolve in the way expected.
It has been found that the surface temperatures of the Indian Ocean influence to some extent winter rain that is partly independent of the Southern Oscillation (Nichols, 1989b). See Indian Ocean Dipole.
The tendency for above average pressures in the Australian region, as well as further west, at times when the SOI is strongly negative is the main reason for reduced rainfall at these times. In years of a strongly negative SOI many strong high pressure systems form over Australia during winter, causing the approaching rain-bearing systems in the westerlies to weaken and be deflected to the south. The low pressure trough over northern Australia in summer is weaker, leading to the weakening of summer rains, as well as often delaying their arrival. The flow of moisture that usually occurs is limited by the high pressure belt. In years when the high pressure belts are strong, and fronts in the westerlies are of low amplitude, the transfer of moisture that normally occurs as strong fronts in the westerlies draw large amounts of clouds and moisture southwards, greatly increasing the rainfall, especially over inland eastern Australia, are reduced and are less common. see Drosdowsky & Williams, 1991).
In strongly positive SOI years, associated with major floods, the circulation patterns are very different. During winter, with lower pressure areas over the continent, weaker high pressure systems that are often further north. The fronts and low pressure systems in the westerlies are stronger, extending further to the north, and often bringing large amounts of moisture over the continent from the tropics. Under these conditions the trough of low pressure over northern Australia is strongly developed, the monsoonal rain can be expected early and to bring more rainfall than at other times.
In El Niņo years, when the SOI is strongly negative, the waters around northern Australia tend to be cooler than normal (and warmer than average in La Niņa years). The temperatures of the waters of the eastern Pacific are opposite to those around Australia, being warmer when the Australian waters are cooler, and vice versa. At these times the lower ocean surface temperatures lead to lower evaporation and so reduced amounts of rain falling on the continent. The existing atmospheric pressure anomalies may also be reinforced by the sea surface temperature anomalies, as mentioned above. Computer modelling has shown that conditions as experienced during El Niņo events lead to reduced rainfall (Smith & Gordon, 1992).
Long term climate variation and Southern Oscillation
Based on the high degree of adaptation of the Australian biota to the erratic climate of the continent, with its climatic extremes that often go from drought to flood and back again, throughout a year in places, and from year to year, it has been suggested that the Southern Oscillation system may have been in operation for a very long time, possibly originating at about the time the continents reached approximately their modern configuration (Nicholls, 1989a).
Whetton suggests there may have been changes to the system over the period of its existence, possibly in regards to the frequency of swings from 'warm' to 'cold' episodes, and these changes may have occurred during the glacial periods when there were probably changes in global circulation patterns. It has been found that over the past 20,000 years there have been changes in the flow of rivers on the 3 southern continents of the western Pacific Ocean-Indian Ocean, the Darling in Australia, the Nile in Africa and rivers in central India, that are so similar they may possibly have been coordinated by a system such as the Southern Oscillation.
Implications for arid Australia of the ENSO intensification
ENSO has functioned in varying amplitude and frequency for the past 130,000 years, as indicated by palaeoclimatic studies (Tudhope et al., 2001), and about 5,000 BP the current ENSO cycle appears to have been established (Donders et al., 2007; Gagan et al., 2004; Moy et al., 2002; Quigley et al., 2010; Rein et al., 2005; Riedinger et al., 2002; Shulmeister, 1999). A range of dates, 5,500-5,000 BP, have been identified as the time of initiation of the latest cycle, by studies from both sides of the Pacific Ocean (Donders et al., 2007; Gagan et al., 2004; McCarthy & Head, 2001; Rein et al., 2005; Sandweiss et al., 1996, 2001; Shulmeister, 1999; Singh & Luly, 1991; Smith, 2009).
An overall increase in effective precipitation and enhancement of monsoon activity initially resulted from the onset of ENSO about 5,000 BP, but prolonged drying and increased variability of climate have characterised the last 4,000 years. This ENSO intensification between about 3,700 BP and about 2,000 BP resulted in conditions in Australia that were significantly drier, and in central Australia, increasing climatic variability (Donders et al., 2007; Gagan et al., 2004; Shulmeister, 2009). ENSO, and specifically El Niņo events, have all been demonstrated by a number of different datasets (Conroy et al., 2008; Huag et al., 2001; Moy et al., 2002; Rein et al., 2005) to be consistently stronger and more frequent during this period. Movement of the Inter-Tropical Convergence Zone (ITCZ) to the north, an increase of anti-cyclonic conditions overlying the region, with the result that the likelihood of monsoon or low-pressure systems, that were rain laden, reached into the interior of the Australian continent would be reduced. Increasing variability of climate would have resulted in occasional significant rainfall in the interior, though the overall effect of ENSO during this period would have been to reduce rainfall (Donders et al., 2007; Kotwicki &Allan, 1998; Sturman & Tapper, 1996; Suppiah, 1994).
The ENSO intensification, specifically El Niņo events, continued until about 1,500 BP, and may even have increased. From 2,000-1,700 BP or later, amelioration has been suggested by several Australian climatic datasets (Donders et al., 2007; Gagan et al., 2004; Lees et al., 1990; Luly, 1993; Mooney, 1997; Smith, 2009). Over the last 2,000 years the climate was considerably more stable than that of the mid-Holocene, though a couple of known climate shifts occurred, 1,200-800 BP, the Medieval Climate Anomaly (MCA), that was wetter, and the 600-200 BP, the Little Ice Age (LIA), a time when the climate was cooler (Dai & Wigley, 2,000; Sturman & Tapper, 1996; Williams et al., 2010).
ENSO - Impact on Maximum Temperature Extremes
Tulas - Are They Linked to ENSO in Australia?
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