Atmosphere Theme Report
Australia State of the Environment Report 2001 (Theme Report)
Lead Author: Dr Peter Manins, Environmental Consulting and Research Unit, CSIRO Atmospheric Research, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06746 9
Climate Variability and Change (continued)
El Nio-Southern Oscillation (ENSO) phenomenon [A Indicator 1.1]
The Southern Oscillation is a seesaw pattern of atmospheric pressure between the tropical central or eastern Pacific and the tropical eastern Indian Ocean or northern Australian region. After the seasonal cycle and the monsoon, ENSO is the most important pattern of variation to affect Australia at periods of less than a decade. El Nio events occur every two to seven years. However, over southern Australia, variations in climate are also caused by variations in the characteristics of frontal activity.
El Nio events are associated with anomalously warm sea surface temperature in the central and eastern Pacific Ocean, as well as higher than normal pressure in the Indonesian-north Australian region and lower than normal pressure in the central Pacific. There is also evidence for the occurrence of extended El Nio events that lasted several years in the 20th century.
El Nio events cause below-normal rainfall, and often drought, over much of northern and eastern Australia. The reverse is true during La Nia events. La Nia events are associated with anomalously cool sea surface temperature in the central and eastern Pacific Ocean, as well as below normal pressure in the Indonesian-north Australian region and higher than normal pressure in the central Pacific. These events usually lead to more rainfall in the Indonesian-north Australian regions and the effects extend well beyond, into most of the tropics and subtropics.
Individual El Nio and La Nia events vary in spatial extent, duration, and timing of onset and cessation. During El Nio years, tropical cyclone tracks shift away from the continent, while during La Nia years, tracks shift towards Australia. The upper panel of Figure 35 shows the atmospheric circulation induced in a vertical plane near the equator under El Nio conditions. The east-west or Walker Circulation covers much of the tropics. The Western Pacific is relatively cool with higher than normal pressure; this reduces the strength of the rain-producing systems, and hence reduces rainfall over northern and eastern Australia. El Nio events are often followed by La Nia events (lower panel of Figure 35). Higher sea surface temperatures over the western Pacific help to produce higher evaporation that leads to high rainfall over northern and eastern Australia. El Nio and La Nia events significantly influence the spatial and temporal patterns of rainfall variability within seasons.
Figure 35: East-west air circulation along the equator during El Nio and La Nia events.
The sea surface temperature variations during these contrasting conditions are indicated.
Source: Allan et al
The pressure difference between Papeete (Tahiti) and Darwin is often used as a measure of the Southern Oscillation, in the form of the SOI (Figure 36). Strong positive SOI values (higher pressure over Tahiti and lower over Darwin) are associated with La Nia events, while strong negative values indicate El Nio events. The SOI can thereby be used to represent the state of ENSO.
Figure 36: Annual variations in the Southern Oscillation Index.
Major floods associated with La Nia events (large positive values of the SOI) in the 1900s are also shown.
ENSO events themselves exhibit long-term variations that appear to be linked to decadal-to-century scale variations of sea surface temperatures in the Pacific (Power et al. 1999) and Indian Oceans. Thus, ENSO variations are modulated on decadal time scales by the Interdecadal Pacific Oscillation and Pacific Decadal Oscillation with resultant effects on the Australian climate over a long time scale. There has been a trend for more frequent El Nio events since 1976 (Trenberth & Hoar 1996, 1997). The long El Nio of 1990 to 1995 was followed by one of the strongest El Nio events on record during 1997 to 1998. The characteristics of El Nio may have changed during the past 25 years. El Nio events are normally associated with relatively dry conditions across eastern Australia, but since the mid-1980s, annual rainfall in eastern Australia has increased slightly, indicating a weakening relationship between ENSO and rainfall over Australia.
Several studies at CSIRO indicate that ENSO explains 30 to 40% of the year-to-year variability of Australia's climate. Variability of sea surface temperatures in the tropics is strongly associated with ENSO. The SOI is used as a tool for estimating the probability of rainfall one season ahead over much of eastern and northern Australia. At present, the values of the SOI and phases of the Index are being used in long-range forecasting of rainfall as well as in agricultural practices and management. Sea surface temperatures over the Indian and Pacific oceans have also been used in long-range forecasting of rainfall. A recent development is the use of global and regional climate models for seasonal forecasting of rainfall and mean, maximum and minimum temperatures.