Rainfall trends

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Australia

Rainfall in Australia is highly variable over time and from region to region. Australian rainfall is driven by many different interacting weather systems and modes of natural variability caused by changes in relative sea surface temperatures and atmospheric circulation patterns. However, underlying longer-term trends associated with global warming are evident. There has been a significant drying across southern Australia since 1970, especially over the cool April to October growing season. This is the strongest recorded large-scale change in rainfall since national records began in 1900.

There has been increased rainfall across northern Australia since the 1970s, with heavy daily rainfall accounting for an increased proportion of rainfall. Because a warmer atmosphere contains more moisture, rainfall extremes are expected to become more frequent and intense as global average temperatures increase. Heavy rainfall events over most land areas have become more frequent in recent decades. However, trends have varied notably between regions and seasons.

Southern Australia

There has been a clear decline in SA’s rainfall since 1970, which has been linked, at least in part, to changes in large-scale atmospheric circulation associated with global warming. There are a variety of mechanisms playing a role in southern Australian rainfall variability and change, the most relevant is the westerly storm tracks. The fronts associated with the band of strong westerly winds and storms encircling Antarctica are a major source of cold fronts and rainfall for southern Australia during winter and spring.

There has been a reduction in the number of fronts impacting upon southern Australia due to the poleward shift of these westerly winds and the expansion of the tropical circulation. Rainfall has declined in the south of SA from April to October, and increased in the north from November to March since 1990.

The Southern Annular Mode (also known as the Antarctic Oscillation and SAM) reflects the north–south movement of the westerly wind belt. Over the past several decades, there has been an increasing tendency for this mode to remain in a ‘positive’ phase, with the westerly winds remaining contracted towards the Antarctic, leading to reduced winter rainfall over southern Australia.

The world’s tropical zones are expanding, pushing winter weather south while bringing increased tropical moisture during the summer months. The Hadley Circulation is a fundamental part of the global climate system. It is the atmospheric circulation that transports warm air poleward from the equator. As this cools and sinks, it creates an east–west band of high atmospheric pressure known as the subtropical ridge over the mid-latitudes of both hemispheres (including southern Australia) and is responsible for the relative aridity at these latitudes.

During the warmer half of the year (November to April), the subtropical ridge sits south of the Australian continent and acts to block southern rain-bearing fronts. It then moves north during the cooler months, allowing autumn and winter rains to reach southern Australia. The intensity of the subtropical ridge has been expanding and increasing in strength since 1970, with the effect that fewer rain-bearing fronts are reaching southern Australia. This correlates well with rising global temperatures over this time.

In southern Australia, the frequency of heavy rainfall, as well as total annual rainfall (Figure 15) has decreased, and this trend is expected to continue for SA.

Decreases in rainfall and rising temperatures are driving an increase in the occurrence of dangerous bushfire weather in spring and summer in eastern and southern Australia, including an earlier start to the fire season. In September 2017, state fire agencies issued severe to extreme fire weather warnings and Forest Fire Danger Index values exceeded extreme levels at many sites, as large parts of SA and New South Wales experienced temperatures that were more than 12°C warmer than the long-term average for this time of year and below average rainfall.

The subtropical ridge is projected to strengthen and move poleward over the course of the century. Cool-season rainfall is projected to decline across much of southern Australia and, although there is high confidence with this finding, the seasonal expression of change remains an ongoing area of research. The drying trend has large implications for natural systems and water security.

The Goyder Institute for Water Research has projected rainfall to continue to decrease in all regions and seasons in SA by the end of the century from 6.5% for the intermediate emissions scenario RCP4.5 in the South East to 26.9% for the high emissions scenario RCP8.5 in the Northern and Yorke regions, as shown in Table 8. However, given the large natural variability of rainfall, there is less confidence in such projections. The range of possible future climate-change scenarios may be larger, and it is advisable to consider multiple sources of climate projection information.

Table 8: Rainfall projections for South Australia to 2090

	Rainfall decline (%)
NRM region	Intermediate emissions scenario	High emissions scenario
Adelaide & Mount Lofty Ranges	7.8	17.4
Eyre Peninsula	10.0	20.9
Kangaroo Island	8.2	18.8
Northern and Yorke	14.1	26.9
SA Arid Lands and Alinytjara Wilurara	7.3	17.9
SA Murray–Darling Basin	11.4	21.7
South East	6.5	15.9

Source: Goyder Institute

Indian Ocean Dipole (IOD) and El Niño

Also affecting southern Australian rainfall are the more remote drivers of natural variability, the IOD and the El Niño, relating to the tropical Pacific Ocean.

The IOD is a measure of differences in the temperatures of the western and eastern equatorial Indian Ocean. A ‘positive’ event is caused by weakening westerly winds along the equator allowing warm water to shift to the west and cool water to rise up from the deep ocean in the east. This means there is less moisture than normal in the atmosphere to the northwest of Australia, often resulting in a reduction in winter and spring rainfall over central and southern Australia.

The number of positive events has been increasing over the past decades, consistent with atmospheric circulation changes caused by global warming. The increase in frequency has manifested as an increase in consecutive events, causing a multi-year accumulative rainfall reduction. The frequency of extreme positive IOD events is projected to increase by a factor of 3 this century.

Global warming is also increasing the likelihood of dry states associated with the El Niño–Southern Oscillation, by causing a weakening of the Walker Circulation in the Pacific Ocean and leading to conditions similar to the El Niño phase of the Southern Oscillation. El Niño events are usually associated with reduced winter and spring rainfall across eastern Australia, including most of SA. Major disruptions to Pacific rainfall have been projected to increase linearly over the 21^st century in response to global warming and continue for the remainder of this century even if global mean temperature has stabilised. When El Niño coincides with a positive IOD, the two phenomena can reinforce their dry impacts.

The CSIRO and BOM in their Projections for Australia's NRM Regions identify the following changes for the southern parts of SA with high to very high confidence:

• Average temperatures will continue to increase in all seasons.
• There will be more hot days and warm spells, and fewer frosts.
• Winter and spring rainfall will continue to decrease. Changes in other seasons are unclear, although results suggest a continuation of the observed autumn declines.
• There will be an increased intensity of extreme rainfall events.
• Mean sea levels will continue to rise and the height of extreme sea-level events will also increase.
• A harsher fire-weather climate will be experienced in the future.

On annual and decadal bases, natural variability in the climate system can act to either mask or enhance any long-term, human-induced trend, particularly for rainfall in the next 20 years.