CLIMATE CONDITIONS

INTRODUCTION

The supply and use of water in Australia needs to be assessed in the context of climate. In particular, the amount of rainfall in the year preceding the Water Account Australia and during the reference period is important, as this plays a large part in determining the amount of water available in the environment as surface and groundwater as well as in dams and other water storages.

COMPARISON OF 2008–09 TO 2009–10

Australian annual mean rainfall (an area weighted annual average of the total precipitation across Australia) in 2009–10 was 503 mm, a 4% decrease from the 522 mm reported in 2008–09.


Maps 1 to 4 show that while the national rainfall levels were similar, there were large differences in the geographical distribution of rainfall during the reference periods. From 2008–09 to 2009–10, rainfall in Victoria increased by 37%, from 498 mm to 682 mm. Western Australia also recorded a dramatic change in rainfall, with rainfall decreasing by 26% from 2008–09 (381 mm) to 2009–10 (281 mm).






CONDITIONS FOR 2009–10

The period from July 2009 to June 2010 saw average to high levels of rainfall for a majority of Australia and below average rainfalls in most of Western Australia. This resulted in the 26^th wettest July-June in 110 years of records (Map 2). The national area rainfall was 503mm, 8% above the 30 year average for 1960–1990.

Five States and Territories recorded rainfall above the 30 year average rainfall in 2009–10: Victoria, Queensland (16%), South Australia (22%), Tasmania (6%) and the Northern Territory (29%). Summer and autumn rains contributed significantly to the above average rainfalls, with Australia recording 28% and 13% above average rainfalls for these seasons. Victoria recorded its highest autumn rainfall since 1995 (8% above average) and New South Wales since 2000 (2% above average), ending long runs of drier-than-normal autumns in both states. Whilst the rains of 2009–10 eased the long term drought considerably, some areas with 3-year rainfalls in the lowest decile remained, particularly in Victoria

However, to highlight the variability of rainfall in Australia, Western Australia had low rainfall, having its 29^th driest July to June in 110 years (Maps 2 and 3).

The south-west was particularly dry, experiencing its 9^th driest July to June on record. The period included an exceptionally warm and dry summer for Western Australia. Much of the west coast experienced very low rainfall levels with several places recording no rain over the summer, including Perth Airport which recorded a 122 day dry spell, and the April to June period was also very dry in the south-west. The heatwave in Western Australia resulted in its hottest summer on record for both daytime maximums (1.52°C above normal) and overnight minimums (1.12°C above normal) as seen in Map 5 below.




Mean temperatures during 2009–10 were much warmer than normal, 0.87°C above the 1961 to 1990 mean, with each month recording warmer than average mean temperatures. It was the warmest July to June period on record for Australia, as well as for states of Western Australia, Victoria and Tasmania, with record mean annual temperatures covering 28% of Australia.

During August, daytime maximum temperature anomalies in the 3 to 5°C range occurred across the much of Australia, with Australia recording its warmest August on record (3.2°C above normal) and the highest temperature anomaly of any month. Both Queensland and the Northern Territory also had their warmest August on record (with 4.24°C and 3.91°C above normal). November was also an exceptionally warm month, particularly in the south eastern states, with Australia recording its second warmest November on record (2.12°C above normal). These hot spells also meant overnight minimums were elevated by up to 4°C during this time, with Australia setting a new November record (1.61°C above normal).


EL NINO / LA NINA PHENOMENA

The Southern Oscillation Index (SOI) calculates the air pressure difference between Tahiti and Darwin. The SOI in conjunction with the Walker Circulation gives an indication of the stage of El Niño or La Niña events in the Pacific Ocean.

A strongly negative SOI (below -10) is characteristic of El Niño, which is often associated with below average winter and spring rainfall over eastern Australia, and a weaker than normal monsoon in the north. A strongly positive SOI (above +10) is characteristic of La Niña, which is often associated with above average winter and spring rainfall over eastern Australia, and an earlier than normal start to the northern monsoon season with above-average rainfall during the tropical wet season.

In 2009-10, Australia experienced a weak El Niño event. The largest impact of El Niño occurred during spring with below average rainfall levels across much of Australia. After a relatively dry winter and spring, the warmer months exhibited rainfall patterns atypical of those normally associated with El Niño. In comparison, for much of 2008–09 (the period of the previous Water Account Australia) Australia was under the influence of La Niña. Consistent with the weather patterns associated with a weak La Niña, large areas of Australia had above average rainfall, particularly in the north of the continent, due to a strong monsoonal trough.