EXPLAINER: Factors driving recent Sydney flood emergencies

CANBERRA, Australia (AP) — Parts of Australia’s largest city have been inundated by four major floods since March last year, leaving weary victims wondering how many times they can rebuild.

The latest disaster follows Sydney’s wettest start to the year with overflowing dams and a soggy landscape unable to absorb more rain which must instead trickle down in swollen waterways.

Here are the climatic, geographic and demographic factors driving the scale of Sydney’s latest flood emergency.

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LEADERS BLAME CLIMATE CHANGE

New South Wales Premier Dominic Perrottet said the government and communities had to adapt to major flooding which was becoming more frequent in Australia’s most populous state.

“To see what we’re seeing all over Sydney, there’s no doubt that these occurrences are becoming more and more commonplace,” Perrottet said on Monday.

Prime Minister Anthony Albanese said the fourth major flood across Sydney following devastating wildfires in the same region during the 2019-2020 Southern Hemisphere summer highlighted the need for climate action.

“What we know is that Australia has always been prone to flooding, bushfires, but we know science has told us that if we continue to fail to act globally on the climate change, so … extreme weather events would be more often and more intense,” Albanese said on Wednesday.

“What we see, unfortunately, is that it’s playing out,” Albanese added.

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LA NINA OUT, OTHER WEATHER PATTERNS IN

Two La Niña weather conditions brought above average rainfall to the east coast of Australia in 2021 and this year. The second was declared last month, but the Bureau of Meteorology predicts a wetter than usual Southern Hemisphere winter for Sydney and a 50 to 50 chance of La Niña returning this year.

The office says two climatic factors have led to flooding in Sydney since Saturday.

The flooding was influenced by the Indian Ocean Dipole, which refers to the sea surface temperature difference between the western and eastern Indian Ocean. In the negative phase, warmer waters concentrate near Australia, resulting in above-average winter-spring precipitation in the Southern Hemisphere, as more moisture is available for weather systems crossing the continent. The IOD has repeatedly dipped into its rainy negative phase over the past month and is expected to remain negative for the next few months.

A second influence was the positive south annular mode. SAM refers to the non-seasonal north-south movement of strong westerly winds that blow almost continuously in the middle to high latitudes of the Southern Hemisphere. In the positive phase, the SAM directs more moisture-filled air than usual into eastern Australia, leading to above-average rainfall and more lows on the east coast in winter.

During the latest rain event, extraordinarily warm waters off the Australian coast of 21 to 23 degrees Celsius (70 to 73 degrees Fahrenheit) provided additional energy and moisture to a deep low and a east coast depression, concentrating heavy rains in 24 hour period. which began at 9 a.m. Saturday.

Several rain gauges in and around Sydney have set July or all-time records.

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GEOGRAPHY AND DEMOGRAPHY

Much of Sydney’s rain runs off into a potentially overflowing river system, but economic interests have largely stalled measures to mitigate flooding.

A 22,000 square kilometer (8,500 sq mi) watershed covering the Blue Mountains on the western fringe of Sydney and the western suburbs of the city of 5 million empties into the Hawkesbury-Nepean river system, which is l epicenter of some of the worst flooding.

The river system faces extreme flood risk as gorges restrict the flow of rivers to the sea, often causing water to rise rapidly and spill across the floodplain after heavy rains, said Jamie Pittock , Professor of Environment and Society at the Australian National University.

The Hawkesbury-Nepean Valley was home to 134,000 people and that population is expected to double by 2050 as Sydney’s population and property prices rise, he said.

“The potential economic returns from property development are a key driver of the lack of effective action to reduce flood risk,” Pittock said.

The state government wants to raise the wall of the Warragamba Dam, Sydney’s main reservoir, to reduce flooding in the valley.

But some say raising the wall would only control half of the floodwaters and not prevent major flooding caused by other rivers in the region, said Dale Dominey-Howes, professor of risk and disaster sciences. disaster risk at the University of Sydney.

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