Explained: What factors lay behind recent flood emergencies in Sydney?

The latest disaster follows the start of Sydney’s wettest year, with overflowed levees and wet landscapes unable to absorb more rain that must instead flow into swollen waterways.

Below are the climatic, geographic and demographic factors behind the scale of the recent flood emergency in Sydney.

Is it the result of climate change?

New South Wales Premier Dominic Beirut said the government and communities need to adapt to the massive floods that are becoming more common in Australia’s most populous states.

“To see what we’re seeing across Sydney, there’s no doubt that these events are becoming more and more common,” Perrottet said on Monday (July 4).

Prime Minister Anthony Albanese said the fourth major flood event across Sydney after devastating bushfires in the same region during the 2019-20 Southern Hemisphere summer was evidence of the need for climate action.

“What we do know is that Australia has always been prone to floods and bushfires, but we know that science has told us that if we continue to not take action globally on climate change, then … extreme weather events will be more often and more intense (“) 6 July).

“What we’re seeing, unfortunately, is that it’s over,” Albanese added.

Or is it due to more specific and well-known meteorological phenomena?

Two La Niña weather patterns have resulted in above-average rain across Australia’s east coast in 2021 and this year. The second was announced last month, but the Met Office predicted that Sydney’s winter will be wetter than normal in the Southern Hemisphere, and a 50-50 chance of a La Niña return this year.

The bureau says two climate drivers have led to Sydney’s flooding since Saturday (July 2).

The flood was affected by the Indian Ocean Pole (IOD), which refers to the difference in sea surface temperatures between the western and eastern Indian Ocean.

In the negative phase, warmer waters are concentrated near Australia, resulting in above-average rainfall in the Southern Hemisphere in winter and spring as more moisture is available for weather systems that cross the continent.

The IOD has repeatedly fallen into its negative rain-carrying phase in the past month, and is expected to remain negative within months.

The second effect was positive southern annular status (SAM).

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 system directs more humid air than usual to eastern Australia, resulting in above-average rain and more east coast lows in winter.

During the last rainfall event, unusually warm waters off the Australian coast, from 21 to 23 degrees Celsius, provided additional energy and moisture to a deep, low basin on the East Coast, with heavy rainfall concentrated over a single 24-hour period that began at 9 a.m. Saturday .

Several rain gauges in Sydney and the surrounding area have been set in July or all records.

What role did geography and demography play?

Much of Sydney’s rainwater drains into a river system prone to sprawl, but economic interests have largely hampered moves to mitigate flooding.

The 22,000 square kilometer watershed covers the Blue Mountains on Sydney’s western edge and the city of 5 million western suburbs drains into the Hawkesbury-Neppen River system, which is the epicenter of some of the worst floods.

Jamie Pittock, a professor of environment and society at the Australian National University, said the river system is at high risk of flooding because gorges restrict rivers’ flow towards the sea, often causing water to bounce back quickly and spill across floodplains after heavy rains.

He said the Hawkesbury-Nepean Valley was home to 134,000 people and that number is expected to double by 2050 as Sydney’s population and property prices grow.

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“Potential economic returns from property development is a major driver of the lack of effective measures to reduce flood risk,” Pittock said.

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

Some argue that raising the wall would only control half of the floodwaters and not prevent major flooding caused by other rivers in the area, said Dale Domini Howes, professor of hazard and disaster sciences at the University of Sydney.

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