Wednesday, 11 September 2019

Northern NSW likely to remain in drought for the foreseeable future

With the Clarence Valley hinterland in drought and water in the upper reaches of the Clarence River system already low, the following article is not good news for valley communities.

Indeed if the comparison with 2002 holds, then there is a possibility that freshwater entering the tidal pool just below the Lilydale gauge will eventually fall from around the current 286 megalitres daily (less than 10% of historical est. average daily flow) to around 50 megalitres a day.

ABC News, 6 September 2019: 

A rare event that took place 30 kilometres above the South Pole last week is expected to impact upon Australia's rainfall outlook. 

The upper atmosphere above Antarctica warmed by as much as 40 degrees Celsius in the course of a few days — and it is continuing to warm. 

This rare phenomenon, known as sudden stratospheric warming (SSW), could deepen one of the worst droughts in Australian history. 

The Bureau of Meteorology's Harry Hendon warned of dry weather ahead. "We will typically see conditions across most of Australia, but primarily concentrated in the eastern part of Australia, become warmer and drier through spring and into early summer," Dr Hendon said. 

SSW is rare in the southern hemisphere with only one major event ever identified, in 2002 — one of Australia's driest years on record.... 

Sudden stratospheric warming over Antarctica causes westerly winds south of Australia to track further north, a pattern meteorologists refer to as a 'negative SAM'. In spring and summer, this negative SAM pattern brings warmer, drier air into southern Queensland and New South Wales. 

"Unfortunately, these are areas already in drought," said a lead author of the BOM's spring climate outlook, Andrew Watkins. 

Dr Watkins said cooler than normal water in the Indian Ocean, a phenomenon meteorologists call a 'positive IOD', has led to a lack of moisture drifting over the continent. 

"This has certainly been a big factor in why winter has been so dry in virtually all of Australia," he said. 

"On top of that, we have the likelihood of prolonged periods of negative SAM, which also brings drier conditions to New South Wales and southern Queensland. 

"So it's a bit of a double whammy in those locations." Dr Watkins said the impact of the SSW may be felt in Australia through to the end of the year. 

"These sudden stratospheric warming events and the patterns that we see from them can go from September [to] October, sometimes persisting through to January," he said. 

Dr Hendon said he was gratified the Bureau of Meteorology's computer models were able to predict the event. 

"In 2002 we didn't even know about it until after it happened, and we didn't know if we would ever be able to predict it," he said. "It's exciting for us now that we have predictive capability that we didn't have in 2002."

NSW Department of Primary Industries, Combined Drought Indicator:


Bureau of Meteorology, 29 August 2019, media release, excerpt: 


Spring outlook shows: 

 • Daytime temperatures are likely to be warmer than average across the entire state. Overnight temperatures are also likely to be warmer than average across most of the state, with the highest likelihood in the north. 

• A higher likelihood of drier than average conditions in the coming three months across most of the state. 

Preliminary winter summary shows: 

 • Temperatures in New South Wales have been above average. Daytime temperatures are likely to rank among the warmest 10 winters on record. 

• Rainfall has been below average. 

• Likely to be among Sydney's three warmest winters on record for daytime temperatures, while rainfall was close to average.

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