MORE INFORMATION- LA NINA

La Niña and its Regional Effects on the Central Coast of NSW

The Central Coast region of NSW is not immune from the impact of the La Niña phenomenon. When the rest of eastern Australia experiences changing atmospheric conditions associated with a ‘sustained, negative, Sea Surface Temperature anomaly’ and with the connected shifts in the Walker Circulation, so does the Central Coast. When the wheat and wool growers of western NSW experience wonderful drought-breaking rains associated with the onset of a new La Niña episode, so, to a greater or lesser extent, does the Central Coast. 

Figure 1 below, records the estimated total annual streamflow in the Central Coast catchment for each year since rainfall records began in 1885. Also shown for comparative purposes are the average annual stream flow estimations over selected periods - i.e. the long term average for the whole period from 1885 to 2006; the average 1900-1949; the average 1950-1990; and the average 1991-2006.



Figure 1:  Central Coast Streamflows

(Source: “WaterPlan 2050 – A long-term water supply strategy for the Central Coast” Plan Adopted August 2007, Gosford-Wyong Councils’ Water Authority. p 2).

The positive impact of the 1954-1956, 1973-1974, 1988-89 and 1998-99 La Niña episodes on the estimated amount of water flowing in the streams and rivers of the Central Coast is clearly in evidence here, as is also the beneficial effects of the more recent drought-breaking rains associated with the developing La Niña that began in mid 2007 and gradually increased in intensity to an SOI value of + 21.3 in February 2008 (see Table 1).

 

Year

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

2004

-11.6

8.6

0.2

-15.4

13.1

-14.4

-6.9

-7.6

-2.8

-3.7

-9.3

-8.0

2005

1.8

-29.1

0.2

-11.2

-14.5

2.6

0.9

-6.9

3.9

10.9

-2.7

0.6

2006

12.7

0.1

13.8

15.2

-9.8

-5.5

-8.9

-15.9

-5.1

-15.3

-1.4

-3.0

2007

-7.3

-2.7

-1.4

-3.0

-2.7

5.0

-4.3

2.7

1.5

5.4

9.8

14.4

2008

14.1

21.3

12.2

4.5

-4.3

5.0

 

 

 

 

 

 

Table 1. Monthly values of the SOI for the years 2004 to 2008
(Source: Australian Bureau of Meteorology)

 

Summarising the pattern of the previous few months, the Australian Bureau of Meteorology at the end of February 2008 made the following remarks:

“The La Niña event in the Pacific basin is mature, and continues to influence the climate of eastern Australia. …

While weak warming has occurred in the far eastern Pacific Ocean, cooler than average sea surface temperatures (SSTs) continue to extend across the central equatorial Pacific. When combined with enhanced Trade Winds, suppressed cloudiness in the central Pacific and a strongly positive (+12) Southern Oscillation Index (SOI), it is clear that the atmosphere and ocean are firmly reinforcing each other, sustaining the La Niña event.

… All the dynamic computer models predict La Niña conditions until at least the end of the southern autumn.”

Up until May 2007, the Central Coast like the rest of NSW had been held in the grip of a particularly vicious drought associated with a prolonged El Niño episode (see the SOI values for 2004-2007 in Table 1 above). This drought had seen combined water storage levels in the Gosford City and Wyong Shire drop to as low as 13% of the total capacity in March 2007. Then, in June 2007, events began to take place which indicated that a change might be in the offing. The most dramatic of these were the intense storms and flooding rainfall experienced in the Central Coast and Hunter River region on the 8th and 9th of June (Queens Birthday weekend) which resulted in nine deaths and thousands of people being displaced from their homes over the next few days.

    

Figure 2: Flooding at Wyong Railway Bridge - 10 June 2007
(Source: NSW State Emergency Service)

The effect of the floods on peoples’ lives was devastating:

“8 June, 2007 will be a date my family and I will never forget. During that night, at about 9.00 pm we had a 25-metre gum tree come crashing through our bedroom, literally resting between my husband and me. My husband was on our computer and I had just gone to bed. The sound of the storm was scary enough but nothing could have prepared us for the sound of the tree crashing in on top of us. Along with the roof tiles, trusses and gyprock falling all around us, it was pouring with rain on top of us. Unbeknown to me at the time, the tree had actually hit my husband on the head. My first thought was to get to our four children who were at the other end of the house. Two thankfully had slept through it but the other two were awake and absolutely petrified. We had neighbours there within minutes helping in all kinds of ways. Some were helping me get the children’s things sorted and others were nailing our bedroom doors shut so no more rain would come in. We had rung the ambulance for my husband, and the SES also to help cover the house."

In the days and months that followed while we were in temporary accommodation we had help from so many different people that we otherwise would never had met. Six months after the storm, we finally moved back home. We will be forever grateful to the people that helped us along the way.”

Vanessa Malone
(Source: Central Coast Advocate, 4th June 2008)

The Queen’s Birthday weekend flooding was an outcome of a combination of low pressure atmospheric conditions off the coast of NSW that developed in connection with a low pressure trough over the northern Tasman Sea in the days immediately before the storms. Over the course of the month of June, five successive ‘east coast lows’ developed off the coast of NSW. The month of June 2007 turned out to be the 2nd coldest June on record for statewide average mean temperatures. There was also above to very much above average rainfall across much of the coast and adjacent ranges with some sites recording the highest June rainfall on record.


   
Figure 3: Satellite image: 8:30am 27 June 2007 - originally processed by the Bureau of Meteorology from the geostationary meteorological satellite MTSAT-1R operated by the Japan Meteorological Agency
(Source: Australian Bureau of Meteorology).   

It would now appear that the weather experienced in June 2007 was the forerunner of a trend towards La Niña conditions which gradually developed over the second half of 2007 and beginning of 2008. The effects of a typical La Niña episode began to manifest in November 2007. By February 2008 the SOI had reached a value of +21.3 which is the highest on record for a February SOI. Associated with the build up of La Niña atmospheric condition, much of Australia experienced a wet summer particularly in the eastern states, northern tropics and most of southern half of WA. Some parts did miss out, such as central Australia down into South Australia.

NSW experienced its wettest summer in 24 years. The statewide average rainfall for summer was 236.9 mm compared to the long term summer average of only 160.0 mm. The Central Coast was especially blessed by well above average summer rainfall without any serious flooding. Near Gosford, the Narara weather station recorded its highest summer rainfall in 88 years of recording; 909 mm fell between 1st December 2007 and 29th February 2008 compared with a long term summer rainfall average of 388 mm (see Table 2). It was also the coolest summer on record based on the mean daily maximums.

 

Recorded highest total summer rainfall

 

 

 

 

 

Total rainfall for summer 2007/08 (mm)

Previous highest for summer

Years of data

Average for summer

Gosford (Narara Research Station) AWS

908.8

907.2 in 1991

88

388.1

Recorded lowest summer mean daily max temp

 

 

 

 

 

Mean daily maximum temp for summer 2007/08 (°C)

Previous lowest for summer

Years of data

Average for summer

Gosford (Narara Research Station) AWS

25.6

25.7 in 1955

54

27.3

Table 2: Rainfall and maximum daily temperature for the period from 1st Dec 2007 to 29th Feb 2008 recorded at Narara Research Station near Gosford compared with long term historical data
( Source: Australian Bureau of Meteorology)

 

As a result of these favourable weather conditions, in the period 1st January 2008 to 10th March 2008, Somersby  received 288 mm more rainfall than in the same period in 2007; similarly,  Mardi received 191mm more and Mangrove Creek Dam 113 mm more than in the same period in 2007.

On 10th March 2008, the total dam storage for the Gosford-Wyong region was 26.6%. This was double the amount that was in storage in March 2007 when the total storage was only 13.9%. Water stored in Mangrove Creek Dam amounted to 22% of its capacity (41,800 ML) on 10th March 2008, compared to 10.4% (19,800 ML) on 12th March 2007.

As a consequence of these much improved storage statistics, Gosford City and Wyong Shire Council’s (http://www.gwcwater.nsw.gov.au/ ) eased water restrictions from 30 March 2008, when the Central Coast officially dropped back from Level 4 to Level 3 usage for the first time in 21 months.  

Clearly a good La Niña episode brings substantial benefits to the Central Coast in terms of summer rainfall.  The only concern once much of the region’s soil moisture reserves have been replenished by summer rainfall is the heightened risk of flooding during the season of east coast lows later in the year.