July 2015
El Nino – what impact does it have in your area?
Matthew Lieschke
Senior Land Services Officer - Livestock
Goulburn
Last month’s article ‘El Nino – what does it mean?’ highlighted the need to look at the whole picture when making
business decisions and not just focus on what’s happening in the Pacific Ocean. The Indian Ocean Dipole is the
other major driver of rainfall in southern NSW and is just as important as the Pacific in terms of spring rainfall. This
important point is often missing from what we hear and read in the news. The other important thing to note is that
not all regions or areas will be impacted to the same degree by an El Nino event. So, which areas are most
susceptible to El Nino? The following looks at the historical impact of El Nino events on rainfall at Yass, Bigga,
Crookwell, Goulburn, Moss vale, Braidwood, Cooma, Berry and Bega.
Weather systems
The two key drivers of spring rainfall in southern NSW are:
1. El Nino Southern Isolation (ENSO). This takes place in the Pacific Ocean.
2. Indian Ocean Dipole (IOD). This occurs in the Indian Ocean.
Each Ocean has three phases: dry, neutral and wet. Therefore, as shown in Table 1 there are nine possible
combinations that can occur. Table 1 also shows the number of years in which the various combinations have
occurred from 1877 – 2006.
Table 1: ENSO and IOD events between 1877 and 2006*
Indian Ocean (IOD)
Pacific Ocean
(ENSO)
Wet
Dry
Neutral
(Negative IOD)
(Positive IOD)
PD/ IW
PD/ IN
PD/ ID
(1 year)
(12 years)
(7 years)
PN/ IW
PN/ IN
PN/ ID
(8 years)
(35 years)
(13 years)
PW/ IW
PW/ IN
PW/ ID
(7 years)
(26 years)
(1 year)
Dry (El Nino)
Neutral
Wet (La Nina)
*The brackets indicate the number of years in which the various combinations have occurred between 1877 and
2006 Source: adapted from Ummenhoffer et al 2010
www.lls.nsw.gov.au/southeast
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All the indicators at present are pointing towards continued El Nino conditions in the Pacific. The IOD is sitting in
neutral territory, but modelling suggests it could creep into the positive zone (dry). So you could say that the
outlook for spring is Dry in the Pacific (El Nino) and Neutral – Dry in the Indian Ocean. This corresponds to the top
right hand corner of Table 1.
Table 1 shows that in the 129 years that have been analysed (1877 – 2006) only 19 years there has been El Nino
conditions combined with either a neutral or dry Indian Ocean reading. So, if we go back and have a closer look at
these 19 years in question, was spring rainfall any different to what we normally get?
To answer this question we have pulled out the data for various locations across the South East and illustrated
the impact via box plots. If you aren’t familiar on how to interpret box plots, take a few moments to read the
following section before moving on to the results – it will make a lot more sense.
Check out the Climate Dogs
NSW DPI has developed some great videos to help explain how these climate systems work – go to:
http://www.dpi.nsw.gov.au/agriculture/resources/climate-and-weather/variability/climatedogs
How to read a box plot
A box plot is a graphical way to show the distribution (or spread) of data. Data is sorted from least to greatest and
then displayed using a box and ‘whiskers’ each side of the box. The key things to note are:
•
The box is where 50% of the data lies.
•
The line in the box is the median value (the middle year if you ranked them from lowest to highest). The
red cross is the average.
•
If the box and whiskers are relatively small (i.e. squashed together) it tells us that the spread between
years is low.
•
If we are concerned about how dry it might be then we focus on how the median changes as well as
where the bottom of the box sits.
Rainfall in local towns
We’ll begin by having a look at Goulburn as an example. The boxplots in Figure 1 illustrate how spring rainfall has
varied over the years depending on what has happened in the Pacific and Indian Oceans. In other words, the box
plots show spring rainfall variation for all the combinations presented in Table 1 for Goulburn. Box plots for the top
left and bottom right corners of Table 1 have been left out as there has only been 1 year in history when this has
occurred (El Nino + Wet Indian Ocean or La Nina and a Wet Indian Ocean).
Looking at Figure 1 you can see that the box plots are jumping around a bit depending on the ENSO/ IOD. The
PD/ IN and PD/ ID box plots (Pacific Dry/ Indian Neutral and Pacific Dry/ Indian Dry) are the major focus as these
conditions are currently in place and expected to continue well into spring. These two box plots are circled in red.
The ‘history’ box plot on the far right shows total rainfall (15 September to 15 November) for all years. We are
concentrating on early spring (15 September to 15 November) as this is the critical period between now and the
end of the year.
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Figure 1: Goulburn rainfall - impact of the Pacific and Indian Oceans on total rainfall between 15 September and 15 November (1877 – 2006)
To help compare the forecasted scenario with historical rainfall I have simplified the graph to just include the first two box plots and the history box plot.
This is show in Figure 2 (graph on the left).
www.lls.nsw.gov.au/southeast
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Figure 2 GOULBURN 15 September to 15 November
Figure 3: CROOKWELL 15 September to 15 November
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Goulburn – for the period 15 September to 15 November
Key points (Figure 2, left side graph)
•
Looking at two box plots on the left, the odds of getting a wet spring are substantially reduced.
•
The Indian Ocean has an impact. Springs when the Indian Ocean are neutral (PD/ IN) are not overly different from history – the median is about the
same, although the bottom line has shifted down a bit.
•
If the Indian Ocean enters into the dry phase (PD/ ID), history shows that spring rainfall suffers to a greater extent. The median and bottom line on the
box plot are both lower than history. Further, the box plot for this scenario is also much more contracted, so there is less potential for upside rainfall.
•
The box plots provide an indication about future rainfall. We also need to consider soil moisture at the end of winter (looking good at present). Work
from Yass shows that if we get 70mm in this period (onto top of a full moisture profile) then we will grow enough grass to get us through to January. If
the soil profile was only half full at the end of August then we would need 130mm of rain to get the same amount of feed. Looking at the plot above, the
odds for getting 70 mm is okay (it is not a certainty), but poor for 130mm.
Crookwell – for the period 15 September to 15 November
Key points (Figure 3, right side graph)
o
o
o
The rainfall box plots for Crookwell show greater spread (i.e. variation) compared to Goulburn, with greater ‘upside’ potential for rainfall
during the two month period.
The chance of getting average spring rainfall is reduced with the drying of the Indian Ocean, particularly if the IOD moves into positive dry
territory (indicated by the PD/ ID box plot). Relatively speaking, this shift or drying of the Indian Ocean has had a bigger impact at
Crookwell than Goulburn on historical rainfall in the 2 month period.
However, the chance of getting rainfall in Crookwell is greater than that for Goulburn - this is indicated by the higher positioning of the box
plots on the graph. Therefore, getting 70mm is more likely for this location, and getting 130mm (worst case scenario) is still within the
realms of possibly.
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Figure 4 YASS
1 September to 31 October
Figure 5: BIGGA
1 September to 31 October
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Yass – for the period 1 September to 31 October
Key points (Figure 4, left side graph)
o
o
o
The rainfall period being examined for Yass and Bigga is two weeks earlier than Goulburn and Crookwell. This is because the critical
period is slightly earlier for these two locations.
The impact of the Indian Ocean is more pronounced for Yass in the critical ‘window’ compared to Goulburn, indicating a greater chance of
being dry.
The chance of receiving 70mm during this window (on top of a full moisture profile at the end of August) is still okay, although the odds are
getting tougher. If we come out of August with a half full moisture profile, getting the 130mm required is looking highly unlikely for Yass.
Bigga – for the period 1 September to 31 October
Key points (Figure 5, right side graph)
o
o
o
The impact of a drying Indian Ocean appears to be having a bigger impact as we move further inland. This is indicated by the middle box
plot in Figure 5 for Bigga – you can see how the entire box plot for PD/ ID drops into the bottom 25% range against the history box plot.
The chances of getting 70mm in the critical window is okay and very similar to Yass, however the odds will get slightly tighter if the Indian
Ocean moves into positive territory. It would certainly be worth keeping an eye on the Indian Ocean during July and August to see what it
does.
If August is a dry month and the IOD is dry, the chances of getting 130mm during the critical window becomes even more unlikely.
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Figure 5 BRAIDWOOD
15 September to 15 November
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Figure 6: COOMA
15 September to 15 November
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Braidwood – for the period 15 September to 15 November
Key points (Figure 5, left side graph)
o
o
o
o
Looking at the box plot on the left (PD/ IN), the median value and the base of the box itself hasn’t changed much when comparing against
the same points on the history box plot. Historically speaking, this indicates that the Pacific Ocean (El Nino) hasn’t had a negative impact
on rainfall during the 2 month period.
Where the Indian Ocean moves into a dry phase (middle box plot), a drying effect is observed. As such, the likelihood of receiving 70mm
plus between mid September and mid November is reduced, but the odds are still pretty good.
If conditions are on the dry side at the end of August (i.e. soil profile is sitting at 50% of capacity) getting 130mm starts to become fairly
unlikely.
However, the major benefit of Braidwood is its location. Even if spring rainfall is well below average and conditions are dry, Braidwood’s
proximity to the coast means that it still has a much greater chance of receiving rain in late spring and into summer. This is a result of east
coast low systems. These act independently of the Pacific and Indian Oceans.
Cooma– for the period 15 September to 15 November
Key points (Figure 6, right side graph)
o
o
o
History shows that dry conditions in the Pacific and Indian oceans reduces the likelihood of rainfall, although the trend is not as strong
when compared to other locations such as Yass and Bigga. In other words, the box plots aren’t pulled as far ‘south’ on the graph in
comparison to the history box plot.
However, Cooma is the driest location of those that we’ve looked at so far, so the odds of getting 70mm + during the two month period are
reduced to begin with. The long term median rainfall for the period is 80mm (refer to the line in the history box plot).
The chances of getting median rainfall in the two month window are reduced under a dry Pacific and Indian Ocean scenario.
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Figure 7 MOSS VALE
15 September to 15 November
o
Key points:
o
o
o
History shows very little impact of El Nino and the Indian Ocean on spring rainfall. If anything, there is a slight
drying trend when the Indian Ocean goes from neutral to dry. This is shown by the lower median and lower
quartile in the middle box plot (PD/ ID).
Even if soil moisture levels are relatively dry at the end of August (i.e. 50% of full capacity) the chance of
getting 130mm is still pretty good.
Getting at least 70mm has even better odds. This would be all that’s needed if the profile is near capacity at
the end of winter.
Like Braidwood, Moss Vale has the ability to pick up significant summer rainfall from the coast. This takes the pressure of spring ra
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Summary
The Pacific and Indian Oceans both have a major influence of rainfall in south eastern Australia during winter and spring. Both have three
phases – dry, neutral and wet.
o
The impact of these weather systems is much stronger for some regions than others. As a general rule, the further you move inland, the
more likely that spring rainfall is going to be negatively impacted by El Nino. This drying effect is likely to be amplified if the Indian Ocean
moves from Neutral to Dry.
o
El Nino is locked in and is predicted to hang around until the end of the year. The main thing to watch now is what happens to the IOD.
o
The other key piece of information is the amount of soil moisture at the end of winter. If your moisture profile is only at 50% at the end of
August and you are located at Yass or Bigga, your risk profile going into summer will be increased.
o
Start thinking through management options now. What does your risk profile look like? What’s going to be your strategy? If your situation
is looking pessimistic it would be a good idea to start selling surplus stock now while stock prices are high. This could include dry cows,
wethers, etc. In terms of feeding stock, do you have enough feed on hand, or do you need to buy some in?
o
It’s critical that you assess your water supply. If your dams are well down at the end of winter your capacity to carry stock over the summer
will be severely compromised.
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© State of New South Wales through Local Land Services 2015. The information contained in this publication is based on knowledge and
understanding at the time of writing July 2015. However, because of advances in knowledge, users are reminded of the need to ensure that
the information upon which they rely is up to date and to check the currency of the information with the appropriate officer of Local Land
Services or the user’s independent adviser.
For updates go to www.lls.nsw.gov.au
[Insert reference number]
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