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Sunday, January 21, 2007

Understanding impact of El Nino

It's often mentioned that 'El Nino' is what causes droughts in Australia and that it's because we're in an El Nino year that it's been so dry. So I thought I'd find out a bit more about El Nino and see if I could find a connection between El Nino and the Maryborough historical rainfall.

Here's what I learned (I wish I'd paid more attention in school now!)

For Australia, the key El Nino metric is the SOI = Southern Oscillation Index. As the bureau of meteorology says:

"The SOI is calculated using the pressure difference between
Tahiti and Darwin. The SOI gives an indication of the stage of
El Niño or La Niña events in the Pacific Ocean. The accuracy of
the SOI in determining the three-month outlook for rain around
Australia is highest between midwinter and midsummer in eastern
Australia. A strongly negative SOI (below -10) is characteristic
of El Niño, which is often associated with below average rainfall
over eastern Australia... A strongly positive SOI (above +10) is
characteristic of La Niña, which is often associated with above
average rainfall over parts of eastern Australia..."

Weatherzone's site expands on this a little:

"A strongly and consistently positive SOI pattern (e.g.
consistently above about +6 over a two month period) is
related to a high probability of above the long-term average
(median) rainfall for many areas of Australia, especially
areas of eastern Australia... Conversely, a 'deep' and
consistently negative SOI pattern (less than about minus 6
over a two month period, with little change over that period)
is related to a high probability of below median rainfall ...
However, it is important to remember that the pattern of
relationship between SOI and rainfall can vary depending on
the particular season and region. Additionally, the change
in SOI over a specified period can be as important in
understanding relationships between SOI and rainfall as is
the absolute value in SOI".

Finally, some more from the bureau of meteorology:

"El Nino events occur about every four to seven years
and typically last for around 12 to 18 months... Each
El Niño event is unique in terms in terms of its strength
(as measured by numbers such as the Southern Oscillation
Index or changes in ocean temperature), as well as its
impact in terms of altered rainfall patterns... For example,
the 1982/83 and 1997/98 events were both very strong as
measured by changes in the Pacific, yet their impacts in
Australia were completely different. Eastern and southern
Australia was gripped by severe drought in 1982/83, but
in 1997 average to above average falls were common in May,
and a dry spell over winter was broken by widespread and
heavy rains in September. Severe drought can sometimes
result from a relatively weak event, as occurred in 2002/03.
Furthermore, changes in the Indian Ocean can enhance the
general tendency for reduced rainfall in eastern Australia,
or mask it by contributing to timely falls".

OK. What I take from this is that the SOI is only one factor to consider so not a fully reliable indicator... but useful to some extent given we're in Eastern Australia.

So I decided to try and see if I could spot any correlation between monthly rainfall and monthly SOI for Maryborough.

Here's a graph showing the monthly SOI's. (Click on it if you want to enlarge). The data came from here.

el nino monthly SOI

A rough scan of this suggests that the most pronounced periods of El Nino and La Nina were as follows. The years in bold are those where the effect was possibly particularly strong, because there was such a prolonged and high spiked period:

El Nino (relating to dryer times):

  • 1878
  • 1881-1882
  • 1885
  • 1888
  • 1896-1897
  • 1905
  • 1911-1912
  • 1914-1915
  • 1919
  • 1923
  • 1925-1926
  • 1934
  • 1940-1941
  • 1953
  • 1957-1958
  • 1961
  • 1965-1966
  • 1969
  • 1972
  • 1977
  • 1982
  • 1987
  • 1991-1994
  • 1997
  • 2002
  • 2005

La Nina (relating to wetter times):

  • 1879-1880
  • 1886
  • 1889-1890
  • 1892-1893
  • 1898
  • 1903-1904
  • 1906
  • 1908
  • 1910
  • 1916-1917
  • 1921
  • 1924
  • 1928
  • 1931
  • 1938-1939
  • 1942-1943
  • 1945
  • 1950
  • 1954-1956
  • 1960
  • 1962
  • 1964
  • 1968
  • 1971
  • 1973-1975
  • 1981
  • 1988-89
  • 1996
  • 1999-2000

Here's what it looks like plotting each of those years against the historical Maryborough rainfall.

El Nino:
rainfall and elnino

La Nina:
rainfall and la nina

Overall, there's some correlation but it's not perfect. There are a few extreme anomalies that I don't understand, but maybe it's to do with the timing of the high SOI's (it's more pronounced in some seasons). Maybe for another iteration I can look at the seasonal factors in more detail, but I'm a bit el-nino'ed out for the moment! It's curious though.

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