Why was 2010 our Third Wettest Year?
Following a decade of below average rainfall, few would have thought that 2010 would have delivered so much rainfall in the fashion that it did. Indeed last year was Australia’s third wettest year on record and Victoria’s wettest year since 1974. The period from July to December was Australia’s wettest ever
The major driver of above average rainfall in 2010 and early this year has been the strong La Nina in the tropical Pacific Ocean. Adding to this, especially during winter and spring was the negative Indian Ocean Dipole (IOD-ve). The last time both a La Nina and IOD-ve combined was 35 years ago in 1975. The 1975 spring rainfall has only been bettered by the spring rainfall of 1992.
Australian Winter/Spring Rainfall for 2010
Australian Spring rainfall for 1992
Australian Spring rainfall for 1975
The 2010/11 La Nina started to show its true colours around July, with the ocean and atmospheric drivers aligning and therefore reinforcing each other. Cooler than average surface and subsurface waters of the Pacific Ocean, sustained high values of the Southern Oscillation Index and a monthly December record (+27), together with strengthened easterly trade winds and strongly suppressed cloudiness around the dateline all signaled the establishment of a strong event.
The IOD-ve developed in early September, driven by warm water along the equator in the eastern Indian Ocean off Sumatra and, to a less degree, cooler waters off Africa. An IOD-ve often gives rise to increased rainfall from northwest cloud bands traveling across Australia from the waters off the northwest WA coast, bringing good rainfall for many Victorian regions north of the divide
The final major player in the big wet has been temperatures off Australia’s northern coasts. Sea surface temperatures in Australia’s tropical regions have been at or near record levels since September and this has contributed to record high humidity over Australia. With such high humidity, it was inevitable that when the right weather conditions occurred, significant rainfall would be the result.
As it always does at this time of year, the IOD-ve event has now dissipated as the Australian monsoon switches the wind patterns around in the eastern Indian Ocean. However, the La Nina remains and models suggest the La Nina will continue well into 2011.
We have analysed the rainfall for the years when La Nina and IOD-ve coincide. The years were classified by Meyers et al. 2007 and adapted by Umenhoffer et al. 2009.
The BOM uses a different method of classifying La Nina and IOD-ve years and their years should also be referred to at:
La Nina years: http://www.bom.gov.au/climate/enso/lnlist/
IOD negative years: http://www.bom.gov.au/climate/IOD/negative/
The La Nina/IOD-ve year that does coincide for the two classification methods is 1975.
Rainfall (mm) and decile classifications for Victorian locations in years when La Nina and IOD-ve coincide
(Meyers et al. 2007 and adapted by Umenhoffer et al. 2009).
| Location |
Duration |
1906 |
d |
1909 |
d |
1916 |
d |
1917 |
d |
1933 |
d |
1942 |
d |
1975 |
d |
2010 |
d |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Longerenong |
Jan-Dec |
563 |
10 |
531 |
9 |
494 |
9 |
424 |
6 |
483 |
8 |
565 |
10 |
535 |
9 |
624 |
10 |
|
|
Oct-Dec |
142 |
4 |
69 |
4 |
173 |
10 |
96 |
6 |
155 |
9 |
145 |
9 |
224 |
10 |
242 |
10 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Swan Hill |
Jan-Dec |
454 |
9 |
305 |
4 |
443 |
9 |
604 |
10 |
356 |
6 |
351 |
6 |
475 |
10 |
582 |
10 |
|
|
Oct-Dec |
63 |
4 |
22 |
1 |
136 |
9 |
185 |
10 |
90 |
6 |
83 |
6 |
248 |
10 |
236 |
10 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Yarrawonga |
Jan-Dec |
744 |
10 |
455 |
5 |
761 |
10 |
705 |
9 |
503 |
6 |
532 |
7 |
743 |
10 |
812 |
10 |
|
|
Oct-Dec |
209 |
9 |
34 |
1 |
268 |
10 |
225 |
10 |
197 |
9 |
175 |
8 |
363 |
10 |
362 |
10 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Hamilton |
Jan-Dec |
767 |
8 |
831 |
9 |
760 |
7 |
826 |
9 |
626 |
4 |
768 |
8 |
822 |
9 |
652 |
4 |
|
|
Oct-Dec |
212 |
9 |
154 |
5 |
239 |
10 |
180 |
7 |
188 |
7 |
101 |
1 |
209 |
8 |
165 |
6 |
Note: ‘d’ denotes decile
Links
To understand more about the drivers of rainfall mentioned in this report watch DPI’s new animation series the Climatedogs,.
You can also watch the December Monthly Climate Update on the DPI website to see what drove our climate during 2010.
Also to receive a monthly or quarterly industry updates on the climate sign up to one of DPI’s industry climate newsletters.
References
Meyers, G.A., P.C. McIntosh, L. Pigot and M.J. Pook, 2007: The years of El Nino, La Nina and interactions with the tropical Indian Ocean. J. Clim., 20, 2872-2880.
Ummenhofer, C.C., M.H. England, P.C. McIntosh, G.A. Meyers, M.J. Pook, J.S. Risbey, A. Sen Gupta and A.S. Taschetto, 2009: What causes southeast Australia’s worst droughts? Geophys. Res. Lett., 36, L04706, doi:10.1029/2008GL036801.
