Changes in extreme weather - Four Degrees of Global Warming - page 28

Drought conditions across large parts of the eastern half of the continent
were dramatically broken by the La Niña event that began in the spring of
2010 (Braganza et al., 2011; Bureau of Meteorology, 2011). La Niña events
are associated with warmer-than-average sea surface temperatures in tropical
waters surrounding Australia and related changes in atmospheric circulation
and typically cause high rainfall in Australia. Due to background warming of the
oceans, the probability of La Niña events being associated with record tempera-
tures is increasing.
During the 2010 and 2011 monsoonal period, a record La Niña event, record
spring and summer rainfall over much of the Australian continent ( Figure 3.7 )
and record high sea-surface temperatures were registered in the Australian region
( Figure 3.8 ). The rainfall resulted in repeated severe flooding, in Queensland
and Victoria in particular. In February 2011, tropical cyclones Carlos and Yasi
brought extremely heavy rainfall around Darwin and the north Queensland
coast respectively. The heavy rainfall of 2010 and 2011 ranks alongside previous
strong La Niña events in the first half of the 1970s, the 1950s and around 1917.
However, the record monsoonal rainfall in the northern tropics was consistent
with a general trend of increased monsoonal rainfall across northern Australia in
recent decades.
Over Australia, trends in heavy rainfall depend on the period of analysis and
the metric of interest. For example, the annual number of days with more than
30mm of rain from 1950-2012 has decreased in much of southern and eastern
Australia, but increased in the north (similar to the trend in average rainfall:
Bureau of Meteorology, 2013). Similar results have been found for other metrics
(Gallant et al., 2007).
Tropical cyclones
For the 1981-2 to 2006-7 cyclone seasons, no significant trends in the total
numbers of cyclones or in the proportion of the most intense cyclones have been
found in the Australian region, South Indian Ocean or South Pacific Ocean
( Figure 3.9 : Kuleshov et al., 2010). Only limited conclusions can be drawn
regarding tropical cyclone intensity and numbers in the Australian region as
a whole prior to 1981, due to a lack of consistent data prior to the start of the
modern satellite period and the related issue of a small sample size of events
from which to analyze changes over time. For the Queensland coast from Cairns
southwards, there has been a decline in the frequency of severe land-falling
tropical cyclones over the last century (Callaghan and Power, 2011). Studies
using more data in the Northern Hemisphere show increased Atlantic hurricane
activity over the past century consistent with increasing sea surface temperatures;
however, definitive attribution of changes to increasing greenhouse gases remains
premature (Vecchi and Knutson, 2007).