Tuesday, June 26, 2012

Singular Spectral Analysis of the Summer (DJF) Median Maximum Temperature for Adelaide between 1888 and 2011

Figure 1 (below) shows the maximum daily temperature 
for the South Australian capital city of Adelaide for the
summer of 2009-10. The data comes from the High-
Quality Australian Daily Temperature Data Set

Figure 1:

Summers in Adelaide are characterized by frequent
hot-spells with maximum temperatures exceeding 35
degrees celsius. These hot-spells generally last for a
day or 
two before temperatures are moderated by 
flows of colder air from the south. 

The highly variable nature of maximum summer 
temperatures in Adelaide means that the best way
to characterize the average summer [December/
January/February (DJF)] maximum temperature 
is to use the median rather than the mean. 
 Figure 2 (below) shows the summer-time (DJF)
median maximum temperature for Adelaide between
1888 and 2011.  

Figure 2

Singular Spectral Analysis (SSA)
Preliminary Results:
If you use SSA to investigate the de-trended maximum 
temperature time series (see figure 2), you find that there 
are spectral peaks at all sub-multiples of the 22.3 year
Hale (H) cycle from H/2 to H/10. The most prominent
sub-harmonics are those at H/3, H/6, H/9 and H/10.

This result strongly suggests that the long-term median 
summer time maximum temperatures in Adelaide are
primarily being driven by factors that are associated 
with the 22.3 year Solar Hale Cycle. 

The presence of sub-harmonics in the temperature record
is indicative of the fact that the ~ 22 year forcing term
must have a broadened temporal structure that is 
triangular like in appearance.     

Figure 3

This result is broad general agreement with the results of
Thresher (2002) who finds that the variability in the strength
of the zonal west winds (along their northern margins) broadly
correlate with the 22 year sunspot cycle [see abstract below]. 

Hence, the most likely causation sequence is:

22 Hale cycle ---> strength of zonal west winds --->
the median summer maximum temperatures in Adelaide

with the strength of the zonal west winds depending directly
on the strength of the wind vorticity around low and high 
pressure cells in the Southern Hemisphere.


Int. J. Climatol. 22: 901–915 (2002)

Royal Meteorological Society.



Atmospheric circulation in the southern mid-latitudes is
dominated by strong circum-Antarctic zonal west winds
(ZWW) over the latitude range of 35 to 60 °S. These 
winds exhibit coherent seasonal and interannual variability, 
which has been related both to Antarctic (e.g. polar ice) 
and low-latitude climate (e.g. El Ni˜no–southern oscillation) 
parameters. Historical and recent studies suggest that, 
at its northern margins, variability in the ZWW also has 
a marked quasi-decadal component. Analysis of sea-level 
pressure and rainfall data for the Australian region, South 
Africa and South America confirms frequent indications 
of quasi-decadal variability in parameters associated with 
the ZWW, which appears to be in phase around the 
hemisphere. This variation broadly correlates with the 
sunspot cycle, and specifically appears to reflect 
sunspot-correlated, seasonally modulated shifts in the 
latitude range each year of the sub-tropical ridge over 
eastern Australia. Sunspot-correlated variability in the 
southern mid-latitudes is likely to have substantial effects 
on temperate climate and ecology and is consistent with 
recent models of solar effects on upper atmospheric climate, 
though the mechanisms that link these to winds and rainfall 
at sea level remain obscure. 

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