21-C Climate: Mackellar cycles

At Manilla, NSW, the anomaly of daily maximum temperature has continued to track, in the opposite sense, that of monthly rainfall.

Graphical log of smoothed rainfall and temperature.

The values shown are anomalies from normal values, smoothed to suppress cycles shorter than 12 months. (See notes below on Normals and Smoothing.)

The pattern is of quasi-biennial cycles that express the insight of Dorothea Mackellar that this is a land “of droughts and flooding rains*. Hot dry times alternate with cool wet times. For temperature, I have chosen the mean daily maximum, as it best matches the rainfall.

This post updates others in the Menu Category “Manilla NSW/21st century climate/Anomalies smoothed”, such as “17 years of ‘Droughts and Flooding Rains’ at Manilla” (29/06/2014).

“Droughts” (hot dry times)

Winter-spring 2002. The drought of 2002 was extreme, having rainfall in the lowest 1% in history. Lowest rainfall anomaly was in the winter and highest temperature anomaly in the spring.

Spring 2009. The temperature anomaly in spring 2009 was as high as in 2002, but the rainfall (as smoothed) barely qualified as “drought”.

Spring-summer 2013. The maximum temperature anomaly in spring 2013 was again like that in 2002 and 2009. This time, the rainfall minimum came later, in the summer. The drought was severe but not extreme.

Autumn-winter 2018. The temperature anomaly peak was higher than the earlier peaks. The minimum rainfall anomaly that followed in the winter was again extreme.

Summer 2018-19. At this time, the temperature anomaly was the highest, and the rainfall anomaly the lowest on this graph.

“Flooding Rains” (cool wet times)

Spring 2005. The spring of 2005 was wet, but the temperature was not cool but rather warm.

Summer 2007-8. Although the summer of 2007-8 was cool, rainfall was normal.

Spring 2010. The curves agreed in spring 2010, which was both cool and wet.

Spring-summer 2011-12. This was the time of the extreme occurrence of low temperature anomaly and high rainfall anomaly.

Winter-spring 2016. The winter of 2016 was wet (like the spring of 2005) and the spring was cool.

Temperature-rainfall relationship

The match between mean daily maximum temperature anomaly and monthly rainfall anomaly on this graph suggests that 10 mm of added rainfall generally corresponds to a fall in temperature of 0.7 degrees.

Trends

To show decadal trends in the data, I have fitted quartic polynomials. At the chosen scales on the two vertical axes, these curves are almost the same. From about 2003 to about 2011, rainfall increased, and daily maximum temperature decreased. The total change in the interval was +6 mm/month and -0.5 degrees respectively. Earlier than 2003 and later than 2011 the trends were for rainfall to decrease and for temperature to increase. These trends were also steeper.

The close match of the two smoothed traces and of the quartic trend lines suggests that the relation between temperature and rainfall did not change in this two-decade time interval at this site. The high temperature in the 2018-19 drought was no higher than would be expected in such a drought.


* By arrangement with the Licensor, The Dorothea Mackellar Estate, c/- Curtis Brown (Aust) Pty Ltd.


Notes

Normals

I established normals for my climate station at Manilla. I based them on daily readings for the decade beginning March 1999. I averaged the values for each calendar day, then fitted a harmonic curve. From the curve, I extracted means for calendar months. For rainfall, the monthly normals are means of the 125 years of readings from 1883 of Manilla Post Office Station 055031.

Smoothing

Smoothing uses a Gaussian function that has a Standard Deviation of 2.5 months, it spans 13 monthly data points, and has a half-width of 6 months, which suppresses cycles shorter than 12 months. The use of this smoothing function makes the last available smoothed data point six months earlier than the latest unsmoothed data.
If unsmoothed monthly data are listed in Column D of a spreadsheet and smoothed values are to appear in Column E, the weightings for the cell E197 are as follows:

E197=(D191*0.009+D192*0.022+D193*0.044+D194*0.078+D195*0.116+D196*0.149+D197*0.164+D198*0.149+D199*0.116+D200*0.078+D201*0.044+D202*0.022+D203*0.009)

[The edit program will not display this function. The function is symmetrical. The highest weighting is 0.164 for Cell D197. That for Cell D198 is 0.149, and so on.]

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s