This year, the south-west of Western Australia has recorded some unexpectedly low temperatures. Has this been due to rainfall, cloud, winds, or the cooler than normal Leeuwin Current and Sea Surface Temperatures in the South West Region?
In this post I examine maximum temperature and rainfall data for Winter in South-Western Australia, and Sea Surface Temperature data for the South West Region, all straight from the Bureau of Meteorology’s Climate Change time series page .
All temperature data are in degrees Celsius anomalies from the 1961-90 average.
Figure 1 is a map showing the various Sea Surface Temperature monitoring regions around Australia.
The Southwest Region is just to the west and southwest of the Southwest climate region, and winter south westerlies impact this part of the continent first. 2016’s winter has seen maxima drop sharply. In fact, it was the coldest winter since 1993:
Fig. 2: Southwestern Australia Winter TMax Anomalies
There is a relationship between rainfall and Tmax- as rain goes up, Tmax goes down, so here south west rainfall is inverted and scaled down by 100:
Fig. 3: TMax and Rain:
The next plot shows TMax and the South West Region’s Sea Surface Temperature anomalies (SST):
Fig. 4: TMax & SST:
Again, related: both have strong warming from the 1970s. Next I check for whether there was a real change in direction in the 1970s, and if so, when. To do this I use CuSums.
Fig. 5: CuSums of Winter TMax and SST compared:
Both have a distinct change point: 1975, with SST warming since, but TMax appears to have a step up, with another change point at 1993 with strong warming since. Rainfall however shows a different picture:
Fig. 6: CuSums of Winter Rainfall
Note the major change at 1968 (a step down: see Figure 3), another at 1975 with increasing rain to the next change point at 2000, after which rain rapidly decreases.
I now plot TMax against rainfall and SST to see which has the greater influence. First, Rain:
Fig. 7: TMax vs Rain:
100mm more rain is associated with about 0.5C lower TMax, but R-squared is only 0.22.
Fig. 8: TMax vs SST:
A one degree increase in SST is associated with more than 1.1C increase in TMax, and R-squared is above 0.51- a much closer fit, but still little better than fifty-fifty.
TMax is affected by rain, but more by SSTs.
I now look at data since the major change points in the 1975 winter. The next three figures show trends in SST, Rain, and TMax.
Fig. 9: Trends in SST:
Warming since 1975 of +1.48C/ 100 years.
Fig. 10: Trends in Rainfall:
Decreasing since 1975 at 89mm per 100 years (and much more from 2000).
Fig. 11: Trends in TMax:
Warming since 1975 at +2.14C per 100 years.
Detrending the data allows us to see where any of the winters “bucks the trend”. In the following plots, the line at zero represents the trend as shown above.
Fig. 12: SST Detrended:
Fig. 13: Rainfall Detrended:
Fig. 14: TMax Detrended:
Note that SST in 2016 is just below trend, but still above the 1961-90 average. Rainfall is only slightly above trend, and still below average. However TMax is well below trend, and well below average, showing the greatest 12 month drop in temperatures of any winter since 1975.
My conclusions (and you are welcome to comment, dispute, and suggest your own):
- Maximum temperatures in winter in Southwestern Australia are affected by rainfall, but to a much larger extent by Sea Surface Temperature of the South West Region.
- The large decrease in winter temperature this year cannot be explained by rainfall or sea surface temperature. Cloudiness may be a factor, but no 2016 data are publicly available. Stronger winds blowing from further south may be responsible.