Tuesday, 19 May 2015

Comparing temperature and rainfall data – with an example

In my previous post, I noted that rainfall extremes and changes are a much more reliable guide than temperature extremes and changes to global warming – because the latter can be distorted from circulation changes produced by the enhanced greenhouse effect:
  1. cooling temperatures via enhanced cloud cover in a super-monsoon 
  2. cooling temperatures via increased outgoing radiation in dry weather under the expanded Hadley circulation
  3. cooling temperatures via enhanced equatorward flow is also potentially possible
The main theme of my previous posts was to show how inverse relationships exist between temperatures for most places at the same latitude. Unlike with rainfall where it is possible for models to show which regions will become drier and which wetter with enhanced greenhouse gases, temperature changes do not show any “preference” so cases of temperature change locally cannot easily detect global warming. The following diagram comparing mean daily temperatures in January for the contiguous US and Alaska North Slope is illustrative:
If you look carefully, you will see that the large anomalies in the Alaska North Slope co-occur with large anomalies of opposite sign averaged over the contiguous US:
  • Spearman’s ρ = -0.49
  • Pearson’s r = -0.56
In contrast, we can see this graph of rainfall over major cities in southwestern Australia and Central Chile. In the previous case we would except co-occurring warmth but never observe it, but here we see synchronous drying, especially in the southern part of Chile between Curico and Valdivia:
The decline here is obvious, although the wet years observed in the “virgin period” are not coherent, the dry years from global warming are much more critically so.

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