Tuesday, 8 November 2016

An examination of CET versus global temperature anomalies: Part III – Conclusions

In our previous two posts relating CET anomalies to global temperature anomalies – for July to December here and for January to June here.

To finish the survey this post will give an overall examination of the observed correlations for each month and for all months. The first step will be to graph the observed CET anomaly versus global temperature anomaly correlation coefficients between 1880 and 1974 for each of the twelve calendar months. They are arranged in fiscal year order in the graph to fit in with my previous research on CET – a choice made because temperature anomalies are larger in winter than in summer.
Monthly Central England Temperature anomaly versus global temperature anomaly correlation coefficients, 1880 to 1974
Before I analysed each month separately, I hypothesised that there would be three plausible patterns of variation in monthly CET anomaly versus global temperature anomaly correlation:
  1. There would be no difference in correlation for all months
  2. There would be a maximum in summer (when natural variance of CET and global temperature anomalies are smallest) and minimum in winter (when they are largest because air advection influences are greatest and most independent of greenhouse gas concentrations)
  3. There would be maximum in autumn (when CET has increased most consistently and the effect of retention of heat by greenhouse gases appears greatest) and minimum in late winter and early spring
If we look at the graph closely, hypothesis (2) appears the most nearly supported of the three, because the maxima in April and June on the extreme right of the graph (which covers one fiscal year) would even if smoothed out with the very low coefficient for May still show a maximum three month mean of around 0.22 – well above any monthly value between November and February when CET and EWP correlate positively. During these months, the correlation coefficient is relatively consistently around +0.09.

A likely conclusions is that, for the summer months when natural variability is lowest, a sample size of ninety-five years is too small to give accurate correlations at the monthly level. When variability is lower at the local level, smaller changes in temperature can have more effect on the anomaly, especially upon its sign.

If this be so, then we can conclude that over an adequately long period, the correlations between CET and global mean temperature would trough in the winter months at around +0.09. What value they would take in months with negative EWP versus CET correlations (April to September) is less certain. The graph above shows +0.11 and +0.21 as the plausible limits, but what to expect is not certain. We cannot test any years before 1880 since I cannot gain access to compatible values of global temperature anomaly. Testing the post-1974 period is a possibility to expand the sample size, but I am too wary that global warming via greenhouse pollution out of Australia, the Gulf States and South Africa will have  distorted the results vis-à-vis the relatively consistent climate of the 1880 to 1974 period.

An examination of CET versus global temperature anomalies: Part II – January to June

Originally when I decided to plot anomaly of Central England Temperature against anomaly of global temperature for the 1880 to 1974 period, I aimed to do the whole project as one; however a single project really is too much memory for the images needed.

Thus, more than halfway through, I decided I would publish the first part from July to December (which is online here) and finish off the second half from January to June as a separate post. I decided upon reflection later that I would do the conclusions as another separate post.


January CET anomaly versus global temperature anomaly in ˚C, 1880 to 1974
January does not present any really outstanding features. Both our hypotheses at the beginning of this post argued the the relationship between CET and global temperature anomaly should weaken from December to January, and this is observed if only slightly. No outliers so striking as Decembers 1910 and 1939 occur, although the four “War Januaries” noted in the section on December show a striking contrast between cold weather over and warmth globally. As this contrast has been substantially described by Stefan Brönimann in ‘The global climate anomaly 1940–1942’ from 2005, I will not discuss it further here, though I might do a post on these four Januaries later.


February CET anomaly versus global temperature anomaly in ˚C, 1880 to 1974
February’s correlation coefficient is slightly higher than that of January, a result which would not be expected under either theory of CET/global temperature r – both of which expected February to be near a minimum correlation.

What’s also notable is that there are some very strong outliers in what is otherwise a reasonably – given the smallness of the CET area – good correlation over the short period of time during which we can compile reasonable data. February 1963, which ended the coldest winter over the CET area since 1740 and possibly – since anecdotal evidence suggests a very strong “latitudinal inversion” during 1740 – the coldest over the UK as a whole since 1684, has been described in an earlier post. Februaries 1970 and 1941, although less extreme, are similar, whilst February 1944 was a highly anticyclonic winter during a global temperature maximum:
Global temperature anomaly for February 1944. This was during the hottest year globally between 1880 and 1974, and featured a high level of warmth in the northern continental interiors, but strongly anticyclonic circulation over Europe.
The character of the winter of 1943/1944 can be seen below: the strong anticyclone centred over the UK produced a circulation cold enough to counter a level of global warmth not exceeded until after the Lonie Report. The northerly circulation over the UK contrasts with strong zonal anomalies virtually everywhere else in the higher latitudes and troughs in the Mediterranean and Baja. The warmth of this winter over high latitudes of North America was similar to the “War Januaries”.
Winter 1943/1944 500 millibar height anomaly in metres. Note the anticyclonic circulation over the UK contrasting with strong westerlies and mild weather over both continental interiors
An opposite situation to February 1944, with warmth in Central England but cold weather globally, comes ironically from another highly anticyclonic UK winter – that of 1904/1905:
Global temperature anomaly for February 1905. Note the unusually cool weather in the northern hemisphere subtropics.
The cool weather over almost all of the northern hemisphere subtropics, except California and Baja California, is quite remarkable, as shown by the zonal means reproduced below. Anomalies of -1˚C extend south of the Tropic of Cancer, where owing to the intense sunlight and consistent anticyclonicity natural variability of mean temperature is much lower than in higher latitudes where air mass variability has much greater influence. Shimla, at only 31 degrees from the equator and sheltered by the Himalayan crest from cold Siberian air, had its only ever subzero monthly mean at minus 0.8˚C, 1.6˚C colder than February 1893. On the plains, Lahore also had its coolest month on record with a mean of 10.1˚C against an 1880 to 1974 February mean of 15.0579˚C.
February 1905 zonal temperature anomaly, showing the extreme cool in the northern subtropics.
In southern North America, February 1905 was also exceedingly cold. Record cold temperatures for Missouri, Arkansas, Oklahoma and Kansas occurred on the thirteenth and fourteenth. Extraordinary rainfall and cloudiness occurred in the normally cloudless desert southwest: Yuma had rain on twelve days and Phoenix on fourteen – a trend that was to continue into March and normally rainless April throughout Arizona and New Mexico.

Owing to the low latitude of the main anomaly centres, it is on the 250-millibar rather than the 500-millibar chart where the anomalous cold flow into the subtropics, anticycloncity over the UK and tropical maritime flow into Arizona is seen most clearly:
250-millibar height anomaly for February 1905, showing the cold air advection over the subtropics (northeasterly from Hudson Bay over the southern US, northwesterly from Central Asia over South Asia)


March CET anomaly versus global temperature anomaly in ˚C, 1880 to 1974
March fits the thesis of summer maximum better than autumn maximum, as the correlation coefficient between CET anomaly and global temperature anomaly is substantially higher than for February. The diamonds one sees on this chart are much more “confined” than was the case for February, where outlying diamonds occurred quite close to the upper left and lower right corners.

The outlying March 1944 with global temperature 0.50˚C above the 1880 to 1974 mean and CET below the normal for that period, is an intensification of the trend of the winter of 1943/1944: with only 11.8 millimetres March 1944 was the driest month between 1939 and 1956 in the EWP series. March 1962 was a classic month of Atlantic blocking with cool throughout Europe and the United States plus warmth in Greenland, Nunavut, Québéc and Central Asia, warmed by enhanced subtropical westerly flow from the Mediterranean:
Global temperature anomaly for March 1962. The pattern of warmth over Greenland and Central Asia and cold over the United States and Europe should be familiar now
There is a notable lack of Marches that were strikingly warm in the CET series but cool at a global level. No doubt this is because the warmest CET Marches between 1880 and 1974 – those of 1938, 1945, 1948, 1957 and 1961 – all occurred when the global temperature had already heated by around 0.4˚C due to industrial development. March 1948 does present a classic map of cold in western Greenland, Nunavut and western North America and warmth in Europe and the eastern United States:
March 1948 global temperature anomaly. This pattern of warmth and cold should be as familiar now as that of March 1962


April CET anomaly versus global temperature anomaly in ˚C, 1880 to 1974
The virgin Pearson product moment correlation coefficient between April CET anomaly and global temperature anomaly fits the thesis of a summer maximum strongly, rather than that of an autumn maximum (and spring minimum). It is indeed stronger than for any other month we have reviewed so far, with the most conspicuous exception being the extremely hot and sunny April of 1893, which was part of a season that decisively ended Central England’s coolest eight-year spell since 1700:
April 1893 global temperature anomaly. Like October 1896, longitudinal bands of heat and cool can be seen over the Northern Hemisphere.
The only Aprils near the other extreme (Central England cool, globe overall hot) are those of 1953 and 1973. April 1953 was part of an El Niño year that did not produce major drought in Australia – unusually it was east of the dividing range where the driest conditions occurred – and this month saw blocking around Hudson Bay produce a combination of very cool weather in the US and warmth in Canada. Indeed, despite the impact of greenhouse pollution from Australia, the Gulf States and South Africa since the 1970s it remains the warmest April on record in the Arctic Archipelago and Nunavik. Eureka, Nunavut averaged -18.7˚C (anomaly +9.06296˚C); Isachsen averaged -17.8˚C (anomaly +8.08˚C); Kuujjuaq averaged 0.3˚C (anomaly +9.8163˚C); Nottingham Island averaged -4.4˚C (anomaly +8.39286˚C); Iqaluit -4.2˚C (+9.90˚C) and Coral Harbour -6.7˚C (+9.74286˚C).

It was cool in the UK, Iceland, and eastern Greenland due to offshore flow in a strong high-latitude westerly pattern:
April 1953 global temperature anomaly. Note the warmth in Canada and northwestern Russia and the cool over the contiguous US
April 1973 had a somewhat similar global pattern to April 1953, though the cool over the United States was more exceptional, as can be seen below:
April 1973 CONUS division temperature ranks. Note the record cool over the Southern Plains grading to hotter than average in the Northeast
Compared to April 1953, the heat over the majority of the globe was never so extreme as over the High Arctic in the former month; however, the interiors of Australia and South America were especially consistent in being hotter than the virgin mean over a wide area, as was European Russia, China and Japan. Only Western Europe, the US, the Arctic Archipelago and Central Siberia were actually cooler than average:
April 1973 global temperature anomaly


May CET anomaly versus global temperature anomaly in ˚C, 1880 to 1974
The May table, contradicting the April one, suggests a spring minimum and autumn maximum in correlation coefficient. In fact, the observed correlation coefficient between CET and global temperature anomaly is less than for any other month, with the distribution of dots 99.91 percent random.

In spite of this, there are no really outstanding cases of a hot May in Central England being a cool May globally. The May of 1911, which proved the beginning of a famous hot summer in Europe (not only in Britain) is the nearest to this, being the second coolest on record globally but quite hot over the UK if not remarkably so. The only other notable hot area was the eastern United States, which saw a contrast with the Western States commonly seen in very hot CET months (e.g. August 1899, July 1911, July 1921, August 1947, July/August 1955 and July 1983):
Divisional temperature rankings for the contiguous United States for May 1911. Note the cool in the West and South and record heat in the Northeast
Unusually for months with heat in the East and cool in the West, May 1911 was very dry over the contiguous United States, being second driest only behind May 1934 and recording record dryness in the hot Northeast:
CONUS precipitation for May 1911. Note how there is no very wet area over (part of) the Mississippi Basin as in most months with a cool West/hot East temperature anomaly pattern
The pattern suggested by this is of an extreme anticyclone over the East and a cyclone over the West, with the eastern high level anticyclone extending so far that Gulf air cannot be advected into the East as is usual under this scenario. Actually, there is very strong easterly flow into Florida – which was consequently very wet and cool – and this is another important difference when compared to most hot East/cool West months. The anomalous easterly flow is quite consistent over the subtropics and meant that dry continental air was advected throughout the eastern United States:
May 1911 northern hemisphere 50kPa height anomaly.Note the strong blocks over the Baltic and Great Lakes


June CET anomaly versus global temperature anomaly in ˚C, 1880 to 1974
June is an extreme contrast to May: whereas the Pearson r for May was the lowest for any month, that for June is the strongest for any month at 0.33318483.

This extreme contrast is only deviated from significantly in two cases – the very hot year of 1944, and June 1972 at the beginning of a strong El Niño. June 1944 was very much akin to July 1993 – flooding rains over the northern Plains (wettest month over Montana since before 1895), very hot in the East, very cool in the West, and dry in the Southeast. On a global scale, the only cooler-than-average regions in June 1944 were the western US, Europe except the southeast, and southern Australia (which was controlled by frosts resulting from extreme drought):
June 1944 global temperature anomaly map. Note general heat – though nowhere excessive – outside Europe, the western US and southeastern Australia
The final, hopefully shorter, post will consider the correlation coefficients observed between CET and global temperature anomaly and discuss conclusions.