synoptic and dynamic aspects of climatic change
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Synoptic and dynamic aspects of climatic change
By E. B. KRAUS
(Read 16 March 1960. Presented by Dr. E. T. Eady. See Q.J., 86, p. 1)
Mr. R. G. VERYARD : This is a very interesting paper and it certainly brings together a lot of useful information about climatic change, but, as in many papers on the subject, there are a number of assertions based on insufficient evidence and, as Dr. Kraus himself admits, there are several assumptions, some of which are very dubious.
In regard to the idea that climatic fluctuations may be related to an increased infra-red cooling rate in the upper air, particularly over the tropics, perhaps I might draw attention to a recent paper by Stranz (1959) who finds quite a high correlation between variations in solar activity and variations in the height of the tropical tropopause. This may lend some support to Krauss suggestion.
Regarding the summary of climatic fluctuations during the last 100 years, from my own reading of the many papers on the subject, I find it very difficult to co-ordinate the results. Different workers have used different lengths of record and different reference periods. (Perhaps I should mention that this is a matter which I hope to get tidied up at the next meeting of the Climatological Commission of World Meteorological Organization). I would like to point out that such changes as have taken place are not all of the same sign. For example, Rubinshtein (1956) found reversed phases of temperature variation in Moscow and Barnaul in Siberia, and for the monsoon area, Pramanik and Jagannathan (1954) found an increase of rainfall in some areas and a decrease in others but with a tendency for deficient rainfall to be more frequent in the semi-arid zones. This latter result supports the findings of Kraus.
In regard to changes during the last glacial period with its interglacial interruptions, and the effort by Kraus to identify these changes with the changes in the last 100 years, I think it is important to get a time-scale clearly in our minds. If we equate the age of the oldest rocks - about 1,500 million years - to one year, then we would get the Quaternary period from about 1800 hr to midnight on 31 December with the last ice-free period a few minutes before midnight, and the period of instrumental meteorological observations and synoptic charts a few seconds before midnight. I cannot help feeling that it requires a lot of imagination to match climatic changes in the last 100 years or so with those in the last 10,000 or 20,000 years, especially when the dating of past climatic events may be in error by several hundreds of years. There may well be long-period fluctuations which we cannot yet identify - as Kraus himself admits when he dis- cusses the effect of COZ.
Regarding the contention of Kraus that a slight decrease in summer temperature might allow for the development of snowfields, this certainly appears to be true for Scotland where a change of only a few degrees is required for there to be permanent snow-beds or not. What is not clear and has yet to be determined is the dynamic effect of a large area of snow-cover on the motion of the atmosphere.
I would like to emphasize that a mean atmospheric circulation can be very misleading. In fact, there appear to be a whole spectrum of circulation patterns which vary in their incidence from day to day, season to season and year to year. Kraus considers that glacial periods were characterized by an intensified Hadley-type circulation in the tropics. But in a paper on related fluctuations of Trade Winds and northern climates, Bjerknes (1958) contends that the warming of the Arctic in the 1920s and 1930s can be understood only if there were an intensification of meridional eddy flux of heat in middle latitudes concurrently with an intensification of the Hadley circulation. Prof. Bjerknes produces Trade-Wind statistics to support his contention. He found a strengthening both of the north and south components.
Regarding the suggestion by Kraus that climatic fluctuations may be related to variations in the ultra-violet and to the ozone and CO, content, his special reference to circulation changes in the stratosphere and troposphere is of particular interest. In the Climatological Research Divi- sion of the Meteorological Office we have recently been looking at upper-air data for the tropics and have been very impressed by the changes, especially at high levels, not only from day to day, and from month to month, but from year to year. At the 50 mb level, i.e., in the lower stratosphere, the year-to-year variations of the monthly mean wind are quite remarkable. In one year there
Mean omfile & the easP-we&wind Christmas1
Figure 1 1950
are westerlies, the next easterlies, and the following year westerlies - with the westerlies or easterlies showing up not only in the monthly mean but throughout the whole month. This is well illustrated by Fig. 1 for Christmas Island, 2"N, 20W, May 1957/8. It will be seen that the text-book picture of the Berson westerlies at 50 mb with the Krakatoa easterlies reaching a maximum speed at 10 mb certainly does not hold good for every year. What we would like to know, of course, is to what extent there is a coupling between stratospheric and tropospheric circulations. Perhaps the I.G.Y. and I.G.C. data will facilitate this.
Rubinshtein, E. S .
Pnrnmik, S. K. and Jagannathan, P. Bjerknes, J.
' Solar activity and the altitude of the tropopause near the Equator,' J. Atmos. Te77. Pkys., 16, Nos. 1-2.
Budyko, M. I., A. I. Voeikov, i Sovremennye Probl. Klirn, Hydrornet Publishing House, Leningrad, pp. 123-174.
' Climatic changes in India,' Proc. I.A.M., Rome. ' Related fluctuations of Trade Winds and northern climates,'
Geophysicu, 6, No. 3-4.
Mr. H. H. LAMB : Dr. Kraus will certainly stimulate everybody's thinking on the subject of climatic change with this paper, as with his earlier ones on secular changes of rainfall. The real patterns of climatic change are, however, a good deal more complicated than the simple, grand design which he depicts in the paper presented today.
I suspect Kraus does less than justice to Simpson's theory, since it does in fact appear that (a) the last Interglacial was warmer than the present (cf. p. 2 of Kraus's paper) and (b) precipitation was heavy in the earlier stages of the last Ice Age. It seems quite possible that at the onset of glacial conditions world temperature was still rather higher than now, as required by Simpson, and only gradually sank, finally falling well below present levels, as the oceans became colder. However, Kraus is obviously right in emphasizing the stronger circulation of the glacial epochs, especially in summer : Simpson's Ice-Age summer-pressure distribution plainly needs correcting in this sense, as we see from consideration of the Southern Hemisphere today.
There seem to be three main points where Krauss presentation may be misleading : 1. It is surely a mistake in principle to assume that all climatic changes must be attributable
to the variations of any one factor. We all know that there are many things, varying all the time, which must cause climatic variations. Kraus himself is not consistently as rigid as this and I would accept that he has produced a good picture of how variations of O3 and COz (amongst other things) could operate to cause climatic changes; I support the notion that changing height of the tropopause is likely and must be important.
The argument which Kraus adduces to discount the influence of volcanic dust (p. 13 of his paper) is contrary to the facts. The general circulation, as demonstrated by many authors, did not decrease in vigour about the end of the last century; on the contrary its vigour increased and reached its maximum in about those years - especially in the 1920s and 1930s and some later years - when volcanic dust is reasonably supposed to have been at a minimum. The increase of circulation intensity can be seen in curves of pressure gradients in the northern and southern westerlies and in the Trades as well as in some measures of the strength of mean meridional currents, the peak intensity generally lying between 1910 and 1940. I would therefore enter a reservation in favour of volcanic dust as possibly an important agent in recent climatic fluctuations.
3. Kraus, in general, overstates the case for contemporaneity of climatic changes in many parts of the world. I think this means that he consistently underestimates the importance of terrestrial causes and terrestrial distribution patterns.
A few examples will show the need for qualification in this regard : (a) The relatively wet period with high lake levels in south-eastern Australia beginning
about 3,500 years ago is about a thousand years before the main increases of rainfall in the sub- Atlantic period given by Brooks for various parts of the Northern Hemisphere, though there do seem to have been brief foretastes of the sub-Atlantic climatic type in Europe around 2,000 and 1,300 B.C.
(b) As regards variations within the last 100 years, our empirical investigations at Harrow show there is no rule about poleward or equatorward movement of the subtropical highs (or the belt of westerlies) with increasing vigour of the circulation.
These highs were at their most intense in both hemispheres during much of the first half of the present century. This did correspond to strong, well-developed westerlies and Trade Winds but coincided more or less with the time when (as Kraus has shown elsewhere) the equatorial rain belt was narrowest and the