RECENT TRENDS IN BRITISH OCTOBER WEATHER

BY I. T. LYALL Newark, Nottinghamshire

VER the last two decades there has been a noticeable trend for October 0 weather in Britain to become warmer and generally drier. This has occurred during a period when the overall trend of climate in the northern hemisphere has been towards lower temperatures. This article discusses the changes in October weather in relation to the main synoptic processes and also in the context of overall climatic cooling.

Clarke (1966) noted that since about 1940 the weather of October in south-east England had become warmer and drier, and related this to the decrease in westerly type and the increase in southerly and anticyclonic types noted by Lamb (1965). Subsequently, Clarke (1973) has also linked this change in October weather with a failure of the 'autumn break' in the Mediterranean (which usually occurs shortly after 20 October). It has also been shown (Mildren 1971) that the trend to October warmth accelerated in the 1960s.

OCTOBER CHANGES

During the years 1921-50, which may be taken as typifying the warm, zonal period of the first half of this century, the mean October temperature at Kew was 10.7OC. For the period 1949-63 (roughly corresponding with that examined by Clarke (1966)) the value rose to 11.2OC, and during 1959-73 to 11.7OC. The trend to drier conditions has also been apparent; the mean rain- fall of the period 1921-50 was 57.7 mm compared with 49.0 mm in the period

Fig. 1 shows the mean October surface pressure in two periods, viz. 1919-38 and 1965-73. (The data for these periods were obtained respectively from pentadal maps published by Lamb (1973) and from the Weather Log.) The following salient points emerge:

(i) the more recent position of the Icelandic low is south-west of its 1919-38 position. The mean pressure of the centre in the later period is 2 mb higher;

(ii) the main isobaric curvature over the British Isles is now anti- cyclonic compared to a cyclonic curvature in the earlier period;

(iii) there is now no low pressure in the western Mediterranean. These changes mean that the deviation component of the mean wind of

1965-73 compared with 1919-38 is south to south-easterly over most of Britain.

In a review of the climate of 1960-69, Perry (1970) noted that the values of the PSCM indices for October were respectively: +2, +4,

1 959-73.

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Fig. 1 . Mean October surface pressure for 1919-38 (solid lines) and 1965-73 (pecked)

Fig. 2. Mean surface Dressure for 3-7 October for 1919-38 (solid lines) and 1965-73 (pecked)

-4, + 14. These compare to values of +5, +4, -2, t 15 in the long term (1873-1964). Whilst this earlier period extends forward into the present epoch and backward into the blocked period of the late 19th century, the values support the above implications that October has become less zonal (or pro- gressive) and more anticyclonic and ‘southerly’. These changes would be expected to produce warmer, drier Octobers; they coincide with the changes noted by Lamb (1965) and show that in general, October is fitting into the overall trend of the current period of being less zonal and more blocked.

ANOMALOUS AUTUMN SEASONAL CHANGES DURING 1965-1973 It is interesting at this point to bring into the discussion the main

seasonal trends of the circulation in autumn as they affect Britain. The major seasonal change occurs from mid-September to late October, as the whole north hemisphere circulation adjusts from its summer to winter pattern. The main change involves a southerly shift of the main westerly belt and an increase in its vigour.

Other circulation changes more relevant to the Atlantic and north-west Europe have been listed by Lamb (1973). These include the following ‘master seasonal trends’:

(i) early autumn sharpening of the north-east Canadian trough and

(ii) progress of the mainstream of the westerlies toward lower latitudes; intensification of the gradient around it;

and one ‘main synoptic process’:

(iii) winter cyclonic activity in the Mediterranean. The first seasonal trend, which culminates usually in mid-September,

results in Atlantic depressions being steered north-east (into the Arctic basin) rather than east; the second brings the first winter storms and usually com- mences by early October.

From the above considerations, three pentads have been chosen for examination, viz. 13-17 September, 3-7 October and 23-27 October. These correspond roughly with (i) the culmination of the first master seasonal trend, (ii) the start of the second master trend, (iii) the Mediterranean ‘autumn break’. In each case the periods 1919-38 and 1965-73 are compared.

13-17 September. The main feature associated with this period (in Lamb’s account which relates to 1919-38) is the steering of Atlantic depres- sions onto a north-easterly track. Comparison of mean pressure maps for the two periods (not shown) indicates the Icelandic low displaced about 5 deg latitude southward during 1965-73. During the same period the most fre- quent latitude for the passage of low-centres at 3OoW was 60°N. At OoW there were two distinct maxima: at SSON and 71°N, with about equal fre- quencies, during the two pentads 13-22 September. This suggests that during the current period a fair proportion of depressions are continuing to track eastwards rather than north-eastwards in accord with the criterion of the first master seasonal trend.

3-7 October. Fig. 2 shows the mean surface pressures for the two periods. The changes are quite dramatic. The main features are:

(i) continental ridge less marked but with the actual pressure value higher (e.g. S mb at 55ON, 10°E);

(ii) a markedly southerly flow over Britain - especially central and eastern England;

( i i i ) in the British sector a much less zonal flow. The pressure differ- ential between 50 and 60”N averaged over 0 to lOoW is 3.2 mb in the present period, as against 5.8 mb in the earlier one.

This map probably typifies the ‘old wives summer’ situation and pre- sents the extreme of the overall change noted for October.

A study of depression tracks for the period 3-12 October in the current period shows the majority crossing 3OoW at 57ON and OoW at 67ON. This suggests that depressions are tracking only north-east by early October.

23-27 October. This is the pentad of the ‘first winter storms’, or ‘autumn break’ of the Mediterranean: its failure in recent years has already been noted (Clarke 1973). Fig. 3 compares the mean surface pressure in the period 1919-38 with that in the period 1965-73. The main changes are:

(i) A ridge Over Europe as in the previous period, but whereas it was formerly an extension of the Azores high, it now extends from the east.

(ii) Weak low pressure over the Mediterranean. Mean pressure Over north Italy is 8 mb higher than hitherto.

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Fig. 3. Mean surface pressure for 23-27 October for 1919-38 (solid lines) and 1965-73 (pecked)

Fig. 4. Mean temperature anomalies (in deg C) at 1000-500 mb for October 1965-73

(iii) The flow over most of the area has a more southerly component. I t is also less zonal in the southern sector. From 50 to 60°N (in 0 to IOOW) the pressure differential has actually increased from 8.0 mb to 11.0 mb, but from 40 to 50°N it has decreased from 6-2 to 2-0 mb.

The general feature to emerge from the study of the seasonal changes is that in the current period they have been delayed. Thus, depressions do not assume a north-easterly track until early October and in later October the Mediterranean ‘autumn break’ has failed (mean October rainfall in Rome for 1959-73 was 73.1 mm, against 101.4 mm for 1921-30 and 1941-50 combined). This late European blocking is well illustrated in Fig. 4 which shows the mean temperature anomalies (in Deg C) at 1000-500 mb for October 1965-73. The isotherms also show a marked zonal gradient between latitudes 60 and 70°N, a feature which encourages depressions to move east- ward or north-eastward well to the north of the British Isles.

OCTOBER AND THE GENERAL TREND

It is now well known that the current trend in the northern hemisphere is to less zonal, more meridional and blocked patterns (Lamb 1974). In fact the years 1960-72 showed a more extreme development of trends noted for 1951-66. Lamb also shows that the main blocking tendency is in latitudes 50 to 70°N, associated in the British sector with an increase in northerly situations.

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In these last two respects, October has been anomalous. The main in- crease in pressure has been north of 65ON in the Denmark Strait sector, and to a lesser extent over Europe south of 50°N. The main increases in pressure taking the whole year have been near the Azores and over Scandinavia for 1951-66 (Lamb 1972) and also 1970-72 (Lamb 1974). For Octobers in 1965-73 these are just the areas with either smallest positive anomaly or, near the Azores, a negative anomaly. The result has been southerly rather than northerly meridional components over Britain in October with the resulting warm trend of that month.

It seems that the recent behaviour of October may be anomalous for that month in an era of low zonality. Kington (1975) has recently compared PSCM indices for each month in 1968-71 against 1781-84. For most months corresponding indices followed very similar patterns, but current Octobers are markedly more progressive and more southerly than in the 1780s.

This anomaly may be related to the very low temperatures over north Scandinavia (decreases of up to 1.6 deg C in the 1000-500 mb layer). Lamb (1974) has pointed out that during the 1960s the 'cold pole' was exceptionally located on the Siberian side of the North Pole - typically near Franz Josef Land. This would relate to the Scandinavian anomaly. Since 1970, it has re- turned to its more usual position in the Canadian sector and Baffin Land has been exceptionally cold. Such factors will influence depression tracks and affect the intensity and longitude of the upper ridge at 50"N in the Europe- east Atlantic sector. This transference has been related to some very unusual winter seasons in Europe, and in the early 1970s there was some evidence of October becoming rather less warm: the lower tropospheric thermal anomaly in October 1973 was quite different from that of previous years in the study.

OCTOBER 1974 This article would be incomplete without reference to October 1974

when the mean temperature over England was 3.8 deg C below average. The month was remarkable for a persistence of northerly types: 22 days were totally or partly northerly or north-westerly (CRUMB 1975). Fig. 5 shows the highly anomalous mean surface pressure for that month. At 50"N the upper ridge was at 20"W. It is suggested in CRUMB that the circulation re- sulted from anomalous cold air over North America in September and a strong Canadian cold trough. By October surface conditions were such as to favour vortices in both Canadian and Siberian sectors and the resultant thermal gradients produced a strong ridge 'locked' at 20"W. The PSCM values were -9, -37, + 1 , +37. Interestingly, the P and S values resembled more those of the 1781-84 sequence (Kington 1975).

CONCLUSION

This article has shown that October has been warm and dry in the period 1959-73 in Britain. This has been mainly due to a European anti-

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Fig. 5 . Mean surface pressure for October 1974 (hatched area differs from 1965-73 average at 5 per cent significance)

cyclone giving southerly and south-easterly flows over Britain. The blocking itself has continued one to two weeks longer than usual in the period 1959-73. The recent changes appear to be anomalous, not only in the present period of climatic cooling but also with respect to those occurring in October in other such periods; October 1974 may #be more typical of October in such circumstances.

REFERENCES

CLARKE, P. C. 1966

1973 CRUMB 1975

KINGTON, J. A. 1975

LAMB, H. H. 1963

1965 1972

1973

1974

MILDREN, M. J. PERRY, A. H. WALKER, J. M.

1971 1970 1972

Temperature and rainfall trends in October. Weather, 21, pp. 364-366

Onset of autumn. Ihid., 28, p. 175 Climatic research unit monthly-data bulletin. Vol. 3,

Issue 5 . Climatic Research Unit, University of East Anglia

An analysis of the seasonal characteristics of 1781- 84 and 1968-71 using the PSCM indices. Weather, 30, pp. 109-1 14

What can we find out about the trend of our climate? Ibid., 18, pp. 194-216

Frequency of weather types. Ihid., 20, pp. 9-12 Climate, present, past and future. Vol. 1. Methuen,

London The seasonal progreasion of the general atmospheric

circulation affecting the north Atlantic and Europe. Climatic Research Unit, University of East Anglia

The current trend of world climate - a report on the early 1970s and a perspective. Climatic Re- search Unit, University of East Anglia

October-a summer month. Weather, 26, p. 548 The 1960s - a climatic review. Ibid., 25, pp. 123-126 Monsoons and the global circulation. Met. Mag.,

101, pp. 349-355

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