OF ZONALLYAVERAGEDTEMPERATURE CHANGES IN DATA AREAS

T h e areally averaged change of mean temperature from a reference time

O

to time

y,

for a region containing a well-distributed network of stations, can be estimated as

(5)

i = l s=1

where dsi is the difference of reported m e a n temperature at station s from time

i-1

to time i, and where ni is the number of stations

with

data sufficient to compute a

&.

T h e standard error of estimate of the regional tem- perature difference, given

by

equation

(5),

is closely approximated

by

Y

i = l i=2

where ni, 1-1 is the number of stations

in

the region supplying temperature differences between times i

- ²

and i

- ¹

and also between times i

- 1

and

i.

Here

s(e) is the standard error of the

Cl,i

as estimators of the regional temperature difference, assumed to be quasi- constant

with

time (see Mitchell,

1961~).

T h e curve6

in

Figs. 1-3 are based on weighted averages of the series of relative temperature

in

100-latitude bands. Each of the latter series, in turn, has been derived

by

means of equation

(5)

applied to five-year (pentadal) m e a n station temperatures, accumulated backward

in

time from the

1955-59

pentad

(zo). The

relative tempe- rature for each pentad

y

of the series possesses an error variance given

by

squaring equation

(6),

and hereafter denoted

by v(Eg)

for brevity. Values of ( T ~

-

^{T ~ ) and}

of

u(E,)

for annual and winter mean temperatures

in

each 100-latitude band are given back to the

1880-84

pentad

in

Tables

5

and

6

respectively.

T h e temperature changes shown in Tables

1 (right-

hand column),

2,

and

3

are the differences between two temporal averages of curves such as those illustrated

in

Figs.

1-3. The

standard errors of such change values, also given

in

Tables

2

and

3,

are derived

by

the equation

SE(A) =

[Zwiav(%-

Ëb)i]t/.,

(7) where wi is the fraction of the total surface area of the ith constituent loo-band, and

v(Ë, - Ëb)

is the error variance of the temperature change in the ith band calculated

by

a procedure noted below, and

in

which

176

World-wide pattern of secular temperature change

T A B

LE

5.

Mean temperature by pentads1, expressed as departures in OF. from 1955-59 pentad, each 100-latitude band from 800

N.

to 600

S.

Winter mean temperature 0.3 0.1

errors of estimate of temperature change in the consti- tuent bands are assumed to b e mutually independent.

T h e error variance

OP

the digerence between t w o was derived from the equation

where the summations extend over the

N

pentads involved in each interval being compared, where .(e) is the square of 4 8 ) previously defined,

and

where n is the average number of stations whose data are used to compute the (7"

-

T ~ ) in these interva1s.l

-

where the numerical subscripts apply to the pentad 1. If the intervnls ore not contiguously chosen,

-

n is the average niimher

of stntiona for the total period of record involved.

177

Clwrngcs of climate

1

Los changomcrtts d e climat

numbers, counted backwards from the

1955-59

pentad (number O).l

The

derivation of the equations

in

this appendix, together

with

a more rigorous discussion of

the

validity of their application, are given

in

[3].

S U P P L E M E N T A R Y D A T A

Various supplementary data are

given for

each 100-latitude band

in

Table

7.

These include : (a) changes of mean temperature and their associated standard errors as estimates

for

the data areas, corresponding to the periods of record shown in Tables

2

and

3;

(b)

values of .(e) and

C, defined in

the text, which are

178

requised for evaluation of standard errors

w(Ey)

given

by

equation

(6) and

of other quantities given

by

equa- tions

(2)

and

(3) ;

and (c)

the

percentage of each latitude band that is land covered, and the percentage of the total earth's surface contained

in

each latitude band [proportional to

the

weights zui

in

equations

(2),

(3), and

(7)l.

1,. Combination OC equations (6) nnd (9) yields

where the n/, 1-1

-

symbols arc as defined under equation (6). W eform of (9) is more uaefui for direct calculation.

T A B L E

6 (continued)

Temperature change

1

stnudard error of estimate for data areas (OF.)

Percentage p ~ ~ { ~ ~

cover =ea of

C land

de)

Band 30-year change 1890-1919 to 1920-49 IO-year change 1940-49 to 1950-59

earth

Changcs ofclimate

/

Les cltangeni.ents de climat

ACKNOWLEDGMENTS In

addition, he thanks Messrs.

H. H.

L a m b and

A. I.

Johnson for providing sea level pressure charts T h e writer is indebted to the Staff of the Weather on which

Fig. 6

is based. Finally,

he

gratefully aclcnow- Bureau National Weather Records Center, Asheville, ledges the assistance of

Mi. J. S.

Barry, Mr.

W. B.

Buck, North Carolina, whose assistance in locating recent Jr., and

Mrs. E. B.

Buchanan, in preparing the temperature data for the

world

m a d e this study possible. manuscript for publication.

R É S U M É

La

répartition mondiale des variations séculaires de température

(J.

Murray Mitchell, Jr.)

L’auLeur examine les changements enregistrés depuis

1880

dans la température moyenne

du

globe et

il

évalue la mesure dans laquelle les données disponibles per- mettent de déterminer ces variations avec exactitude.

La

tendance a u réchauffement observée depuis quelques dizaines d’années dans le monde entier paraît s’être renversée peu après

1940. Les

températures moyennes semblent avoir évolué de manière parallèle dans les deux hémisphères, mais l’insuffisance des

données rela tives aux régions océaniques empêche de déterminer avec précision l’ordre de grandeur de ces changements.

L’auteur expose et compare la répartition géogra- phique des variations annuelles et hivernales enregistrées de

1900

à

1960

durant diverses périodes de

dix

et de vingt ans. Les variations de la circulation générale sont nettement démontrées c o m m e la cause principale de ces écarts régionaux.

Les variations observées dans la température mondiale seraient d’une grande importance pour étudier les causes définitives des changements séculaires

du

climat.

D I S C U S S I O N

E. KRAUS.

Perhaps the most interes% part of the evidence presented by

Dr.

Murray Mitchell,

Dr.

Rodewald and some of the other speakers is the way in which it falls into a pattern.

Not only air temperature, but also subtropical rainfall, the tendency of hurricanes to move along certain tracks or sea- surface temperatures, show a reversal of the preceding climatic trend during the last one or two decades. The true physical signiñcance of

Dr.

Murray Mitchell’s result lie? perhaps in the combined evidence, based on so many different variables.

B.

DZERDZEEVSKII.

I think

that

Dr.

Mitchell’s paper is very interesting and that the results he obtained are extremely important.

I

a m certain that progress can be made on investi- gations into climatic change only by using world-wide data and by analysing the “ world-wide connexions ” among the meteorological observations and circulation characteristics embracing at least one hemisphere.

I

also believe that dividing the long-period data into shorter periods is very useful

;

but it seems to m e that it is preferable to use the natural periods or epochs ” of different duration.

The dividing of climatic data can be fulfilled by analysing the circulation data. Indeed it is not easy. Although such analyses considerably increase the amount of work, the results obtained prove to be more valid.

J. MURRAY

MITCHELL. Dr. Dzerdzeevskii’s remarks are greatly appreciated.

With

regard to his suggestion to parti-

180

tioning the climatological data according to natural periods,

I

a m very sympathetic to this view and, in a way, this is what

I

have tried to do.

I

believe that my curves of world mean temperature (see Fig. 1) might be interpreted as part of a rhythm somewhere between 60 and 90 years in period. Evidence for periods near 80 or 90 years has of course been suspected in the past. It may indeed be worth while to study changes of world-wide climate in and between the various phase8 of such a presumed rhythm. However, if the pre-eminence of such rhythms is ever refuted by new information, as it may well he, w e should,

I

think, he willing to abandon this approach without further ado.

J. NAMIAS. In

examining Dr. Mitchell’s figures 5(f) and 6(a),

I

would

like

to point out an important connexion between the pressure distribution and its change and the field of temper- ature change-both for the decades between the 1940s and 1950s. The areas of warming over the United States and Europe which

Dr.

Mitchell mentioned and indeed associated with the strong development of the Gulf of Alaska centre action following repeated outburst of cold Alaskan air.

It

is well known from my own studies that this situation produces fast westerlies, rapid zonal movements of cyclones along the Canadian border, and flooding of the United States by mild Pacific air-the polar air being “contained” in Canada.

The

European warmth is associated

with

a well-developed Icelandic low, which is in turn associated with cold outflowing

World-wide pattern of secular temperature change

air masses from the east Canadian tundra.

This

system drives mild Atlantic air masses into Europe. Hence the cooling in northern laiitudes especially over Alaska and the Yukon, and the development of this cold air over the Gulf of Alaska in effect sets up areas of warmth which lessen the degree of

apparent hemispheric or global cooling.

J.

MURRAY MITCHELL.

Dr. Namias’ interpretations are a valuable addendum to my own brief comments on this matter, und

I

thank him.

C. C. WALLEN.

Like oLher speakers

I

wish to express my great interest in and admiration for the contribution given by Dr. Mitchell. There are just two points which

I

would like to raise because they have bearing on my o w n home area.

It

was surprising to m e to find that Dr. Mitchell refers to the Scandinavian area as one where pressure has fallen during the recent period of decreasing world temperature indicating rather an increase of temperature. This necessarily must he clue to the selection oí‘ the period of comparison : in this case the forties and the fifties. Already seven years ago, using temper- ature records €rom Sweden,

I

pointed out that the so-called

“ recent climatic fluctuation ” upwards trend seemed to have changed into a decline.

A

recent study of the differences in temperature and pressure between the periods 1901-30 and 1931-60 shows a definite decline of temperature in Scandinavia coupled with an increase of 9 mh. in pressure in the northern parts. Splitting up the period 1931-60 and looking separately at the forties and fifties evidently means that a slight recovery in tbe overall downwards trend that has occurred in the

ñfties, in northern and central Europe, seems to bave been given too much emphasis.

I

wish also to state how much

I

agree with Dr. Kraus that this downwards trend in temperature should be considered significant from a physical point of view although it m a y not be so from a statistical point of view. It certainly renders itself extremely well to studies of the relationship between changes in climate elements on one side and fluctuations in the general circulation.

In this connexion

I

should wish to mention that in the recent study for Scandinavia mentioned above

I

have also tried to find some causes for the temperature decline in winter from 1901-30 to 1931-60 in terms of changes in the general circu- lation. Evidence definitely shows that the decline is coupled with an increase of easterly to north easterIy circulation types indicating a shift westwards in the mean position of ridges and troughs. This seems to go well with the explanation for overall decline in temperature during historical periods that has been given earlier by Dr. Lamb.

J. MURRAY MITCHELL.

Let m e comment on Dr. Wallén’s first point merely by observing that, in studies of the relation between circulation changes and climate changes, one fre- quently comes to somewhat different conclusions depending on which averaging periods are compared. In many parts of the world, this is probably due to the use of inhomogeneous data, but

I

doubt very much that this is the case in Scandinavia.

W h e n such discrepancies occix, there seems to be no recourse but to make a more refined analysis of available data, of the exemplary kind for which Dr. Wallén himself is well known.

B I B L I O G R A P H Y / B I B L I O G R A P H I E

CALLENDAR,

G. S.

1961. Quart. J. R. met. Soc., vol. 87, p. 1.

- .

1961b. Solar variations, climatic changes, and related LANSBERG, H.

E.

; MITCHELL, Jr.,

J. M.

Quart.

J.

R. met. Soc., geophysical problems, Ann. N.

Y.

Acad. Sei., vol. 95, MITCHELL, Jr.,

J. M.

1961a. Researchpaper, no. 43. Washington,

WILLETT,

H. C. 1950. Centenary proceedings. London, Royal

vol. 87, p. 435.

D.C., United States Weather Biireau.

p. 235.

Meteorological Society, p. 195.

181

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