GEOGRAPHICAL PATTERN OF TRENDS - CHANGEMENTS DE CLIMAT OF CLIMATE

It is patently clear that the zonally integrated trends thus far discussed are not representative of trends at all longitudes

within

the zones.

In

order to reveal the pattern of changes,

the

data for solid-record stations have

been

differenced between certain

10-

and 20-year periods, and these differences mapped as shown in

Fig. 5

(a$).

Figs.

5

(u) and

5 (b)

show the 20-year changes of annual-average temperature and winter-season temperature, respectively, from

1900-19

1920-39.

These charts are directly comparable to those published

by

Willett

(1950, his

Figs.

5

and

6),

but they have been analysed independently. Comparison of the two sets of analyses reveals a number of digerences, primarily

in

areas of poor data coverage. These differences, which give an indication of the degree of subjectivity involved

in

making such analyses, are not, however, sufñcient to obscure

the

salient features of the change

field

evident in both these and Willett's figures.

The

greatest changes are seen to have been the warming of

the

Arctic, parti- cularly

in

the Atlantic sector, and,

in

the winter season,

TABLE

4. Hypothetical temperature change

(OF.)

in no-data areas,

Arc,

such that planetary average change estimated by observed change in data areas,

A,

is not significantly different from zero;l and evaluation of reasonableness2

Thirty-year change 1890-1919 to 1920-49 Ten-year change 1940-49 to 1950-59

Annual Winter Annual Winter

Latitude zone

World

800 N.-60"

S. <

-0.27

(U) <

-0.4'6 (U) >-0.09

( ?)

>-0.30

(R)

600 N.-600

S. <

-0.12

(U) <

-0.08 (U) >--0.10

(R)

>-0.34

(R)

Northern Hemisphere

800 N.-OO

<

-0.68

(U) <

-1.22

(U)

>-0.23

( ?)

>-o.go

(R)

600 N.-Oo

<

-0.35

(U) <

-0.28

(U)

>-O21

(R)

>-0.92

(R)

Tropics

Southern Hemisphere

300 N.-300

S. <

+O.ll

( ?) <

+O.il

( ?)

>-0.09

(R)

>-0.19

(R)

00-600

S. <

+0.243

(R) <

+0.39

(R)

>-0.21

(R)

>-0.25

(R)

1. Computed by equation (4) with 1,

=

2.58, corresponding to 99 per cent signiEcance level. Values of A for comparison arc ahown in Tables 2-3.

2. R ?

= =

mnrginnlly definitely reasonable reaaonahle

{

^hence,^trueplnneiary trend m a y have been zero.

=

unreasonnblc, hence true planetary trend WEIS non-zero.

166

World-wide pattern of secular temperature change

a broad zone of cooling throughout southern Eurasia.

S o m e cooling is also evident over Canada, over m u c h of South America and southward to Antarctic, and over parts of southern and western Africa. Warming extended in a more or less continuous zonal belt between about

200

and

400N.,

being interrupted

only

in the Asian sector. This warming took in most of the arid zones of the Northern Hemisphere. T h e proportion of the total earth’s surface which experienced warming

during

this period was evidently quite large, perhaps as m u c h as

85

per cent for the case of annual temperatures.

Figs.

5

(d) show the subsequent 20-year changes from

1920-39

1940-59. Although his

period coincided

with

a very small net change of world mean temperature, as can be inferred from

Fig. 1, it

is clear from these figures that m a n y parts of the world were experiencing appreciable net temperature changes at the time. T h e earlier warming o€ the ArcLic had apparently terminated, and a fairly extensive area of cooling also appeared over (or at least surrounding) the southern Indian Ocean. Cooling also developed

i n

a meridional band approximately parallel to the principal mountain ranges of the Americas.

While

the

Arctic as a whole appears to have cooled since

1940

the degree of cooling has not been geogra- phically uniform. T h e greatest cooling appears to have been confuied to northern Siberia,

the

west coast of Greenland (probably including Bafñn B a y and the eastern Canadian archipelago), and

the St.

Elias Range- Rocky Mountain region in Alaska and extreme western Canada. Another area of marked cooling included western South Africa, and this m a y have extended southward over a considerable portion of the sub- Antarctic.

Fig.

5

(e) indicates that, in all, about

80

per cent of the total earth’s surface has probably been involved in the net annual cooling since

1940.

Four or five areas of net warming nevertheless stand out in Figs.

5

(e) and

5(f3.

These include the

United

States and south- eastern Canada, eastern Europe,

the

Pacific coast of Asia, the Brazilian Plateau, and western portions of the Indian Ocean.

Comparing the period of net world-wide warming [Figs.

5

(a) and

5

(b)]

with

the period of net cooling [Figs.

5

(e) and

5 (J)],

w e are drawn to the following conclusions.

First, although, as w e have previously noted, zonally integrated trends over large portions of the earth appear to have been parallel,

the

trends in individual geographical locations have borne very little systematic relationship to these. That is to say, the trends

in

most individual localities are not well correlated-either posi- tively or negatively-with the global average trends.

Second, the geographical patterns of change are not predominantly a matter of oscillation of temperature from one time interval to another, as one might expect

if

a simple strengthening and weakening of the general circulation were responsible. Rather, the patterns appear to have changed in more or less independent modes.

In order to clarify the extent to which the temperature change fields are related to concomitant changes of the general circulation, the writer has differenced the January and the

July

decadal mean sea level pressure charts for

194049

and

1950-59,

copies of which were winter temperature change from

194049

1950-59

shown in Fig.

5 (f).

F r o m such a comparison, it is evident that most of the areas of strong interdecadal m e a n temperature changes can be attributed with little difficulty to interdecadal m e a n wind-field changes.

Specifically, one m a y note the following.

T h e marked winter-season cooling

in

south-eastern Alaska and the Yukon evidently coincided

with

an equally impressive change of pressure gradient in that area

in

January, which favoured an increased local frequency of Arctic air masses from the Canadian interior.

T h e warming

in the

north-eastern United States and south-eastern Canada coincided

with

a weakening of the normal north-westerly gradient

wind

from the Hudson B a y region, which permitted a higher incidence of w a r m maritime air masses to reach the area in the

1950s.

M u c h

the

same relationship can be noted between the warming and the circulation change in the vicinity of Japan from

the 1940s

to the

1950s.

T h e change of the pressure field across North America, like that across Europe, w a s such as to favour a n inten- sified zonal flow in those areas.

In

the case of North America, this m a y have resulted in increased pseci- pitation and downslope heating consistent with the observed warming over and eastward of the Rocky Mountain Plateau

in

the

United

States. In the case of Europe, the strengthened westerlies probably de- creased the incidence there of polar outbreaks from Siberia.

In the Southern Hemisphere, similarly direct rela- tionships between the winter temperature and

July

circulation changes can be noted. However, even rela- tively large pressure changes are insufficient to produce radical changes of the enesgetic zonal circulation of the Southern Hemisphere. Consequently, it is not sur- prising that the temperature changes associated with the pressure changes have been smaller