OF THE GENERAL CIRCULATION OVER THE NORTH ATLANTIC

DURING THE DECADE 1951-60

It

should not be overlooked that the behaviour of sea- surface temperature-which is so important

in

view of the fact that heat stored in the sea is fed back to the atmosphere-depends upon quite a number of different factors.

The

precise influence of these various factors is difficult to determine and, moreover, the effects m a y be different between various ocean areas (and, perhaps, not constant

with

time).

FIG.

13. Pressuxe change (mb.) from the period mean 1958-60.

FIG.

14. Mean barometric slope along 300

W.

longitude from 400 to 60O.N. latitude, for the periods 1951-53 and 1958-60.

103

Changes of climate

I

Les changements de climat

-1

-1

-

^-3

-2

-

-3

-

FIG.

15. Meridional profiles through the field of pressure change.

Difference of the three-year averages (1958, 1959, 1960) minus (1951, 1952, 1953).

T h e distinct decadal trend of sea-surface temperature

in 1951-60

suggests an

inquiry

into possible trends of other elements, and some preliminary results

in

this field conclude this study.

Fig. 16

reveals a clear decadal trend of increasing windiness over the North Atlantic, the ''windiness'' being defined as the percentage frequency of strong winds plus gales

(222

knots). For the collective of nine ocean weather stations the increase from

1951-53

%

Decadal mean

36p

^3s

1551/53 1952/54 195. 34

! I

~

i5 1954/56 1955/57 1956/58 1957/59 1950/60

FIG.

16. Three-year running means of windiness over the North Atlantic Ocean (average of all nine ocean weather stations).

Windiness

=

percentage frequency of winds

2

22 knots (strong winds

+

gales).

104

(34.2

per cent) to

1958-60 (39.3

per cent) is

5.1

per cent.

Given a n initial value of

100,

the increase amounts to

14.8

per cent.

This increase in windiness was not limited to only some of the ocean weather stations

but

occurred, more or less, at all of

them. A

similar result is obtained

if

the m e a n

wind

velocities at the ocean weather stations are considered, and

Fig. 17

gives a tentative picture of the distribution of the change

(in

knots) of m e a n

wind

speed from

1951-53

to

1958-60.

For the collective of all nine stations the increase from

1951-53 (18.4

knots) to

1958-60 (19.8

knots) is

1.4

knots (or

7.5

per cent).

There is certainly no clear local correlation between change in sea temperature and change in

wind

force, but the net result of spatial integrating

is

that the cooling trend is related to a trend of increasing atmospheric circulation

during

the last decade.

A t a first glance, this is a result which seems to contradict the findings

in

some former studies which suggested that the ocean warming could be explained

by

an increased atmospheric circulation. But a "sub- polar warming" has also been found

in

the recent decade.

Undoubtedly more extended and more detailed research is necessary to

ñnd

more conclusive answers to all the m a n y questions regarding oceanic climatic change.

Finally, it m a y be mentioned that the decadal variation of air temperature in

1951-60

w a s very similar to that of sea temperature at

the

ocean weather stations. But though

the

individual signs of change at

FIG.

17. Mean wind speed change (in knots) from the period 1951-53 to the pe130d 1958-60.

Sea-surface iem2neraíures of the North Atlantic Ocean

1951-53 1952-54 1353-55 1954-55 1955-57 1956-58 1957-59 1958-60

I l l , I

FIG.

18. Comparison of the change in air and sea-surface tem- perature, period 1951-53 to period 1958-60.

23.8

22 22 22 21.0

18.8

FIG.

19. Thee-year moving means of precipitation frequency over the North Atlantic Ocean.

each of the stations were the same, the amounts (air-sea) were not the same.

The

particular distribution for air-sea differences is shown

in Fig. 18. For

the collective of nine ocean weather stations the sea-surface cooling from

1951-53

to

1958-60

was a little greater (-0.210) than the air cooling

(-0.210).

T h e h a 1 figure

(Fig. 19)

suggests that there was a n increase

in

precipitation frequency

during

the decade.

In

the field of interrelations between sea and atmosphere there are m a n y problems.

It

is the author’s opinion that one cannot speak yet of a definite turning point

in

the recent Atlantic warming, but the present decade of the sixties should be carefully watched for further development.

R E S U M E

L a température superJicielle de l’océan Atlantique au cours de la décennie 1951-1960, ses anomalies et son évolution, en relation avec la circulation atrnosph6rique

(M.

Rodewald) D’après les observations de la température de la mer effectuées par les navires météorologistes stationnaires

(NMS)

et représentatives de l’état thermique de l’océan Atlantique en général, la décennie

1951-1960

a été plutôt chaude, l’anomalie moyenne étant de

+0,4

O C (moyenne des neuf

NMS). L’étude

de quelques données supplé- mentaires conduit

à

un ordre de grandeur de

+O’S

O C

pour l’océan dans son ensemble.

L a tendance au cours de la décennie

1951-1960

a cependant été à un net refroidissement. L’anomalie moyenne de l’ensemble des

NMS

s’est abaissée de

0,55 OC

pour les trois années

1951-1953 à +0,28

O C pour

1958- 1960. Un

nouveau refkoidissement décennal de cette importance annulerait l’anomalie positive.

En

tenant compte d’autres données le résultat obtenu est analogue mais plus positif.

L a tendance décennale de

1951-1960

a été loin d’être uniformément répartie. L a tendance sur le nord-est de l’Atlantique a été nettement

à

l’inverse

de

celle de l’ouest et

du

sud. Ce phénomène a été examiné en le comparant aux variations de la circulation atmo- sphérique.

D e manière générale, on a obseivé une large extension vers le sud

du

tourbillon polaire (dépression d’Islande) et cela semble avoir

été

la cause

du

refroidisement décennal observé et de ses singularités.

105

Changes of climate

}

Les changements de climat

D I S C U S S I O N

€1.

H. LAMB.

Studies of the pressure Geld from month to month and year to year indicate that it is essential to dis- tinguish between changes produced by variations of the intensity and by alterations of the position of key features.

Most pressure difference indices in the Northern Hemisphere seem to show decreasing, not increasing, circulation intensity during the 1950s. On the other hand there has been a rcmar- kable progressive southward displacement of the Iceland low towards the position of ship

C

in mid-Atlantic-i.e., towards about 500N.300-40oW., a9 shown by some of Dr. Rode- Wald’s diagrams. This southward trend is so far unexplained : it began in the winter months some years before the modest increase of the sea ice north of Iceland became noticeable, and it now appears to affect most times o€ the year. Thus the increased windiness in the temperate zone of the Atlantic 400-650N. m a y perhaps be due to the Iceland low pressure activity moving to a position central in the region, the increased intensity of circulation appearing as a somewhat localized exception. Quieter circulation appears particularly to have set in in higher latitudes.

M. RODEWALD.

Certainly my findings are preliminary and are only for the area covered by the ocean weather stations.

I

a m inclined to say that mean pressure difference indices m a y not always indicate the real intensity of circulation.

For instance, in the case of a more frequently and/or more sharply changing circulation pattern, the mean pressure difference over a certain period m a y not reflect the real strength of circulation.

In

this respect, the mean wind speed-derived from about eight observations per day-seems to m e to give a more direct and better index of the circulation intensity.

As

to the mean wind speed, there was an increase from the period 1951-53 to 1958-60 at all nine North Atlantic Stations.

This

increase was greatest at the position of ocean weather station

C,

but was also well pronounced at the northern stations

A

and

M.

L. A.

-AS. Surface temperature is immediately controlled by evaporation from the ocean surface.

W i l l

increased evapo- ration account for the lowering of temperature and vice versa?

M. RODEWALD.

Certainly evaporation, depending, to a great extent, on wind velocity, is an important factor in determining the temperature of the surface layer of the sea. Increasing wind forces will act in lowering the surface temperature and vice versa.

But

there are many other factors which deter- mine the behaviour of sea-surface temperatures, such as, for example, radiation, cloudiness, and advection in

the

air as well as in the sea.

J.

NAMIAS.

In view of the diEerence of atmosphere-ocean effects according to season it would seem desirable to stratify the data by seasons. This might offer more clues to the physical causes of the ocean surface temperature variations.

Incidentally, the circulation fluctuations shown by

Dï.

Rode- Wald produced m a n y weather and climate fluctuations in eastern United States, e.g., increased hurricanes, increased meridional movement of extratropical cyclones, and increased warmth.

All

of these phenomena appear to he dependent (and interactive with) the general circulation.

M. RODEWALD.

Breaking down the study into seasons certainly m a y clarify some questions.

It

should and m a y easily be done in a further study.

C.

C. WALL~N. I

wonder

if

possibly the increase of temperature during the 1950s in the north-eastern Atlantic might have been smaller than during earlier decades and therefore only shows up as a relative maximum compared with the decline in the south and south-west. If that is the case, the cooling in the south and south-west may be connected with a long- term change in the general circulation which is gradually spreading in its effects upon sea temperatures.

M. RODEWALD.

No comparison has been made between the warming trend during the Hties to that of former decades in the north-eastern Atlantic area. But it m a y be said that the year 1960 was by far the warmest year within the last decade at ocean weather station

M

in the Norwegian Sea.

One has to wait for the next years in order to see whether the West Atlantic cooling willspread to the north-east, or not.

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

BJERKNES,

J.

1959. The recent warming of the North Atlantic, in : Bert

Bolin

(ea.), The atmosphere and the sea in motion.

N e w York, p. 65-73. (Rossby Memorial Volume.)

BULLIG, II.

J.

1954. Atlas der Monatswerte von Wasscrtempera- tur, Wind und Bewölkung auf dem Seeweg Europa-Siid- urnerika. Hamburg, Deutscher Wetterdienst, Seewetteramt.

(Einzeveröff., no.

5.)

DEUTSCHER WETTERDIENST.

1951-1960. Die Groscwetterlagen Mitteleuropas. Jahrg. 4-13, no. 13, Offenbach/Main.

GOEDECKE,

E.

1952. Das Verhalten der Oberflächentemperatur in der Deutschen Bucht während der Jahre 1872-1950

und der zusammenhang mit dem der nordwesteuropäischen Meere, Ber. dtsch. Komm. Meeresforsch. Stuttgart, vol. XIII, no. 1, p. 1-31.

INSTITUT

P Ü R

METEOROLOGIE

UND

GEOPHYSIK,

Berlin. 1953.

Normalwerte des Luftdruckes auf der Nordhemisphärc für die Periode 1900-1939,

Met.

Abhandl., vol. II, no. 1.

LAUZIER,

L. M.

1953. Coastal station data, Atlantic coast.

Fisheries Research Board of Canada. (Manuscript report series (biological stations), no. 537.)

- .

1958. Surface sea-water temperatures along the Canadian Atlantic coast, 19548-1957, Progress reports of the Atlantic

106

Sea-surface temperatures of the North Atlantic Ocean

coast stations of the Fisheries Research Board of Canada, no. 71, p. 8-12.

-; HULL, J. H.

1961. Temperatures along the Canadian Atlantic Coast, 1950-1959. (Fisheries Research Board of Canada, Manuscript report series (oceanographic and limnological, no. 39.)

RODEWALD, M.

1952. Normalwerte und Abweichungen der Meerestemperatur bei den nordatlantischen Wetterschiffen, Dtsch. hydrogr. 2. Hamburg, vol. 5, no. 2-3, p. 131-140.

- .

1953. Die Abweichungen der Obefiichentemperatur des Ozeans bei den nordatlantischen Wetterschiffen 1951-1953, Dtsch. hydrogr. 2. Hamburg, vol. 6, no. 4-6, p. 170-185.

- .

1956a. Die nordatlantische Temperaturanomalie in den Jahren 1954 und 1955, Dtsch. hydrogr.

Z.

Hamburg, vol. 9, no. 3, p. 137-142.

- .

1956b. Beiträge zup IUmaschwankung im Meere. 5.

Beitrag : Die rezente Envärmung der atlantischen Küsten-

gewässer Nordamerikas, Dtsch. hydrogr.

Z.

Hamburg, vol. 9, no. 4, p. 182-186.

-1956~. Die Wassertemperaturen und ihre Anomalien bei den japanischen und nordamerikanischen Wetterschiffen im Nordpaziûschen Ozean, Dtsch. hydrogr. 2. Hamburg, vol. 9, no. 6, p. 281-298.

SMED, J

1949. The increase in the sea temperature in northern.

waters during recent years. Cons. Penn. Int.pozcr l'Explor.

de lu Mer, Rapp. et Proc.-verb., vol.

CXXV,

p. 21-25.

(Copenhagen.)

- ^.

^{1951. Variation of} the temperature of the surface water at station Myggenaes (Faroes) during the years 1914-51, Ann. Biol., Copenhagen, vol.

VIII,

p. 20-21.

SOUTHWARD, A. J.

1960.

On

changes of sea temperature in the English Channel,

J.

Mur. biol. Ass.

U.K.,

Cambridge, vol. 39, p. 449-458,

107

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