OBSERVATIONAL STUDIES OF GENERAL CIRCULATION PATTERNS 
in the centers of action. Even at 300 mb the Icelandic 
and Aleutian lows of winter have not lost their iden- 
tity although they are displaced considerably north- 
westward. The Siberian wintertime anticyclone appar- 
ently gives way to westerlies aloft. 
The contrast between winter and summer shows up 
chiefly in diminished gradients and northward migra- 
tion. With the approach of summer the sea-level sub- 
polar lows lose much of their strength, the subtropical 
oceanic highs become much stronger, and over Eurasia 
the well-known monsoonal reversal of pressure sets in. 
The comparison of circulations between the Northern 
and Southern Hemispheres is much more difficult in 
view of the relatively scant supply of data in the South- 
ern Hemisphere—particularly at upper levels.? Certain 
attempts have recently been made to improve on 
Shaw’s Southern Hemisphere charts [47] notably by 
the British Meteorological Office [9] for upper levels 
and by Willett [60] for sea level using Serra’s world 
charts [46]. From these data a few seemingly reliable 
statements appear to be possible: 
1. The zonal circulation at sea level appears to be 
stronger in the Southern Hemisphere than in the North- 
ern Hemisphere. This is strongly indicated in a table of 
average circumpolar zonal wind speeds computed by 
Willett [60] who made use of Serra’s monthly mean 
maps [46] for the months of January, April, July, and 
October from 1879 through 1934. (See Table I.) 
Tas.e I. SenecteEp Norman Crrcunation INpIcES 
AT Sea Lrye.* 
Wie 1 Maximum 
swesterlies | SUprovea 
Wind Wind 
See teat, || oceecaLat, 
sec!) sec!) 
Winter 
Northern Hemisphere (Jan.)....| 2.25 | 54°N | 5.60 | 20°N 
Southern Hemisphere (July)....| 7.10 | 50°S | 8.15 | 13°S 
Summer 
Northern Hemisphere (July)....| 1.10 | 53°N | 1.25 | 25°N 
Southern Hemisphere (Jan.)....| 5.60 | 50°S | 3.10 | 22°S 
* Determined by choosing the 20° latitude band of greatest 
meridional pressure change; converting to geostrophic 
speeds, and noting the central latitude of the 20° band. 
Clearly, the Southern Hemisphere zonal wind system 
at sea level is relatively stronger, displaced farther 
equatorward, and shows a smaller percentual seasonal 
variation than that of the Northern Hemisphere. 
Willett also points out the surprisingly large seasonal 
fluctuation of the subtropical easterlies and their great 
strength as compared even to the zonal westerlies. This 
is indeed a fact which must be of importance in treating 
2. Subsequent to the preparation of this article two papers 
dealing with conditions at upper levels in the Southern Hemi- 
sphere have been published: ‘‘A Meridional Aerological 
Cross Section in the Southwest Pacific’? by F. Loewe and 
U. Radok, J. Meteor., 7:58-65 (1950); and ‘‘A Meridional 
Atmospheric Cross Section for an Oceanic Region’? by J. W. 
Hutchings, J. Meteor., 7:94-100 (1950) —Ed. 
555 
the general circulation, for the area covered by this 
wind system is far greater than that embraced by other 
branches of the zonal circulation. 
A comparable table for upper-level circulations for 
both hemispheres is impossible to present at this time. 
From the British upper-level normals [9], however, it 
appears that the contrast of zonal wind speeds, so 
evident at sea level, is much less pronounced or perhaps 
even lacking at mid- and high-tropospheric levels. For 
example, the geostrophic zonal wind speed computed 
between latitudes 20° and 40° from the British 300-mb 
charts for both hemispheres during winter (December-— 
February in the Northern Hemisphere, and June—Au- 
gust in the Southern Hemisphere) gives 29.5 m sec 
for the Northern Hemisphere as compared with 29.7 
m sec for the Southern Hemisphere. 
2. The undulations in the circumpolar vortex of the 
Southern Hemisphere appear to be much less pro- 
nounced (7.¢c., have much less amplitude) than in the 
Northern Hemisphere. In fact, according to the British 
charts at 300 mb, it seems questionable even to talk of 
troughs and ridges in the Southern Hemisphere. 
3. The cellular character at sea level is correspond- 
ingly less pronounced in the Southern Hemisphere than 
in the Northern Hemisphere, although ‘‘centers of ac- 
tion” are plainly present. 
Various attempts have been made to characterize the 
state of the general circulation by numerical values. 
Thus certain circulation indices have come into use. 
Among the most prominently used are the geostrophic 
zonal wind speed, the “maximum ”’ index, the meridional 
index, and zonal wind-speed profile. Definitions follow: 
1. The geostrophic zonal wind speed within different 
latitudinal bands is computed either regionally or for 
the entire hemisphere. Such quantities as the ‘zonal 
index” express numerically the zonal wind speed aver- 
aged between two latitudes (generally 35°N and 55°N) 
and characterize the temperate-latitude westerlies. The 
polar easterlies (55°N to 70°N) and subtropical easter- 
lies (35°N to 20°N) are similarly computed. These 
indices may also be evaluated for upper levels of the 
atmosphere. 
2. The ‘‘maximum index” expresses the peak strength 
of some particular branch of the zonal circulation, as 
well as the latitude at which it appears. 
3. Meridional indices express numerically the total 
north-south flow across a particular latitude circle. 
4. In the last few years another effective form of rep- 
resenting important characteristics of the general cir- 
culation has come into use. This is the zonal wind-speed 
profile, on which the mean zonal speed within 5° lati- 
tude zones is plotted against latitude. 
The seasonal behavior of Northern Hemisphere zonal 
indices computed for monthly periods is shown in Fig. 
7. Recent work [35] with hemisphere-wide upper-air 
maps suggests that, at least at certain times of the year, 
normals computed for periods as long as a month con- 
tain smoothing sufficient to obscure certain important 
shorter period variations. Most important of these is 
the primary decline and subsequent rise of the mid- 
troposphere zonal westerlies which usually takes place 
