[Read before the Wellington Branch, September 16, 1942; received by the Editor, September 17, 1942; issued separately, March, 1943.]
Summary: This paper deals with the upper wind conditions on the 28th-30th November, 1929, as shown by pilot balloon soundings at Little America. From the soundings it is deduced that for at least a week a warm anticyclone extending from the surface to beyond 5 km. was located over Antarctica, and it is suggested that such anticyclones are more frequent in the area than the theory of the polar vortex would lead one to suppose.
In 1893, taking the distribution of mean pressure at the earth's surface and assuming values for the mean lapse rate of temperature, Teisserenc de Bort constructed pressure maps for levels at various heights in the upper air. From these and similar maps it may be inferred that the average circulation in the upper part of the troposphere is controlled by two gigantic cyclones, centred one at each pole (the so-called polar cyclones or polar vortices). The circulation in the Southern Hemisphere appears to be simpler than that in the Northern, and this was, and still is, attributed to the more uniform distribution of land and sea south of the equator. The Southern circulation, then, should be more representative of average conditions, such as would exist on a globe uniformly covered by water. If, as it seems reasonable to assume, the winds in extra-tropical regions are on the average geostrophic, the maps show that between about 30° S. and the Antarctic coast westerly winds must prevail throughout the atmosphere from the surface to the tropopause. This is partly confirmed by surface and upper wind observations that have been made in South America, South Africa, Australia and New Zealand, since the construction of the original maps. But observations on the Antarctic Continent have shown that there the surface winds are predominantly easterly or southeasterly in direction, at all events near and on the coast—i.e., that the surface circulation is controlled, not by a polar depression, but by an anticyclone, co-extensive with the continent. However, most meteorologists assume that this anticyclone is shallow and that, at no great height above the continent, it is replaced by the inner portions of the polar vortex—that here also the upper winds are predominantly westerly.
It is obvious from its derivation that the idea of a “polar vortex” is an abstraction, adequate, perhaps, as a summary of average conditions, but misleading if applied to the analysis of individual synoptic situations in the far south. To find, by the manipulation of mean values, that the Antarctic anticyclone is on the average a shallow pressure feature may be justified, but to
deduce from this that all easterly, south-easterly and southerly winds, and in particular the blizzards, are “katabatic” or are shallow surface flows is to obtain a result that it really beyond the capacity of climatological methods.
In a recent publication (5) I have shown that easterly blizzards may occur in the South Indian Ocean at some distance off the Antarctic coast, that these easterlies are similar to those described by Meinardus at the “Gauss” winter station on the coast (3), and that they are not “katabatic flows,” but are essential parts of vigorous cyclonic circulations in the South Indian Ocean. It is probable, of course, that these easterlies are shallow and give place to northerlies or north-westerlies at upper levels, since an occlusion is usually associated with the depression. But it is pertinent to ask whether the easterlies are shallow at some distance south of the low-pressure centre, say, at 80° S. South of the Indian Ocean there is no information which would enable us to answer this question, but as depressions are frequently found at the mouth of the Ross Sea or in the sea itself, a clue, at least, to the answer may be found in the beautiful series of pilot balloon runs made in 1929 and 1934 at Little America (Latitude 78° 34° S.). (2).
Analysis of these pilot balloon ascents shows that winds from an easterly quarter are surprisingly frequent at all levels up to
10 kilometres. The frequencies (as given by Grimminger) of different wind directions at the surface, 2.5 km., 5 km. and 10 km., from the combined data for 1929 and 1934 are summarised in Figure 1 in the form of wind roses. The rose for 5 km., especially, is worth study. It shows that at this level winds from all directions are approximately equal in frequency. We may suspect from this alone that deep easterlies—i.e., easterlies extending from the surface up to or beyond 5 km.—are far more frequent than would be expected from the theory of the polar vortex. The suspicion has been confirmed in the following manner: For the 22 complete months (1929–30 and 1934–35) for which pilot balloon flights are available I have taken the number of days when at least one flight reached 5 km.; then, if a deep easterly be defined as a day on which, from the surface geostrophic level to 5 km., the wind remained between N.N.E. and S.S.E., these deep easterlies amounted to 38% of the 5 km. flight-days. If the flights reaching 3 km. or more be treated in the same way it will be found that deep easterlies amounted to 44% of the 3km. flight-days. It may be contended that 3 or 5 km. flights are more likely on those days when deep easterlies prevail. Fortunately Grimminger (1) has made an analysis of the conditions preceding and accompanying “clear” periods, when long flights are most likely. He defines a “clear” period as one during which there was not more than five-tenths of upper clouds, not more than two-tenths of middle clouds and not more than one-tenth of low clouds. Figure 2 shows a wind rose giving the percentage frequencies of winds from different directions at 5 km. during “clear” periods.
Comparison of Figures 1 and 2 and perusal of Grimminger's tables reveals that “clear” periods are probably dependent on the direction of the wind in the lowest layers; winds off the ice, that is, with a southerly component, tend to be free of low cloud. The 5 km. winds are still more or less equally distributed.
Little insight into deep easterlies can be gained by a study of frequencies alone. Before any interpretation of the upper
circulation is possible a detailed investigation of individual flights and the relative synoptic situations would have to be carried out. Unfortunately, synoptic material for the periods covered by the expeditions is almost entirely lacking, and the New Zealand and Australian reports that are available are of too poor a quality to serve for reliable frontal analysis. For the period November-December, 1929, however, I have been able to obtain a private weather log maintained by Captain W. W. Stuart on board the whaling ship “Southern Princess” in the Ross Sea. My best thanks are due to Captain Stuart for permission to extract information from this log and to use it in the analyses shown in Figures 5, 6 and 7. It should be understood that these analyses are probably correct only in broad outline, representing the best means of explaining the changes in the meteorological elements at Little America, at the “Southern Princess” (for which readings of the barometer and thermometer at four-hour intervals are given in the log) and at stations in New Zealand keeping autographic records at that time. The analyses provide a background, as it were, for a different and more searching method of attack, recently developed by Rossby and his pupils. (4, 6.)
To illustrate this method, which should be of very great value in future Antarctic studies, I have chosen the deep easterlies that prevailed at Little America during the period 00.00 G.M.T. 29th November, 1929, to 00.00 G.M.T. 1st December, 1929, as shown by the two remarkable balloon soundings at 09.42 G.M.T. and 15.52 G.M.T. on the 29th. The hodograph for the former of these flights is shown in Figure 3. From the hodograph it is evident that the geostrophic wind at the top of the friction layer (given by the Ekman spiral) was 4.7 m/s from 68°; that between that level and 1.7 km. the wind shear vector was directed approximately from north to south, indicating a similar trend in the mean isotherms and, consequently, that the air in the lowest 1.7km. was warmer to the east of the station than to the west; that from
1.7 km. to 5.3 km. the shear vector was directed approximately from south-east to north-west, indicating that the air in that layer was warmer to the south-west—in other words, that the gradient of mean temperature ran from south-west to north-east. It is also evident that up to 5.7 km. there is no sign of winds with a westerly component, such as would be expected if the theory of the polar vortex were always applicable to individual synoptic situations. If the upper winds are geostrophic, and there is every reason to
suppose that they were, a pressure gradient for south-easterly winds existed even at 5 km. and at that level isobars and mean isotherms were parallel.
From the shear vectors below 5 km. it may be deduced that, while the air to the north-west, south-west and south-east of the station was relatively stable, a relatively unstable vertical stratification lay to the north-east. Some light may be thrown on this by the surface synoptic analysis. Reference to Figures 5 and 6 shows that at the time of the flight an old quasi-stationary low-pressure area covered the Ross Sea. In association with the southern portion of a meridional front, this low had moved into the Ross Sea on the 27th November (G.M.T.) after a period of nearly a week during which pressure had been abnormally high both at Little America and the “Southern Princess.” The northern part of the meridional front (see 5 in bibliography) had meantime been stationary near Tasmania and had been deformed on the 28th, giving rise to the wave, which, partly occluded, is seen on Figure 5 approaching the “Southern Princess.” The snow reported by the ship at 21.00 G.M.T. on the 28th is warm front precipitation. Captain Stuart has a note in his log for this day: “Very poor chasing weather. Incessant fog and snow.” The log shows that the barometric minimum occurred at about 04.00 G.M.T. on the 29th, after which the barometer rose rapidly and the temperature (0° C. at the time of passage of the occlusion) fell, to –4° C. at 16.00 G.M.T. on the 29th. The subsequent history of this depression and its amalgamation with the Ross Sea low may be inferred from Figures 6 and 7 and from the meteorogram for Little America shown in Figure 8. According to the meteorogram, the occlusion was very weak when it passed over Little America (“very light snow in the evening”) and the depression had lost much of its previous intensity.
From this brief survey of the synoptic situation it will be seen that the surface geostrophic wind at 09.42 G.M.T. on the 29th was controlled by the pressure gradient on the southern side of an old quasi-stationary low in the Ross Sea. Apparently there was still a little activity in this low, since the distribution of the shear vectors in the lowest five kilometres indicates that a relatively unstable stratification existed in the air lying to the north-east of Little America. Nevertheless, the rise in pressure at the station shows that the low was filling up. Rossby and his pupils (4) have shown that it is possible from pilot balloon data alone to compute pressure changes due to advection in the lower atmosphere in extratropical regions. With an appropriate choice of units the three-hour tendency due to advection in the lowest 3km. is equal to the product of twice the area swept out by the horizontal wind vectors between the top of the friction layer and 3km., when the vectors are plotted in polar co-ordinates, and a multiplier that is constant for a given latitude. It is obvious from Figure 3 that advection in the lowest three kilometres would bring over the station relatively cold air from the north-east quadrant; it is easily computed by Rossby's method that the change in surface pressure induced by this advection would amount to + 0.014″.
The actual rise in pressure at Little America in the three hours preceding 10.00 G.M.T. as given in the tables of hourly values of pressure was — 0.02″. It will be noticed that between 3 km. and 3.7 km. the shear increases rapidly; if, instead of calculating the pressure rise due to advection in the lowest 3 km., we compute that attributable to flow in the lowest 3.7 km. we find that the pressure rise would be + 0.026″. The hodograph shows that no change in the mean temperature isotherms is to be expected from advection between 3.7 km. and 5.3 km. These facts justify the hypothesis that the three-hour pressure change was wholly due to advection of cold air in the lowest 3.7 km. and display excellently the power of Rossby's method.
The actual horizontal gradient of mean temperature for the lowest 3 km. in the neighbourhood of Little America is also calculable from the hodograph, and the calculation is facilitated by the fact that a temperature sounding to 3 km. was made about that time, on the South Polar Flight; from the observed temperatures the mean temperature has been estimated at –15° C., and this gives a horizontal mean temperature gradient of 5° C./735 km. from 205°. (South = 180°.)
At 15.52 G.M.T. 29th November, the time of the second long pilot balloon sounding, the hodograph of which is shown in Figure 4, the winds up to about 6 km. had decreased considerably in force
(on the average by about 9 m/s.) and had veered 20°–30°. The direction of the horizontal mean temperature gradient in the lowest 3 km. was still approximately the same (195°), but its magnitude had decreased to 5° C./2840 km. The hodograph shows that advection in the lowest 3.3 km. would have brought about a pressure fall; computation by Rossby's method gives — 0.006″ in three hours. The actual pressure fall at the surface in the three hours preceding 16.00 G.M.T. was — 0.01″. An interesting feature of the flight is the intrusion of a shallow layer of winds from a westerly quarter between 1 and 2 km. Evidently a relatively unstable atmospheric stratification still persisted somewhere northeast of the station. The westerlies appear to be associated with the approach of the depression from the north-west, as the meteorogram for Little America shows an accelerated pressure fall after 16.00 G.M.T., associated in its later stages with the movement of a bank of stratocumulus from the west and, during the period of snowfall, from the north. By 09.39 G.M.T. on the 30th, when the third balloon flight of this series was made, there was an almost complete overcast of stratocumulus, moving from the north. The flight was a short one as follows:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|09.39 G.M.T. 30th November, 1929.||0° = North. 270° = West.|
|414 m:||018°||8.8||"||Cloud: 9/10 StCu. North.|
|612 m:||006°||8.4||"||Remarks: Few flakes of snow.|
|801 m:||357°||7.9||"||Disappearance: Entered StCu 1,230 m.|
Wartime limitations of space prevent my giving a hodograph or table of the fourth flight, at 21.34 G.M.T. on the 30th, but the main features of the analysis are as follows:—The flight reached 3,150 m. and indicated that winds between east and north-east prevailed from the surface up to that level; speeds varied between 3.8 and 11.8 m./s., the mean speed being 6.4 m./s. The hodograph shows that the horizontal gradient of mean temperature in the layer explored by the flight was weak and had changed direction to 5° C./2370 km. from 130°. In the layer from the surface to 2 km. there were indications of an unstable stratification to the north of the station, while the layer above 2 km. still showed the effects of instability in the north-east. Advection in the lowest 3 km. should have produced a fall of pressure, but in fact the three-hour tendency at 22.00 G.M.T. was — 0.01″. We must therefore conclude that above 3 km. there was marked anticyclogenesis. It should be pointed out, however, that pressure began to fall at 22.00 G.M.T., but not, as may be seen from the meteorogram, with any rapidity.
For five days after the situation which is the topic of this paper winds from an easterly quarter prevailed in the upper atmosphere. Some of the soundings reached 10 km. and the lowest flight reached 3.8 km. In no flight during this period were there westerly components in the winds above 2 km., though some showed westerlies below that level.
Before passing to the explanation of the facts already elucidated, it may be as well to point out a few interesting features off the main track of inquiry:—
(a) There is a well developed friction layer when the wind in the lowest layer is greater than about 4 m./s. A modified Ekman spiral then appears in the hodograph. See Figure 3.
(b) When the surface and lower winds are light there is a katabatic flow from some southerly point. The winds veer with height, increase at first, then decrease, and finally show a very shallow layer of backing before settling down at about 1 km. to the geostrophic value. Figure 4 is a good example.
(c) Even in those flights with a well-developed friction layer, if the surface wind is off the Barrier, there will be distortion of the Ekman spiral, which I have taken to be due to a “katabatic” effect. The effect shows as a rapid increase in the wind at about 0.2 km. but without much backing, after which the wind backs rapidly without much increase in speed. (See Figure 3.) In the rare cases when the wind is off the Ross Sea this katabatic-effect is not present.
(d) In long flights in the light season the tropopause can usually be located from the hodograph. The shear vectors show an abrupt change of direction as in Figures 3 and 4. At this time the tropopause seems to have been at about 5 km.
The upper wind regime and the surface changes in the meteorological elements during the 29th and 30th November, 1929, can best be explained in the following manner:—
(a) During the period, and for some time afterwards, an anticyclone of great vertical extent was located south of Little America. This anticyclone was warmer than its surroundings, up to great heights. The observations force one to the conclusion that it extended at least to the tropopause.
(b) The changes in the upper winds and horizontal mean temperature gradient on the 29th indicate that the anticyclone was becoming less intense and was probably re-oriented to extend off the continent somewhere in the north-west. This is partly confirmed by the high pressures at the “Southern Princess” on the 29th and 30th.
(c) To account for the position and structure of the anticyclone it must be assumed that it was maintained by inflow of air at high levels and that its warmth was due to subsidence in the whole air mass of which it was constituted. I would suggest that the source of the inflowing air at high levels was in the northeast, and lay not above the weak depressions over the Ross Sea but above the more vigorous and extensive disturbance over New Zealand.
(d) The depressions that moved into the Ross Sea appear to have little effect on the anticyclone. The second, which was quite active when it passed the “Southern Princess,” was insignificant by the time it reached the Ross Sea. The front associated with it, likewise, rapidly lost intensity; it is doubtful, of course, whether any front, in the strict sense passed over Little America. However,
any remaining concentration of solenoids associated with the depression appears to have been effective only below 3 km. Evidently the dominating influence on the circulation was the polar anticyclone.
(e) October and November, 1929, were remarkable for the number of deep depressions that formed in the Tasman Sea—New Zealand area. This is now recognised in the Meteorological Office, Wellington, as a sign of low circulation index in the South-west Pacific. During periods of low index the eastward movement of the atmosphere and of the secondary circulations is much smaller, and the meridional flow, both at the surface and aloft, is more pronounced than usual. October and November, 1929, were also remarkable for the number of days when deep easterlies prevailed at Little America. This suggests that the inner portion of the polar vortex during periods of low index is not merely weak, as the M.I.T. School maintains, but is absent, being replaced by a warm, high anticyclone.
In conclusion, I wish to thank Flight Lieutenant Gabites and Pilot Officer Hutchings for reading and criticising the manuscript of this note and my wife for preparing the drawings for publication.