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DISCUSSIONS 551.524.733 (99) Antarctic stratospheric warming reviewed in the light of 1967 observations By H. R. PHILLPOT (Read 21 May 1969; see Q.J., 95, p. 329. Presented by Mr. R. A. Ebdon) (This is a fuller version than printed in the July 1970 Quarterly Journal) Dr. R. J. MURGATROYD : In reply to Professor Sheppard's question concerning knowledge on Arctic stratospheric warmings, it seems that the onset or ' triggering ' of a sudden warming in the higher latitudes of the Northern Hemisphere stratosphere may well be determined by events in the troposphere. In the late winter the polar regions are very cold and there are strong zonal winds and large values of polewards heat transfer both in the troposphere and lower stratosphere. In these conditions tropospheric disturbances may produce a substantial boundary flux of energy BE into the stratosphere mainly through the pressure interaction (flux of geopotential) term. This energy input can be large compared with the energy exchanges already occurring within the stratosphere. Although the winter lower stratosphere may not be unstable initially to small perturbations the isentropes at high latitudes slope upwards towards the poles and it seems that when the boundary input is unusually large conditions are suitable for a primarily baroclinic type of growth i.e. AZ + AE + KE + Kz + AZ (where A, K represent available potential and kinetic energy respectively and the suffixes z and E denote their zonal and eddy components) to be initiated. The process is not entirely baroclinic, however, as Kz falls rapidly suggesting a baro- tropic contribution to the growth of KE. Hence the instability is probably a mixed type. After the peak warming and reversal of the mean meridional temperature gradient the stratosphere will slowly revert to its ' normal ' forced energy sequence BE -+ KE + AE -+ Az and KE --+ Kz. Mid-winter warmings are more frequent in the mesosphere and upper stratosphere than in the lower stratosphere and also well observed in the Northern Hemisphere but not in the Southern Hemisphere. The factors which may be important are (i) the vertical stability (ii) the horizontal temperature gradient (iii) the wind shear in the horizontal and (iv) the eddy energy input from the troposphere. Baroclinic instability is favoured by low values of (i), growth of baroclinic disturbances by large values of (ii) and barotropic instability by large values of (iii). In the mean the factors (ii) and (iii) are most favourable in the upper stratosphere and above and the vertical stabilities (i) of the lower stratosphere and lower mesosphere may not be very different. It is not possible, however, without more data and detailed case studies to be more definite on the relative importance of these factors. As regards (iv), however, it seems likely that, since the polar vortex of the Southern Hemisphere is more symmetrical than that of the Northern Hemisphere and there is no counterpart there of the ' Aleutian High ' and not such a well marked wave-number two standing wave system, the comparative lack of large-scale eddy activity must be an important factor in accounting for the relatively small number of mid-winter warmings. It is not yet clear how the factors (i), (ii) and (iii) compare at the different levels between the two hemispheres and further data are also required to study this aspect more thoroughly. Mr. H. H. LAMB (communicated) : The author is to be congratulated on a useful contribution to knowledge in an area of atmospheric behaviour where previous data were lacking and a remark- able demonstration of what can be done in the way of hemispheric-scale analysis on the basis of only about 20 stations south of latitude 40"s. Contrast the abundance of stations in the Northern Hemisphere, e.g. typically about 150 reports for the 30 mb level on any given day north of 40"N. Nevertheless coverage is still very poor in the ocean zone, and especially in the Pacific sector, of the Southern Hemisphere, and one wonders what prospect there is even now of stratospheric observations there. Inevitably these vast ocean areas remain somewhat of a gap in Phillpot's discussion of the events in 1967. Yet there are several hints that the Pacific sector may be impor- tant : e.g. (1) the high values of 30 mb geopotential from September onwards at 65"s from 150"E to 12O"W (compare the Aleutian high in much the same sector and its importance in the Northern Hemisphere) also (2) the tendency when there is a third trough in the Southern Hemi- sphere circulations for it to be near the coast of South America. One should remember that the whole of the Pacific sector of the globe, north and south of the Equator, is clearly involved in 131

Antarctic stratospheric warming reviewed in the light of 1967 observations

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DISCUSSIONS 551.524.733 (99)

Antarctic stratospheric warming reviewed in the light of 1967 observations

By H. R. PHILLPOT

(Read 21 May 1969; see Q.J., 95, p. 329. Presented by Mr. R. A. Ebdon)

(This is a fuller version than printed in the July 1970 Quarterly Journal)

Dr. R. J. MURGATROYD : In reply to Professor Sheppard's question concerning knowledge on Arctic stratospheric warmings, it seems that the onset or ' triggering ' of a sudden warming in the higher latitudes of the Northern Hemisphere stratosphere may well be determined by events in the troposphere. In the late winter the polar regions are very cold and there are strong zonal winds and large values of polewards heat transfer both in the troposphere and lower stratosphere. In these conditions tropospheric disturbances may produce a substantial boundary flux of energy B E into the stratosphere mainly through the pressure interaction (flux of geopotential) term. This energy input can be large compared with the energy exchanges already occurring within the stratosphere. Although the winter lower stratosphere may not be unstable initially to small perturbations the isentropes at high latitudes slope upwards towards the poles and it seems that when the boundary input is unusually large conditions are suitable for a primarily baroclinic type of growth i.e. AZ + AE + KE + Kz + AZ (where A , K represent available potential and kinetic energy respectively and the suffixes z and E denote their zonal and eddy components) to be initiated. The process is not entirely baroclinic, however, as Kz falls rapidly suggesting a baro- tropic contribution to the growth of KE. Hence the instability is probably a mixed type. After the peak warming and reversal of the mean meridional temperature gradient the stratosphere will slowly revert to its ' normal ' forced energy sequence BE -+ KE + AE -+ Az and K E --+ Kz.

Mid-winter warmings are more frequent in the mesosphere and upper stratosphere than in the lower stratosphere and also well observed in the Northern Hemisphere but not in the Southern Hemisphere. The factors which may be important are (i) the vertical stability (ii) the horizontal temperature gradient (iii) the wind shear in the horizontal and (iv) the eddy energy input from the troposphere. Baroclinic instability is favoured by low values of (i), growth of baroclinic disturbances by large values of (ii) and barotropic instability by large values of (iii).

In the mean the factors (ii) and (iii) are most favourable in the upper stratosphere and above and the vertical stabilities (i) of the lower stratosphere and lower mesosphere may not be very different. It is not possible, however, without more data and detailed case studies to be more definite on the relative importance of these factors. As regards (iv), however, it seems likely that, since the polar vortex of the Southern Hemisphere is more symmetrical than that of the Northern Hemisphere and there is no counterpart there of the ' Aleutian High ' and not such a well marked wave-number two standing wave system, the comparative lack of large-scale eddy activity must be an important factor in accounting for the relatively small number of mid-winter warmings. It is not yet clear how the factors (i), (ii) and (iii) compare at the different levels between the two hemispheres and further data are also required to study this aspect more thoroughly.

Mr. H. H. LAMB (communicated) : The author is to be congratulated on a useful contribution to knowledge in an area of atmospheric behaviour where previous data were lacking and a remark- able demonstration of what can be done in the way of hemispheric-scale analysis on the basis of only about 20 stations south of latitude 40"s. Contrast the abundance of stations in the Northern Hemisphere, e.g. typically about 150 reports for the 30 mb level on any given day north of 40"N.

Nevertheless coverage is still very poor in the ocean zone, and especially in the Pacific sector, of the Southern Hemisphere, and one wonders what prospect there is even now of stratospheric observations there. Inevitably these vast ocean areas remain somewhat of a gap in Phillpot's discussion of the events in 1967. Yet there are several hints that the Pacific sector may be impor- tant : e.g. (1) the high values of 30 mb geopotential from September onwards at 65"s from 150"E to 12O"W (compare the Aleutian high in much the same sector and its importance in the Northern Hemisphere) also ( 2 ) the tendency when there is a third trough in the Southern Hemi- sphere circulations for it to be near the coast of South America. One should remember that the whole of the Pacific sector of the globe, north and south of the Equator, is clearly involved in

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132 DISCUSSIONS

the quasi-biennial Southern Oscillation which demonstrates the existence of a pattern of inter- hemispheric relationships. The Pacific sector is clearly one place where Southern Hemisphere data, if they were available, might be important to Northern Hemisphere forecasting.

One notices with much the same thoughts in mind that Phillpot found the years 1964 and 1967 were alike in being the warm years (and alike in other aspects of their behaviour during the Antarctic spring) and that the years in between followed a different pattern. It was 1964 and 1967 that produced the best summers of the decade so far in Europe, preceded in both cases by early warming of the polar stratosphere in the northern spring. If there be some global link between these events, as the data seem to suggest, the signs in both Northern and Southern Hemispheres are that for the present the quasi-biennial oscillation has slowed up and has a nearly three-year time scale.

Mr. H. R. PHILLPOT (communicuted reply) : Whilst I appreciate the opportunity of com- menting on Mr. Lamb's remarks on my paper which was very kindly presented by Mr. R. A. Ebdon, I do not feel that much can be said particularly about possible inter-hemispheric relation- ships, and even about behaviour over the South Pacific sector.

It is true that an examination of stratospheric temperatures, derived from TIROS VII radiation measurements in the 15-micron band, in the (southern) spring of 1963 showed that over almost the whole of the Western Hemisphere in Lat. 60"s the temperature remained low later than over other areas at this latitude; and also that a persistent and very well defined feature of satellite cloud photographs is a cloud band oriented from north-west to south-east across the central and south-eastern South Pacific Ocean.

On the other hand, upper air temperature soundings made from the United States oceano- graphic research vessel Eltanin, through the years 1962-66 when the ship frequently operated in high latitudes in the Western Hemisphere, have not revealed any features or evidence to suggest that the stratospheric behaviour described in the paper requires modification.

It appears to me that a most significant feature of the southern polar area is the location, and movement of the vortex. If, as was established previously, this is centred, and persists, near the geographic pole (as in the spring of 1959 and 1961) then the warming is (a) less dramatic i.e. with reduced wave activity around the vortex, (b) more symmetrically distributed around the hemisphere, and (c) without marked change in rate through the season. If, however, the vortex moves even a relatively short distance away from the geographic pole then the warming can appear more dramatic - partly because the effect of any migratory waves may be observed through short period temperature fluctuations (these appear to be most pronounced at stations in sub-Antarctic latitudes, and on the Antarctic continental coastline particularly of east Antarctica, as in 1964); and also through a small temperature change in the general level accompanying the shift of the (cold) centre. Precisely what bearing these variations might have on the global circulation is hard to suggest.

It would be useful to establish whether the mean temperature at the centre of the vortex varies significantly from year to year, but again this is difficult without a chart series.

As Dr. Murgatroyd has pointed out more case studies are required. Unfortunately, the disposition of the reporting stations is such that only in the South American sector does there appear to be any hope of assessing features like horizontal wind and temperature gradients. This sector is an interesting one and a detailed study of the available observations might be rewarding, but it would appear that the only real hope in the other sectors, particularly off the east Antarctica coast, lies in information which may become available from satellites.

551.507.352 : 551.521.2 : 551.465.755

Microwave observations of sea state from aircraft

By W. NORDBERG, J. CONAWAY and P. THADDEUS

(Read 21 January 1970; see Q.J. 95. p. 408. Presented by Professor P. A. Sheppard)

Mr. J. S. SAWYER : Is the reflected solar radiation at 1.55 cm sufficiently strong to contribute

Dr. R. FRITH : The great difference in emission between a smooth surface and a rippled to measured radiation if observations are carried out during the day 1

surface rather suggests that the emission comes mainly from quite near the surface.