Stratospheric final warmings in the Southern Hemisphere and their energetics

Meteorol Atmos Phys 83, 35–49 (2003)DOI 10.1007/s00703-002-0558-6

CPTEC Instituto Nacional de Pesquisas Espaciais, INPE, S~aao Jos�ee dos Campos, SP, Brazil

Stratospheric final warmings in the Southern Hemisphereand their energetics

V. B. Rao, M. B. Rosa, J. P. Bonatti, and S. H. Franchito

With 8 Figures

Received October 3, 2001; revised June 5, 2002Published online: November 25, 2002 # Springer-Verlag 2002

Summary. Using 9 years (1985–1993) data, final strato-spheric warmings in the Southern Hemisphere are studied.Interannual variations in the onset date and the temperaturesare noted. In 1985 the stratosphere was colder by about 5 Kand the wave activity was less. This year the final warminggot delayed. In contrast in 1988 the final warming occurredearlier when compared with the mean picture and the waveactivity was more. An examination of Eliassen-Palm fluxesshowed the important role of planetary waves in the wave-mean flow interaction. In the energetics the most spectacu-lar change is the reduction of zonal kinetic energy. Beforethe warming the energy exchanges were Pz! Pe!Ke!Kz Pz and after the warming they were Pz Pe Ke!Kz Pz. The dramatic reduction of zonal kinetic energyseems to be due to two effects: the reduction in Ke!Kz

conversion and the weakening of direct meridional circulation.

1. Introduction

Stratospheric sudden warming is one of the mostspectacular phenomena in the atmosphere. In theNorthern Hemisphere (NH) these warmings oc-cur during the winter time, where there is noapparent heat source such as solar radiation.Temperatures increase rapidly several tens ofdegrees in a few days and the normal temperaturegradient (with higher temperature in mid-lati-tudes and lower values at the pole) is reversed.Associated with the reversal temperature gradi-ent, the strong westerly circumpolar jet thatexists before warming gets completely destroyed

creating easterlies. Many observational and the-oretical studies (Labitzke, 1981; McIntyre, 1982and many others) revealed that the warmings arecaused by planetary waves, mainly waves num-bers 1 and 2 generated in the troposphere. Justbefore the warming, planetary waves becomevery efficient in transporting heat and momentum(Rao and Bonatti, 1981). These planetary wavesinteract with the mean flow to decelerate thepolar-night westerlies, which in turn gives riseto reversal of the meridional temperature gradi-ent and this was first shown by Matsuno (1971).

In the case of the Southern Hemisphere (SH)midwinter warmings are weak and the most dra-matic events occur at the end of the austral winterand begining of the spring. Strong planetary wavesdevelop in September and October, which affectthe radiative balance in the stratosphere and alterthe distribution of temperature and winds. Thewesterlies of the winter get destroyed finally gen-erating easterlies. This warming is called finalwarming after which the summer circulation getsstablished.

Although midwinter warmings in the NHreceived a great attention, the final warmings inthe SH received comparatively less attention.Nevertheless, some studies have been made.Godson (1963) studied final warming in 1961over Byrd, Antarctica. He reported that in this

year final warming occurred in a very gradualfashion. Hirota et al (1983) noted that the eddyactivity during the spring and summer is morevigorous in the SH than in the NH. Farrara andMechoso (1986) briefly studied final warming inthe SH for the years 1978–1983. They noted thatthere is substantial interannual variation in thetime evolution of the warming. Shigtani et al(1990) examined eastward travelling waves dur-ing the spring of 1983. Examining the phases ofwave numbers 1 and 2 they noted that the wave 1amplitude reaches its maximum when the ridgeof stationary wave 1 and eastward travellingwave 2 overlap. Mechoso (1990) compared finalstratospheric warmings in the NH and SH. Henoted important interhemispheric differences inthe location and magnitude of the largest tem-perature increase over the polar region duringthe spring. Those in the SH are in the lowerstratosphere and those in the NH are in the upperstratosphere, the former being almost twice aslarge as the latter. This suggests that the SHwarmings can be studied using only lower strato-spheric data. Randel and Newman (1998) de-scribed characteristics of the stratosphere in theSH and discussed briefly the final warming in1995. They also noted that the highest interannu-al variability in 50 hPa zonal mean temperatureoccurs during the spring warmings in the SHwhile in the NH highest variability occurs inthe midwinter.

In the present article we study the interannualvariation of final stratospheric warming in the SHfor 9 years (1985–1993). Compared to the briefstudy of Farrara and Mechoso (1986) for 1978–1983, we use data for a different period. Further,Farrara and Mechoso (1986) and Mechoso (1990)used geostrophic winds, which are of question-able accuracy (Elson, 1986; Boville, 1987). Inthis study we propose to use actual wind data.Interannual variation in middle atmosphere is asubject of current interest (Hamilton, 1995). Inaddition, we calculate energetics of the finalwarming in the SH which they did not discuss.To our knowledge, energetics of final warmingin the SH have not been discussed earlier. Studyof energetics is important because it eluci-dates different physical processes at work and 9years of data will give an idea of interannualchanges in the magnitudes of various energyparameters.

2. Data sources and method of analysis

The data used in this study is obtained fromEuropean Centre for Medium-Range WeatherForecasting (ECMWF). The variables used are:geopotential height, temperature, vertical veloc-ity, zonal and meridional velocity components.The area of the study is between 25� S and 85� Sat all the longitudes with a grid of 2.5� � 2.5�

(latitude and longitude). The pressure levels are100, 70, 50, 30 and 10 hPa. The data are the dailymeans of 00:00 and 12:00 UTC for the period1985 through 1993.

In the present study we propose to calculatethe Eliassen-Palm (E-P) vector and its diver-gence, which can be used as a diagnostic forthe interpretation of wave-mean flow interaction(Edmon et al, 1980).

The E-P vector in pressure coordinates is givenby (Andrews et al, 1983):

F ¼ ðF’;FpÞ;where

F’ ¼ � a cos’½u0v0 þ @u=@p�; ð1Þ

Fp ¼ � a cos’

� u0v0

þ �

1

a cos’

@ðu cos’Þ@’

f

��;

ð2Þ

¼ v0�0

@�@p

¼ v0T0�RTcpp @T

@p

� ; ð3Þ

where ’ represents the latitude, p the pressure,(u, v,!) the velocity in (longitude, latitude, pres-sure) coordinates, � the potential temperature,T the temperature, a the radius of earth, f theCoriolis parameter, R the gas constant, � the airdensity and cp the specific heat at constant pres-sure. Overbar denotes zonal averages and primesdenote departure of bar.

The effect of eddies and zonal mean flow issuch that

@u

@t¼ 1

a cos’r � F; ð4Þ

where the operator r is given by

r ¼ 1

a cos’

@

@’þ @

@p: ð5Þ

36 V. B. Rao et al

Note that in (1)–(5) an extra cos’ arises becausewe are implicitly dealing with angular momen-tum which is proportional to u cos’ and v cos’.

In this study we also propose to examine theenergetics of the atmosphere during the finalstratospheric warming in the SH. The energyequations after Saltzman (1957) are written as:

@Ke=@t ¼ ðKe! KzÞ þ ðPe ! KeÞ þ Te; ð6Þ

@Kz=@t ¼ ðKe ! KzÞ ðKz ! PzÞ þ Tz; ð7Þ

@Pe=@t ¼ ðPz ! PeÞ ðPe ! KeÞ þ Qe; ð8Þ

@Pz=@t ¼ ðPz ! PeÞ þ ðKz ! PzÞ þ Qe; ð9Þ

where Kz and Ke are respectively the zonal andeddy energies, Pz and Pe are respectively the zonaland eddy available potential energies, Tz and Te

are respectively the zonal and eddy energy fluxes,Qz and Qe are respectively the zonal and eddypotential energy generations by diabatic heating,(Ke!Kz) is the conversion of eddy kineticenergy (KE) to zonal KE, (Pe!Ke) is the conver-sion of eddy available potential energy (PE) toKE, (Kz! Pz) is the conversion of zonal KE tozonal available PE, (Pz! Pe) is the conversion ofzonal PE to eddy PE. All terms are calculated forthe atmosphere from 25� S to 85� S and 100 hPa to10 hPa.

3. Results

3.1 Evolution of temperature and zonal wind

Figure 1 shows the distribution of temperature (K)from 1 September through 30 December at 60� Sfor each year (1985–1993) and the mean of allyears. Compared to the mean there are modestinterannual variations at this latitude (60� S).The behaviour of the atmosphere in 1985 wasvery different. The isotherms are more verticalshowing that the variations are abrupt. In all theyears the warming of the atmosphere from 1 Sep-tember to the end of the year can be seen. In 1988the atmosphere was much warmer (by about 5 K)during the early September compared to themean. This warming was probably associatedwith midwinter warming noted by Hirota et al(1990). During the early September, 1987 and1989 temperatures were colder by about 5 K inthe lower stratosphere compared to the mean.

During early 1986, the atmosphere was warmersimilar to that noted for 1988. During the laterpart of the spring and beginning of the summerthe differences are small except in 1985, whichwas colder.

Figure 2 shows the meridional variation of tem-perature at 10 hPa level. This figure gives a niceview of the final warming and its interannual var-iation. In the mean from about 1 September up toabout 10 October temperature decreases towardsthe pole from the subtropics. After this date tem-perature increases towards the pole showing thechange from winter to summer circulation. Asso-ciated with this change in the meridional gradientof temperature, the strong westerly zonal windchanges to summer easterlies (Fig. 3).

There are large interannual variations in theonset date of final warming as given by thechange in the meridional temperature gradient.In 1985 the onset date was delayed very muchand occurred around 1 November. In the mean itoccurs in the middle of October. Thus in 1985 theonset date of warming got delayed by almost twoweeks. As will be seen later, the wave activitywas very weak in 1985. In addition, the atmo-sphere was colder in 1985 than in the mean. Incontrast, the higher latitude temperatures werehigh at 10 hPa level during 1988, 1991 and1992, at least 15 K more during 1991 and 1992in comparison with the mean values.

Figure 3 shows the time-latitude cross-sectionof the zonal wind at 10 hPa level. In the mean theeasterlies first appear in low latitudes. The wes-terly jet core moves polewards reducing in inten-sity and finally giving way to easterlies. Thisseems to be the typical characteristic of finalwarming in the SH (Hartman, 1976; Mechosoet al, 1988 and others). Compared to this meanpicture, there are large interannual variations,particularly in the timing of appearance of east-erlies at higher latitudes. Compared to the meanzonal wind value in our case (maximum wind of70 m s 1), the values in Fig. 4 of Farrara andMechoso (1986) are higher. This difference isdue to the fact that geostrophic zonal winds (theircase) are overestimated compared to the ob-served winds (Boville, 1987). Although in themean there is a gradual decrease in the intensityof westerlies, in individual years there seems tobe periodical intensification and weakening ofwesterlies in September and October months.

Stratospheric final warmings in the Southern Hemisphere and their energetics 37

Fig. 1. Zonal mean temperature at 60� S in K for different years

38 V. B. Rao et al

Fig. 2. Meridional variation of temperature in K during the spring at 10 hPa


Fig. 3. Time-latitude cross-section of the zonal wind at 10 hPa. Contour interval: 10 m s 1

40 V. B. Rao et al: Stratospheric final warmings in the Southern Hemisphere and their energetics

Fig. 4. Eliassen-Palm (E-P) flux and its divergence for wave-1 at 10 hPa

As we shall see later, these periods can be relatedto the events of wave-zonal flow interactions(divergence and convergence in E-P fluxes, Figs.4 and 5).

During 1985, westerlies around 70� S at 10 hPalevel continued even up to 16 December. Asnoted earlier, the final warming in this year (asseen in the first reversal of temperature gradient)got delayed by about a month compared to themean. During 1988, however, the easterlies firstappeared at 70� S and 80� S by as early as 1November. In 1986 easterlies of the summermonths are stronger (about 30 m s 1).

Figures 4 and 5, respectively, show the E-Pvectors and their divergence (m s 1day 1) bywave numbers 1 and 2. In both Figs. 4 and 5the mean shows convergence (negative contours)and the values are higher for wave number 1 thanfor wave number 2. From Eq. (4) the conver-gence of E-P vector indicates deceleration ofzonal wind. This is in agreement with the decel-eration of zonal wind seen in the mean. Also thewave activity ceases after 16 November whenthe easterlies appear in the mean agreeing withthe well known Charney and Drazin (1961) con-dition for the vertical propagation of planetarywaves from the troposphere into the stratosphere.Another interesting feature seen in Fig. 4 and to alesser extention in Fig. 5 is the periodic behav-iour of about 2 weeks. A periodicity of about 2weeks in geopotential height of wave num-bers 1 and 2 has been noted by Mechoso et al(1988).

Large interannual variations are also seen inboth Figs. 4 and 5. Activity of wave number 2is very weak in 1985 when compared to its activ-ity in other years. As noted earlier, the finalwarming got delayed this year, suggesting theimportant role of planetary waves in the devel-opment of final warming in the SH. Upwardpropagating of planetary waves interact withthe mean flow producing the final warming (seeFig. 7, E-P fluxes before, during and after, alsoFarrara and Mechoso, 1986). In contrast to thecase of 1985, during 1988, the warming occurredearlier compared to the mean and the strongactivity of wave number 1 can be noted duringthis year in Fig. 4. Easterlies in high latitudesappear earlier this year. A careful examinationof Figs. 4 and 5 shows that in many years whenthe activity of wave number 1 is maximum,

activity of wave number 2 is minimum and viceversa. This suggests a nonlinear interaction be-tween wave number 1 and 2. The linear correla-tion coefficient between the time series of mean(9 years) kinetic energy of wave numbers 1 and 2is 0.46 for 60 days from 1 September.

3.2 Mean circulation and E-P fluxes

Figure 6 shows the mean (9 years) circulationand temperature at 10 hPa level before, duringand after the final warming. Before the warminga very cold westerly vortex can be seen clearly,with a low over Antarctica. The temperature atthis level before warming over Antarctica isaround 201 K. Maxima values of the zonal windare found over the South Atlantic and Indianoceans, near the Antarctica coast.

During the warming, the vortex is weakenedand a wave number 1 configuration can be seenin the geopotential height with a high over theSouth Pacific and a low over the South Atlantic.In all the 9 years (figure not shown) this picture isseen. The atmosphere at this time is warm with amaximum temperature of around 250 K overAntarctica. Strong easterlies are seen over theSouthwest Pacific and weak westerlies are seenover western Antarctica.

After the warming, the cyclone over the SouthAtlantic disappeared and a high pressure center isseen over the southern polar region. Over theCentral Antarctica temperature decreased com-pared to the earlier period. However, in the mid-dle latitudes temperatures are warmer than in theprevious period. Strong eaterlies are seen overthe entire SH.

Figure 7 shows the mean (9 years) E-P fluxesand their divergence (left) and the distribution ofzonal wind (right) before, during and after thewarming. These days are around the mid-Sep-tember (before), mid-October (during) and mid-November (after). Before the warming, a strongvertical E-P fluxes are seen over middle and highlatitudes of the SH. The westerly jet is clearlyseen with a maximum value of 70 m s 1 at10 hPa level over 60� S. Also a strong conver-gence of E-P flux is seen between 30 and10 hPa levels. During the warming (middle fig-ure) there is a strong decrease of the zonal wind.This deceleration of the zonal wind is in agree-ment with the convergence of E-P fluxes seen in

42 V. B. Rao et al: Stratospheric final warmings in the Southern Hemisphere and their energetics

Fig. 5. Same as Fig. 4, but for wave-2

the earlier period (Eq. 4) showing the importantrole of planetary waves in the wave-mean flowinteraction. The convergence of E-P fluxes seenin the earlier period is mostly due to wave num-bers 1 and 2 (figure not shown) and the contribu-tion of higher numbers is small. In the middlefigure on the right the presence of easterlies

can be noted at higher levels over low latitudes.Comparing the figures of the zonal wind beforeand during the warming, the poleward and down-ward movement of the westerly jet can be noted.This is in agreement with the results of the earlierauthors (Hartman, 1976, and others). After warm-ing the E-P fluxes are substantially reduced

Fig. 6. Mean geopotentialheight (m), mean zonal wind(m s 1) and temperature (rightpanels, K) at 10 hPa before,during and after the warming

44 V. B. Rao et al

particularly at levels above 100 hPa and easterliesat all latitudes of the SH can be seen in the higherlevels. Westerlies are seen at low latitudes in thelower levels.

3.3 Energetics

Based on the calculations of energy parameters,Lorenz block diagram can be prepared for twostages of final warmings in the SH, namely

before and after warming. Figure 8a shows theblock diagram for the SH stratosphere beforewarming (mean for 15 days, 1–15 September)and Fig. 8b for the stratosphere after the warming(mean for 1–15 November). From these figuresit can be seen that in the (9 year) mean themost spectacular change is the reduction of zonalKE, from 584 m2 s 2 to 54 m2 s 2. Although inthe mean this destruction of zonal wind appearsto be uniform in time, in individual years the

Fig. 7. Mean E-P fluxes and their divergence (left panels) and the zonal wind (m s 1) (right panels) before, during and afterthe warming


decelerations are not uniform (Fig. 3) and theseseem to be associated with increased eddy fluxesin the stratosphere (Fig. 4). This decelerationoccurs when the zonal mean jet moves polewardand downward (Fig. 7). The vertical eddy fluxTe seems to be important before the warming,although there are increases and decreases fromthe mean value of about 22� 10 5 m2 s 3.Before warming Ke!Kz conversion and Pz!Kz

conversion (direct mean meridional circulation)seem to maintain the zonal KE against frictionaldissipation.

Another change that occurs between the twostages is the change in Pz, Pe and Ke conversion.Before the warming it is Pz! Pe!Ke andchanges to Pz Pe Ke after the warming. Thechange in conversion between Pz and Pe after thewarming is due to the change in the meridionaltemperature gradient (with temperature increas-ing towards the pole) after the warming. The slowincrease of temperature during November overthe polar region seems to be due to radiative heat-ing. This builds up the zonal available PE.The dramatic destruction of zonal KE (from584 m2 s 2 before warming to 54 m2 s 2 afterwarming) seems to be due to two effects: thereduction in Ke!Kz conversion and weakeningof direct meridional circulation.

Tables 1 and 2 show the interannual variationsin the energy parameters before and after thefinal warming. There are large variations in mostof the quantities and in some of exchange quan-tities even the sign changes. The KE of wavenumber 1 varies from a maximum value of92 m2 s 2 in 1988 to a minimum 12 m2 s 2 in1993. This is reflected in the total eddy KE.The KE of wave numbers 2 and 3 is in generalmuch smaller. Eddy available PE also shows asimilar feature. The interannual variation in Kz

and Pz are relatively smaller. The conversion ofeddy KE to zonal KE is smallest in 1988 in the 9years considered. This seems to be associatedwith early deceleration of the westerly jet andan early appearence of easterlies in the high lati-tudes. The early appearence of easterlies in 1988was already mentioned while discussing thecharacteristics of Fig. 3. The stronger decelera-tion of the westerly jet in 1988 seems to be asso-ciated with stronger wave activity, particularlywave number 1. The weak eddy activity in1985 and 1993 seems to be associated with adelay in the first appearence of easterlies.Another interesting aspect is the very low (almostzero) value of Pz!Ke conversion in 1988. Thisalso helped the deceleration of the westerlyjet.

Fig. 8a. Lorenz energy diagram before warm-ing, and b Lorenz energy diagram after warm-ing. Energy parameters in m2 s 2 and exchangesin 10 5 m2 s 3

46 V. B. Rao et al

The conversion between eddy and zonal avail-able PE is accomplished mostly by wave number1. This conversion is small in 1988 and 1993.

Vertical eddy flux (Te) is also mostly accom-plished by wave number 1. In the case of gen-eration of eddy available PE, in some years

Table 1. Energy parameters before warming

1985 1986 1987 1988 1989 1990 1991 1992 1993

K1 36.8 80.2 19.4 92.3 25.3 53.9 69.8 60.3 12.6K2 22.4 6.4 6.4 9.7 15.9 7.1 4.5 11.1 10.1K3 2.8 5.5 6.0 3.8 3.1 1.1 15.0 4.2 13.5

P1 21.3 65.6 12.9 106.3 25.1 33.7 64.9 63.2 19.1P2 2.8 2.4 2.4 3.8 5.6 2.3 1.9 2.8 3.4P3 0.3 0.4 0.6 1.1 0.1 0.2 0.7 0.3 0.4

K1!Kz 91.3 101.0 59.7 35.3 67.4 153.5 118.8 83.6 19.1K2!Kz 25.4 11.3 8.4 31.5 28.2 11.6 4.0 28.5 17.8K3!Kz 0.3 7.3 7.9 6.6 0.9 1.4 17.2 2.0 7.1

P1!K1 5.9 11.7 6.6 11.9 14.1 15.6 10.9 11.1 4.7P2!K2 1.7 2.2 2.9 1.6 0.2 3.6 0.2 2.3 5.6P3!K3 1.8 0.8 0.4 0.1 0.3 0.7 3.0 0.2 0.4

Pz! P1 5.5 14.8 4.7 4.8 9.6 11.1 8.5 11.2 0.6Pz! P2 0.7 0.3 0.2 1.7 4.2 0.2 0.7 0.5 2.5Pz! P3 0.1 0.2 0.2 0.8 0.1 0.1 0.5 1.1 0.1

1 14.9 33.1 12.3 17.8 16.6 18.3 28.8 31.0 2.42 7.3 2.6 0.4 4.1 12.2 0.6 2.8 7.4 0.53 6.2 1.9 2.2 5.1 0.1 0.1 3.9 5.6 0.3

Q1 59.2 53.1 4.7 1.3 21.7 42.4 31.2 55.9 6.2Q2 0.4 7.4 13.7 20.0 27.7 16.2 6.1 17.3 8.9Q3 5.7 2.5 1.0 2.0 1.0 1.1 7.8 1.6 0.2

Table 2. Energy parameters after warming

1985 1986 1987 1988 1989 1990 1991 1992 1993

K1 2.8 3.5 4.8 1.1 6.1 8.2 1.6 8.0 6.4K2 2.1 1.1 2.3 2.7 0.5 1.7 2.8 1.0 1.6K3 1.6 1.8 1.4 1.8 2.0 1.2 0.6 0.9 0.7

P1 0.7 1.3 1.5 0.8 1.9 2.6 1.1 2.0 2.0P2 0.9 0.3 1.1 1.3 0.3 0.8 1.6 0.4 0.5P3 0.4 1.2 0.5 0.9 1.4 0.6 0.3 0.7 0.5

K1!Kz 0.5 0.7 1.6 0.1 1.2 2.0 0.5 0.1 2.5K2!Kz 0.9 0.4 0.2 1.1 0.1 0.9 0.9 0.6 0.2K3!Kz 0.0 1.2 1.4 0.7 1.8 0.7 0.2 0.1 0.0

P1!K1 0.3 0.5 0.2 0.1 0.1 2.1 0.3 1.7 0.8P2!K2 0.7 0.5 0.8 0.1 0.3 1.2 0.1 0.2 0.4P3!K3 0.7 0.8 0.0 0.1 0.7 1.2 0.4 0.6 0.1

Pz! P1 0.1 0.1 0.1 0.0 0.0 0.1 0.0 0.1 0.0Pz! P2 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0Pz! P3 0.1 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0

1 1.9 0.1 1.7 0.1 1.8 3.7 0.7 2.5 2.02 0.2 2.0 1.9 0.4 0.1 1.8 0.5 0.3 0.33 1.0 1.2 0.8 0.2 1.2 1.5 0.3 0.6 0.1

Q1 0.1 0.1 2.6 0.2 1.7 2.3 1.0 2.0 1.4Q2 5.0 3.5 2.6 1.6 1.1 3.2 0.6 2.3 3.6Q3 0.7 1.0 0.8 0.7 2.3 1.4 0.0 1.6 0.1


(1987, 1988 and 1989) the contribution of wavenumber 2 is quite high and more than that ofwave number 1. In the other years the contribu-tion of wave number 1 dominates. After thewarming (Table 2) the conversions are verysmall. Pz and Ke values are seizable.

4. Summary and concluding remarks

In the present paper we used 9 years (1985–1993) data to study the final stratospheric warm-ings in the SH. In the SH largest temperaturechanges are associated with warmings in thelower stratosphere (Mechoso, 1990). Thus westudy warmings using 100, 70, 50, 30 and10 hPa. We found substantial interannual varia-tions in the characteristics of final warmings inthe SH. In 1985 abrupt variations of temperatureare noted. In 1988 the stratosphere was warmer(by about 5 K) during early September comparedto the mean. This seems to be associated withunusual midwinter warmings noted by Hirotaet al (1990). During the early September, 1987and 1989 temperatures were lower by about 5 Kin the lower stratosphere compared to the mean.

There are large interannual variations in theonset data of final warming as given by the rever-sal of meridional temperature gradient. In 1985the onset of final warming at 10 hPa got delayedand occurred around 1 November. In the mean itoccurs in the middle of October. This seems to beassociated with weak wave activity in 1985 andalso the stratosphere was colder in 1985 com-pared to the mean.

In the mean the westerly jet core moves pole-ward reducing in intensity and finally giving wayto easterlies. Compared to this mean picture thereare larger interannual variations, particularly inthe timing of the first appearence of easterlies.In individual years there seems to be periodicalintensification and weaking of easterlies inSeptember and October. These periods arerelated to the events of wave-zonal flow interac-tion (divergence and convergence of E-P fluxes,Figs. 4 and 5). In the E-P vectors and their diver-gence a periodic behaviour of two weeks isnoted. Mechoso et al (1988) noted a periodicityof about two weeks in geopotential height ofwave numbers 1 and 2. The delayed final warm-ing in 1985 and early warming in 1988 are asso-ciated with weak and strong wave activity

respectively. Also a strong negative correlation( 0.46 for 60 days) was noted between thekinetic energy of wave numbers 1 and 2. Anexamination of E-P fluxes, their divergence andthe distribution of zonal wind showed the impor-tant role of planetary waves in the wave-meanflow interaction.

In the energetics, the most spectacular changeis in the reduction of zonal KE (in the mean from584 m2 s 2 to 54 m2 s 2). In individual years thedeceleration are not uniform and are associatedwith increased eddy fluxes. The decelerationoccurs when the zonal mean jet moves polewardand downward. Before the warming the energychanges are Pz! Pe!Ke!Kz Pz. After thewarming the energy exchanges are Pz Pe Ke!Kz Pe. This is similar to what Reed et al(1963) noted for the midwinter warmings in theNH. The slow increase of temperature duringNovember over the polar region seems to bedue to radiative heating. This does not happenin the midwinter (polar night) NH warmings.The dramatic destruction of zonal KE seems tobe due to two effects: the reduction in Ke!Kz

conversion and weakening of direct meridionalcirculation.

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Authors’ address: V. B. Rao, M. B. Rosa, J. P. Bonatti andS. H. Franchito, Instituto Nacional de Pesquisas Espaciais,INPE, CP 515, 12201-970, S~aao Jos�ee dos Campos, SP, Brazil


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Stratospheric final warmings in the Southern Hemisphere and their energetics