• Introduction
• EOF analysis of the SH stratosphere
• Composite analysis of the upper tropospheric pattern
• EOF and SVD analysis of the troposphere
• Concluding remarks

FIGURES

• Figure 1
• Figure 2
• Figure 3
• Figure 4
• Figure 5

1.Introduction

The seasonal march and the year-to-year variation of the stratospheric circulation are significantly different between the northern and southern hemispheres. In the northern hemisphere(NH), monthly mean (steady state) planetary waves show the maximum amplitudes in mid-winter associated with the occurrence of sudden warmings, but in general the phase of east-west wavenumber 1 is almost fixed due to the topographic effect of the surface.

On the other hand, planetary waves in the southern hemisphere(SH) stratosphere show large amplitudes in late winter or early spring (Sept.and Oct.), and the off-pole pattern of the polar vortex which means the phase and amplitude of wavenumber 1 varies from year to year, as can be seen in the "Antarctic ozone hole"

In view of the fact that the topographic conditions are quite different between the two hemispheres, it is interesting to examine the generation mechanism of planetary waves in the SH stratosphere, as well as their interannual variability, in terms of the dynamical coupling between the troposphere and stratosphere.

In this study a statistical analysis is made of interannual variations of planetary waves in the wintertime SH stratosphere with the aid of NCEP/NCAR reanalysis data-set for 20 years(1979-1998), in conjunction with the upper tropospheric circulations such as the mean zonal flow and short-period disturbances.



2.EOF analysis of the SH stratosphere

In order to see the year-to-year variation of the planetary waves in the winter SH stratosphere, first we applied the EOF analysis to the monthly mean geopotential height anomaly (deviations from the zonal mean) of 10hPa for September and October of 1979-1998. The result of the analysis for the set of 40 months is shown in Fig.1.

Fig.1a gives the climatology of the 10hPa height deviation, in which the dominance of zonal wavenumber 1 is observed with the ridge and trough located at 150oE and 50oW respectively along the latitude belt of about 60-70oS.

With the long-term variations of this pattern, the first component of EOF1 (Fraction of variance 58.7%, Fig.1b) indicates the east-west fluctuation of the phase, while the second component EOF2 (29.1%, Fig.1c) denotes the variation of wave amplitudes as its pattern is almost similar to Fig.1a. Note that the positive (negative) score of EOF1 shows the westward (eastward) drift of the center of the trough, and the positive (negative) score of EOF2 corresponds to the increase of wave amplitudes.



Figure 1. (a)Average of the 40 months(1979-1998 Sep,Oct) for Z* at 10 hPa
and the (b)first,(c)second mode of the empirical orthogonal function of Z*10 .


3.Composite analysis of the upper tropospheric pattern

Next, to see the relationship between the stratospheric wave pattern and the characteristics of upper tropospheric circulations, the composite analysis was made of 300hPa monthly-mean zonal winds U, together with horizontal heat flux T'v'associated with the high-pass filtered disturbances with a time-scale less than 10 days, according to the positive and negative category of the stratospheric EOF1 and EOF2 shown above, for September of 1979-1998. Both of the 300hPa zonal winds and heat flux are considered to be representative of tropospheric circulations relating to the activity of baroclinic unstable waves.

As is shown in Fig.2a and 2b, the westward (EOF1>0) and eastward (EOF1<0) drift of the stratospheric wave correspond to the single and double jet structure of the upper troposphere over the eastern hemisphere.

As for the hemispheric pattern of heat flux (Fig.2c,d), the wave activity is dominant over the Indian Ocean in general. The difference between the EOF1 positive and negative category is the intensity.



Figure 2. Composite monthly mean zonal wind U(a,b),and horizontal eddy heat flux T'v'(c,d) for the group with the positive (a,c,),negative (b,d) PC1 index.


Fig.3 shows the correspondence of U and T'v'to the stratospheric wave amplitude (EOF2). The difference between the composite of positive and negative category appears in the heat flux pattern, not only the location of the maximum but also the intensity. This suggests that the intensity of planetary waves in the SH stratosphere is mainly controled by the transient troposphric waves through their vertical energy fluxes.


Figure 3. Same as Fig 2, but for the group with the positive (a,c),and negative(b,d) PC2 index.



4. EOF and SVD analysis of the troposphere

In order to confirm the dynamical coupling between the troposphere and stratosphere in more detail, we applied the EOF and SVD analysis to the tropospheric zonal winds and 300hPa heat fluxes.

Fig.4a shows the first mode of tropospheric U (TUEOF1, Fraction of variance 55%), which is the dipole pattern throughout the whole troposphere, indicating the appearance of single and double jet structure. The classification by this principal component TUPC1 sign (Fig.4b) confirms the separation of the east-west drift of the phase of stratospheric waves.

On the other hand, the first component of 300hPa T'v' pattern (VTEOF1, Fig.5a) reflects the wave activity over the Indian Ocean, though the fraction of variance is rather weak (9.3%), and the stratospheric wave intensity is clearly separated by the sign of TVPC1 (Fig.5b).

Figure 4 . Latitude height section (1000-100 hPa) of the first EOF mode for monthly mean zonal wind averaged longitudinally for the eastern hemisphere, (b) Scatter diagram of the value and the longitude of the minimum Z* (trough) at 10hPa.
Red (blue) circle denotes the positive (negative) PC1 index for zonal wind U .



Figure 5 . Same as Fig.4 ,but (a) the first EOF mode for monthly mean horizontal eddy heat flux at 300 hPa , (b) green (orange) circle denotes the positive (negative) PC1 index for T'v' .


Furthermore, the result of the SVD analysis for the 10hPa height field anomaly Z* and 300hPa T'v', though not shown here, shows that the first and second mode explain the fluctuation of the phase and amplitude for Z* respectively. The temporal correlation coefficients between the expansion coefficients is high for the first two mode.(r=0.78,0.77 ,respectively). This also confirms the dynamical control of planetary waves in the stratosphere by the tropospheric transient disturbances.


5.Concluding remarks

Throughout the present analysis of the NCEP/NCAR data over 20 years, some characteristic features of the interannual variations of the wintertime SH stratosphere and their connection with the tropospheric dynamics are found. They are summarized as follows:

i. The year-to-year variation of monthly mean planetary waves at 10hPa can be characterized by the east-west drift of the phase, as well as by the intensity of the wave amplitude.
ii. The variation of stratospheric wave pattern is well correlated with that of tropospheric circulations, in such a way as the phase with the upper tropospheric jet structure and the amplitude with the heat flux pattern.
iii. The result of SVD analysis for the stratospheric planetary wave and the tropospheric heat flux confirms that their year-to-year variations are tightly connected each other.

All of these results strongly suggest that the generation and maintenance of SH stratospheric waves are quite sensitive to the transient wave activity in the troposphere. In this regard, the generation mechanism for the SH stratospheric wave by the forcing below should be further investigated, as well as the nature of interannual variabilities in the troposphere.