CHAPTER 6. THE YEAR 2002 SOUTHERN HEMISPHERE SSW 91
of late September (Azeem et al., 2010). This figure clearly indicates that at the start of the sequence the basic cyclonic polar vortex is well established and is centred just off the pole at 60◦S. Thereafter, the distortion of the polar vortex can be seen to commence on 18 August due to minor warmings. Other distortions are also seen in early September due to the presence of minor warmings. On 25 September the polar vortex splits into two cyclonic cells indicating the commencement adapted from a major stratospheric warming.
Azeem et al.: MLT anomalies during 2002
Aug 13(Day 225)
120 W o
60
oW 0o
60o
E
120
oE
180oW 72oS 54oS 36oS 18oS
Aug 18(Day 230)
120 W o
60
oW 0o
60o
E
120
oE
180oW 72oS 54oS 36oS 18oS
Aug 23(Day 235)
120 W o
60
oW 0o
60o
E
120
oE
180oW 72oS 54oS 36oS 18oS
Aug 28(Day 240)
120 W o
60
oW 0o
60o
E
120
oE
180oW 72oS 54oS 36oS 18oS
Sep 04(Day 247)
120 W o
60
oW 0o
60o
E
120
oE
180oW 72oS 54oS 36oS 18oS
Sep 07(Day 250)
120 W o
60
oW 0o
60o
E
120
oE
180oW 72oS 54oS 36oS 18oS
Sep 14(Day 257)
120 W o
60
oW 0o
60E o
120
oE
180oW 72oS 54oS 36oS 18oS
Sep 22(Day 265)
120 W o
60
oW 0o
60E o
120
oE
180oW 72oS 54oS 36oS 18oS
Sep 25(Day 268)
120 W o
60
oW 0o
60E o
120
oE
180oW 72oS 54oS 36oS 18oS
K
800 820 840 860 880 900 920 940 960
Potential Temperatures Pressure Level = 10hPa Altitude ~ 30km Year 2002
Figure 6.4: Southern Hemisphere potential temperature maps at 10-hPa pressure level for selected days in August and September of year 2002 (from Azeem et al. (2010)).
state of the middle atmosphere in the late Southern Hemisphere winter of year 2002, it is essential to study the wave-mean flow interaction in the Southern Hemisphere. Details of previous studies of the wave-mean flow interaction in the Southern Hemisphere are given by Hartmann et al. (1984) and Semane et al. (2006). The Eliassen-Palm (E-P) flux diagnostics is used to interpret planetary waves in terms of their propagation and trajectory in the meridional plane. In this study, the theoretical formulation given by Edmon et al. (1980) was used to deduce the E-P vector (F) and its divergence (∇.F) from the ECMWF ERA-40 reanalyses data. The E-P flux vector and its divergence is defined by the following equation (Edmon et al., 1980; Andrews et al., 1987)
F=
F(φ),F(z) =n
−ρ0acosφ v0u0
, f ρ0θacosφ
v0θ0 θ0
o and
∇.F= 1
acosφ(Fφcosφ)φ+ F(z)
z. (6.1)
whereφrepresents latitude, (u, v, w) the ’velocity’ in (longitude, latitude, pressure) coor- dinates,θ the potential temperature,athe Earth’s radius.The over-bars denote the zonal mean and the primes denote the deviations with their respective means. Parameters. The other symbols have their usual meaning. Further explanations are given in Chapter 1, section 1.31, and also by other authors (e.g. Edmon et al. (1980); Andrews et al. (1987)).
The orientation of the the E-P flux vector indicates the direction of the planetary wave propagation (Andrews et al. (1987)). Generally, in the mid-latitudes and high-latitudes, planetary wave activity is usually observed to propagate from the winter troposphere up into the stratosphere and mesosphere and towards the equator (Eliassen and Palm, 1961).
In some cases the wave planetary activity originating from the winter troposphere is obser- ver to propagate up into the stratosphere and mesosphere and towards the pole (Eliassen and Palm, 1961).
For the purpose of studying the year 2002 SSW dynamics, the days before and during the onset of the SSW were selected. The E-P cross-section diagrams for the Southern Hemisphere plotted for the dates of 1, 11, 20, 21, 23, and 25 September 2002 are shown in Figure (6.5). The E-P flux vectors (F) are represented with arrows. The contours represent ∇.Fin m.s−1; negative values are shaded. The contour interval is 2 m.s−1 per day, and the dashed contours correspond to the values of more than 10 m.s−1 per day or less than -10 m.s−1 per day. The zero line has been overlaid (thick black contour). The direction of the E-P flux vectors indicate the active vertical propagation of the wave flux for planetary wave propagation with height and from one latitude to another. Planetary wave breaking is identified by the convergence of the E-P flux (i.e. negative values). The 2002 major stratospheric warming started approximately on 18 September 2002 and persisted for about a week, thus the 1st and 11th of September represents the days before the SSW whiles the 20th, 21st, and 25th of September represents the days during the SSW.
CHAPTER 6. THE YEAR 2002 SOUTHERN HEMISPHERE SSW 93
Figure 6.5: Eliassen-Palm cross-sections in the meridional plane for selected days of 1, 11, 20, 21, 23, and 25 September 2002.
During 1 September 2002 it is evident that there are few E-P arrows in the troposphere, indicating less planetary wave activity propagating from the troposphere. However, at the stratospheric heights the E-P arrows with a strong equatorward component are observed.
This E-P flux component seems to be responsible for the minor SSW observed in late August 2002 (see Figure (6.1)). On 11 September there seem to be a strong activity of upward propagating waves indicated by longer length of E-P arrows. During the few days of the beginning of the major stratospheric warming (20 and 21 September), there is strong upward movement in the mid-latitudes lower stratosphere and upper stratosphere, with longer E-P arrows.
On the 21stof September, the driving of the mean flow in the stratosphere seems to exhibit a dipole structure with some wave flux vectors indicating acceleration of the mean flow on the poleward flank of the stratospheric jet and deceleration equatorward of the jet ore.
On 25 September a strong equatorward E-P flux component was observed in the upper stratosphere. When comparing all the figures, there seems to be a tendency of an increase of the stratospheric wave activity during the period from 1 to 25 September. The strong upward planetary wave propagation seem to be dominant over the mid- and high-latitude regions. The E-P flux vectors are observed to have a tendency of bending equatorward with height. The equatorward directed E-P flux vectors are known to generate convergence of the E-P flux (e.g. Semane et al. (2006)). In this study, the convergence of the E-P flux is observed to be dominant over the high-latitudes, mid-latitudes and subtropics, indicating intense wave penetration and enhance wave driving in stratosphere. The convergence associated with the wave penetration and wave driving is observed to reach minimum values lower than -8 m.s−1 during the period from the 11 to 25 September.
The E-P flux figures above illustrate that much of the acceleration of the zonal wind in troposphere during the year 2002 winter was provided by strong planetary wave activity.
The planetary waves of zonal wavenumbers=1 have previously proved to produce almost all of the upward wave flux at approximately 20 km and above, and all eddy-driving of the mean flow in the upper stratosphere (e.g. Hartmann et al. (1984)). The lowest wavenumbers (e.g. wavenumber 1, 2 and 3) are most likely to produce rapid deceleration of zonal mean circulation and associated increase of temperature in stratosphere, which is thereafter referred as the major SSW. The middle atmosphere wave activity during the 2002 winter SSW is discussed in details in the next sections.