J F M A M J J A S O N D J 180
190 200 210 220 230 240 250 260
Temperature (K)
Year: 2002
J F M A M J J A S O N D J
-20 0 20 40 60 80 100 120
Zonal Mean Wind (m/s)
Year: 2002 Temperature (at 80° S)
Zonal Mean Wind (at 60° S)
Figure 6.1: 2002 records of daily zonal mean wind at 60◦S (solid line) and temperature at 80◦S (dotted line) from NCEP reanalysis at the height of 10-hPa (32 km).
Apart from using the SuperDARN HF radar wind data, the UKMO wind and temperature data was also used to study the wind, temperature, and planetary wave activity during the year 2002 winter period. The Eliassen-Palm (E-P) flux diagnostics was used to interpret planetary waves in terms of their propagation and trajectory in the meridional plane. The E-P divergence and vector was calculated using the ECMWF ERA-40 reanalyses data.
The SABER instrument data on board the TIMED satellite measurements, is used to examine temperature structure in the middle atmosphere during the 2002 SSW event.
CHAPTER 6. THE YEAR 2002 SOUTHERN HEMISPHERE SSW 89
180 200 220 240 260 280 300 320 340
-20 -10 0 10 20 30 (b)
Meridional Velocity (m/s)
Day
2002 Mean & Std
180 200 220 240 260 280 300 320 340
-20 0 20 40
Zonal Velocity (m/s) 2002 Mean & Std(a)
Figure 6.2: Time evolution of daily averaged (a) zonal and (b) meridional velocities obtained from SANAE HF radar observations from 1 July to late November 2002. Each figure is superimposed with the 2003-2006 mean profile with standard deviation.
During the period of July to November, one can see from Figure (6.2a) that eastward winds are primarily dominant in the polar winter in the MLT region particularly during August and September. The reversals of mean wind begin at around the beginning of July, which is approximately two months before the occurrence of the SSW in late September.
The reversal of the zonal wind which is associated with the SSW event is well outside the standard deviation of the 2003 to 2006 average, and starts on the 18 September (day 261, from Figure (6.2a)). The reversal persisted for a few days before it returned to eastward direction on 27 September (day 270). However, in the stratosphere the zonal wind reversed on the 25th of September 2002 (Figure (6.1)) leading to the unprecedented major SSW in the Southern Hemisphere. This is consistent with results from Dowdy et al. (2004). Dowdy et al. (2004) used MF radars at polar locations to study the 2002 SSW and reported that the zonal wind in the mesosphere became westward on day 261 (18 September), approximately a week earlier than the 10-hPa (∼32 km) winds turned westward. The meridional mean winds during 2002 also show an enhancement during the period of the occurrence of the SSW event and this feature is not seen in the other years (see Figure (6.2)). Maximum reversals of meridional mean winds are seen on day 242 (30 August 2002) and 270 (27 September 2002).
6.2.2 Mean Wind Reversal at the Stratosphere and MLT
Figure (6.3) shows the time evolution of the zonal wind at 10-hPa (∼32 km) derived from the NCEP at 60◦S during the period from 250 to 300 Julian days in 2002. The zonal wind observations from SANAE HF radar (for ∼94 km MLT height) are superimposed with a solid black line. The SANAE HF radar data is smoothed by a 4-day running mean.
Zonal Winds (m/s)
Day of Year
250 255 260 265 270 275 280 285 290 295 300
-40 -20 0 20 40 60
80 Zonal winds (MLT)
Zonal mean Zonal wind (10 hPa)
Figure 6.3: Zonal wind evolution in the MLT region (∼94 km) obtained by SANAE HF radar (solid black line) during the period from 250 to 300 of 2002-Julian days. The red dashed line illustrates the zonal-mean zonal wind variation at 10-hPa (∼32 km) derived from NCEP reanalyses at 60◦S for the same period of time.
The figure shows that the zonal wind started to reverse (became westward) in the MLT region on the 261th 2002-Julian day (18 September), i.e., about a week earlier than the reversal of the zonal wind in the polar stratosphere at∼10-hPa pressure level. The reversal in the MLT reaches a maximum magnitude of about -10 m/s on day 263 (20 September 2002). The MLT zonal winds reversal lasted for a few days and thereafter returned to the eastward direction for more than 7 days, before another reversal which starts on day 282 (9 October 2002) and persists until the beginning of summer. The reversal in the stratosphere began on the 268th day and reached its maximum amplitude of -20 m/s on day 270 before it turns back to the eastward direction on day 273. It is noted here that the daily variability of the zonal wind may vary from day-to-day and time-to-time, but it is found to be within±8 m/s.
6.2.3 Potential Temperature (∼32 km)
Figure (6.4) shows the Southern Hemisphere NCEP potential temperature maps at 10-hPa (∼32 km) for selected days in August and September 2002 leading up to the major SSW
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)).