NMOC Quarterly Summary October-December 2001

Anticyclonicity-Cyclonicity During Spring 2001

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Most of Australia experienced above average rainfall during spring as shown in Fig.1. Large areas of central Australia and Western Australia experienced very much above average totals. Nowak and Leighton (1997) show that anticyclones centred south of latitude 45 S lead to significant rainfall over central Australia and this was the case last spring. Figure 2 indicates the anticyclonicity anomalies for spring. Maximum anticyclonic anomalies occur over the south east Indian Ocean across waters to the south of Western Australia, and to the south of the Tasman Sea and New Zealand. Apart from the western Tasman Sea the anticyclonic maxima anomalies are south of the average anticyclonic position for spring.

Figure 1
 
 

Negative cyclonicity anomalies (Fig.3) are evident from the Kimberley to south of Adelaide, and in the eastern Tasman Sea. Apart from the eastern Tasman Sea the negative anomalies occurred for each month of spring. The close proximity of a strong positive and negative cyclonicity anomaly in the tropics can be explained by episodic strong ridging of the subtropical high into the Kimberley. Cyclonicity anomalies for spring are positive over interior region of the Northern Territory and over waters to the south of the Bight across Tasmania and into the central Tasman Sea. The sea level pressure composite anomaly (Fig. 4) obtained from the NCEP/NCAR data reanalysis (Kalnay et al. 1996) shows the anomalous southerly position of the anticyclones was such that cyclogenesis occurred to the north of the anticyclones throughout the period.

Figure 2

Figure 3

Figure 4

Sea Level Pressure (mb) Composite Anomaly 20-26 October 2001 NCEP/NCAR Reanalysis

Figure 5

An extended period showing such a pattern commenced on 20th October. A waving cold front approached Western Australia from the Southern Ocean with a low over the interior. Deep ridging behind the cold front advected low thickness values northwards aiding the process of frontal cyclogenesis, which occurred by 00 UTC on the 21st with a low south of Albany. A secondary low also developed ahead of the cold front. On 22nd October the low complex became cut off (i.e. discrete circulation) and moved slowly eastwards. The immobility of the anticyclones assisted in maintaining this synoptic pattern for several days. For the period 21st to 26th October sea level pressure anomalies (Fig. 5) from the NCEP/NCAR data reanalysis show a blocking pattern with the northern part of the block centred in the Bight. Due to the immobility of the synoptic pattern, rainfall for the week ending 26th October (Fig. 6) was widespread in the south of the continent. Northern Australia experienced large amounts of rainfall due to a persistent active trough line extending from Charleville to Tennant Creek.

Figure 6
 
 

References

Nowak, H. and R Leighton, 1997. Relationships between east Australasian anticyclonicity, the Southern Oscillation and Australian rainfall. Aust. Meteor. Mag., 46, 267-276.

Kalnay, E. et al.1996. The NCEP/NCAR Reanalysis 40-year Project. Bull. Amer. Meteor. Soc.,77, 437-471