Monsoon and Teleconnection Variability over Australasia During the Southern Hemisphere summers of 1973–77

Robert S. Allan Department Of Geography, University of Adelaide. Adelaide, South Australia 5001

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Abstract

Interactions between monsoon and teleconnection circulations Over the greater Australasian region 35°N–35°S, 85°E–175°E, are observed in 850 and 200 mb monthly mean velocity potential (¯χ) and streamfunction (¯ψ) fields during the peak of the Southern Hemisphere summers (January and February 1973–77) prior to the Winter Monsoon Experiment (WMONEX). Aspect of the spatial and temporal Australasian monsoon and teleconnection systems are presented through a closer examination of the January 1973–75 sub-period, using vertical crow sections of monthly mean divergence and monthly mean surface streamfunction (¯ψsfc) fields.

Most importantly this study suggests that 1arge-scale Southern Oscillation/walker Circulation (SO/WC) teleconnection patterns are associated with variations in components of the Asian monsoon system. Interannual and intraseasonal fluctuations in the Near Equatorial Troughs of the Northern (NETNH) and Southern (NETSH) Hemispheres appear to be related to large-scale atmospheric circulation changes. Over Australia the southward displacement of a strong section of the NETSH is observed during the “wet” summers of 1974 and 1976. It is suggested that, at least in the lower levels, the Australian monsoon circulation in 1974 was almost entirely contained and maintained within the Southern Hemisphere. In the equatorial western Pacific, fluctuations in the NETNH broadly coincide with northeast trade wind variability and possibly the El Niño cycles of 1972–73, 1975 and 1976.

At 200 mb, circulation patterns suggest that changes in anticyclonic maxima (¯ψ minima) in the Australian region are associated with teleconnection overturnings. In the 1973 summer, 200 mb velocity potential fields appear to be synonymous with a SO/WC “breakdown” and the development of a strong El Niño phenomenon.

The variable structure of the SO/WC overturning which would accompany some of the above events is observed in a series of vertical cross sections of monthly mean divergence between the surface and 200 mb levels during January 1973–75. Evidence is presented to show that the concept of the SO/WC as a simple large-scale vertical cell which fluctuates in magnitude and direction may be something of a simplification.

In January 1973–75, ¯ψsfc fields suggest that NETNH activity in the Asian sector is not homogeneous between the surface and 850 mb levels. The extent of cross-equatorial Bow into Australia seems to be related to the coupling between the NETNH, and the cyclonic activity over the northwest of the continent. In fact, variability in this flow may result from either a low-level, quasi-permanent heat low or deeper and more structured NETSH conditions being dominant in this region. Rainfall and monsoonal activity in Australia appear to be related to extremes in this phenomenon.

Abstract

Interactions between monsoon and teleconnection circulations Over the greater Australasian region 35°N–35°S, 85°E–175°E, are observed in 850 and 200 mb monthly mean velocity potential (¯χ) and streamfunction (¯ψ) fields during the peak of the Southern Hemisphere summers (January and February 1973–77) prior to the Winter Monsoon Experiment (WMONEX). Aspect of the spatial and temporal Australasian monsoon and teleconnection systems are presented through a closer examination of the January 1973–75 sub-period, using vertical crow sections of monthly mean divergence and monthly mean surface streamfunction (¯ψsfc) fields.

Most importantly this study suggests that 1arge-scale Southern Oscillation/walker Circulation (SO/WC) teleconnection patterns are associated with variations in components of the Asian monsoon system. Interannual and intraseasonal fluctuations in the Near Equatorial Troughs of the Northern (NETNH) and Southern (NETSH) Hemispheres appear to be related to large-scale atmospheric circulation changes. Over Australia the southward displacement of a strong section of the NETSH is observed during the “wet” summers of 1974 and 1976. It is suggested that, at least in the lower levels, the Australian monsoon circulation in 1974 was almost entirely contained and maintained within the Southern Hemisphere. In the equatorial western Pacific, fluctuations in the NETNH broadly coincide with northeast trade wind variability and possibly the El Niño cycles of 1972–73, 1975 and 1976.

At 200 mb, circulation patterns suggest that changes in anticyclonic maxima (¯ψ minima) in the Australian region are associated with teleconnection overturnings. In the 1973 summer, 200 mb velocity potential fields appear to be synonymous with a SO/WC “breakdown” and the development of a strong El Niño phenomenon.

The variable structure of the SO/WC overturning which would accompany some of the above events is observed in a series of vertical cross sections of monthly mean divergence between the surface and 200 mb levels during January 1973–75. Evidence is presented to show that the concept of the SO/WC as a simple large-scale vertical cell which fluctuates in magnitude and direction may be something of a simplification.

In January 1973–75, ¯ψsfc fields suggest that NETNH activity in the Asian sector is not homogeneous between the surface and 850 mb levels. The extent of cross-equatorial Bow into Australia seems to be related to the coupling between the NETNH, and the cyclonic activity over the northwest of the continent. In fact, variability in this flow may result from either a low-level, quasi-permanent heat low or deeper and more structured NETSH conditions being dominant in this region. Rainfall and monsoonal activity in Australia appear to be related to extremes in this phenomenon.

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