Blocking in the Southern Hemisphere

Kevin F. Trenberth National Center for Atmospheric Research, Boulder, CO 80307

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K. C. Mo M/A COM Sigma Data Inc., NASA/Goddard Space Flight Center, Greenbelt, MD 20771

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Abstract

The focus of this Paper is on the frequency and spatial distributions of blocking and persistent anomalies of geopotential height over the Southern Hemisphere. The analysis is based upon daily height fields at 1000 and 500 mb for both summer and winter. Histogram frequency distributions of height anomalies and maps of the skewness and kurtosis have been computed. Blocking events are objectively defined by requiring a large positive anomaly to exist for 5 days or more. Composite flow and anomaly fields for several cases are presented and examined in detail. The geographical distribution of the frequency of lame amplitude ⩾150 gpm anomalies at 500 mb that persist for only 1–3 days is very similar to that of the high-frequency band (2–8 day period) variances that identity the storm tracks in the Southern Hemisphere.

The primary location for blocking in the Southern Hemisphere is in the New Zealand sector and blocking occurs through a local enhancement of the climatological split in the mean westerlies on a spatial scale of 60° longitude. Other maxima occur southeast of South America and over the southern Indian Ocean. Sporadically, multiple blocking events occur at more than one location and one triple blocking event is examined in detail. Zonal wave 3 is strongly evident in such cases; it also plays a dominant role in the majority of blocking events. However, on most occasions, blocking occurs in isolation as a local phenomenon, and it appears that the local wave 3 may be but part of a wave train with a great circle rather than zonal orientation. Transient eddies appear to play an important role in sustaining a blocking event by either continually reinforcing the anticyclone on its western flank or by quickly reestablishing a new anticyclone as the old one breaks down or moves away.

Abstract

The focus of this Paper is on the frequency and spatial distributions of blocking and persistent anomalies of geopotential height over the Southern Hemisphere. The analysis is based upon daily height fields at 1000 and 500 mb for both summer and winter. Histogram frequency distributions of height anomalies and maps of the skewness and kurtosis have been computed. Blocking events are objectively defined by requiring a large positive anomaly to exist for 5 days or more. Composite flow and anomaly fields for several cases are presented and examined in detail. The geographical distribution of the frequency of lame amplitude ⩾150 gpm anomalies at 500 mb that persist for only 1–3 days is very similar to that of the high-frequency band (2–8 day period) variances that identity the storm tracks in the Southern Hemisphere.

The primary location for blocking in the Southern Hemisphere is in the New Zealand sector and blocking occurs through a local enhancement of the climatological split in the mean westerlies on a spatial scale of 60° longitude. Other maxima occur southeast of South America and over the southern Indian Ocean. Sporadically, multiple blocking events occur at more than one location and one triple blocking event is examined in detail. Zonal wave 3 is strongly evident in such cases; it also plays a dominant role in the majority of blocking events. However, on most occasions, blocking occurs in isolation as a local phenomenon, and it appears that the local wave 3 may be but part of a wave train with a great circle rather than zonal orientation. Transient eddies appear to play an important role in sustaining a blocking event by either continually reinforcing the anticyclone on its western flank or by quickly reestablishing a new anticyclone as the old one breaks down or moves away.

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