The Seasonal Cycle of Blocking.pdf (4.56 MB)
The seasonal cycle of blocking and associated physical mechanisms in the Australian region and relationship with rainfall
journal contribution
posted on 2023-05-19, 08:22 authored by Pook, MJ, James RisbeyJames Risbey, McIntosh, PC, Ummenhofer, CC, Marshall, AG, Meyers, GAThe seasonal cycle of blocking in the Australian region is shown to be associated with major seasonal temperature changes over continental Antarctica (approximately 15°-35°C) and Australia (about 8°-17°C) and with minor changes over the surrounding oceans (below 5°C). These changes are superimposed on a favorable background state for blocking in the region resulting from a conjunction of physical influences. These include the geographical configuration and topography of the Australian and Antarctic continents and the positive west to east gradient of sea surface temperature in the Indo-Australian sector of the Southern Ocean. Blocking is represented by a blocking index (BI) developed by the Australian Bureau of Meteorology. The BI has a marked seasonal cycle that reflects seasonal changes in the strength of the westerly winds in the midtroposphere at selected latitudes. Significant correlations between the BI at Australian longitudes and rainfall have been demonstrated in southern and central Australia for the austral autumn, winter, and spring. Patchy positive correlations are evident in the south during summer but significant negative correlations are apparent in the central tropical north. By decomposing the rainfall into its contributions from identifiable synoptic types during the April-October growing season, it is shown that the high correlation between blocking and rainfall in southern Australia is explained by the component of rainfall associated with cutoff lows. These systems form the cyclonic components of blocking dipoles. In contrast, there is no significant correlation between the BI and rainfall from Southern Ocean fronts.
History
Publication title
Monthly Weather ReviewVolume
141Issue
12Pagination
4534-4553ISSN
0027-0644Department/School
Institute for Marine and Antarctic StudiesPublisher
Amer Meteorological SocPlace of publication
45 Beacon St, Boston, USA, Ma, 02108-3693Rights statement
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- Open