The Seasonal Cycle of Blocking and Associated Physical Mechanisms in the Australian Region and Relationship with Rainfall

Headnote

ABSTRACT

The seasonal cycle of blocking in the Australian region is shown to be associated with major seasonal temperature changes over continental Antarctica (approximately 158-35°C) and Australia (about 88-17°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 cutofflows. 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.

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1. Introduction

Atmospheric blocking is broadly recognized by synopticians as a situationwhere a synoptic-scale anticyclone becomes quasi stationary well poleward of the mean subtropical ridge position for the particular time of year. More specifically in the Australian region, it presents as a strongly involuted tropospheric flow pattern with a clearly defined split-jet structure. Blocking is one of the most significant phenomena encountered in the day to dayweather cycle yet the prediction of its onset, duration, and dissipation in numerical weather prediction (NWP) remains problematic at the medium-range time scale (Watson and Colucci 2002; Renwick 2005). Furthermore, the problem of prediction becomes more challenging in coupled general circulation models (GCMs) run on the seasonal and longer time scales. Palmer et al. (2008) have demonstrated that the frequency of...