Blocking Episodes in the Southern Hemisphere: Impact on the Climate of Adjacent Continental Areas

This work presents an updated climatology of blocking episodes for the Southern Hemisphere between 1960 and 2000, based on data from NCEP/NCAR reanalysis. Five contiguous areas of blocking activity are considered; Southeastern Pacific, Southwestern Pacific, Atlantic, Indian and Oceania. The impact of the three most important areas of onset blocking episodes (Southeastern Pacific, Atlantic and Oceania) upon the climate of the adjacent continental areas (South America and Australia) was evaluated. Composites of the meteorological variables (temperature and precipitation) were obtained for periods of diagnosed blockings. The impact of the blocking episodes over the climate of South America and Australia is highlighted whenever anomaly fields of temperature and precipitation are significant at the 5% and 10% levels, respectively. Impacts of Southeastern Pacific and Atlantic blockings are observed on the temperature field over several regions of South America. Significantly higher (lower) temperatures than climatology occur in southern Brazil, northern Argentina, Uruguay and Paraguay, and lower (higher) than climatology in the extreme south of South America for the Southeastern Pacific (Atlantic) blocking episodes. Precipitation over South America is also affected by the Southeastern Pacific and Atlantic blockings in different ways. The Southeastern Pacific blocking has higher impact on precipitation in summer (dry conditions in northeast Brazil) and spring (wet conditions in central and southern Brazil), while the Atlantic blocking affects precipitation in autumn and winter (wet conditions in parts of central and southern Brazil). The blocking cases over Oceania affect southeastern Australia with normal to higher than climatological precipitation and with negative temperature anomalies in that region. Finally we provide a detailed analysis of a South Atlantic blocking episode, which occurred between the 4 th and the 8 th of June 1997. This event shows clearly the split of the jet stream into two branches (subtropical and polar) surrounding the anticyclonic sector, and satellite imagery revealed the presence of transient systems in the periphery of the blocking anticyclone responsible for high values of precipitation in the southeastern sector of South America.