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

Abstract

Abstract-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.

The role of cold fronts in the onset of the monsoon season in the South Atlantic convergence zone

“…However, composites of rainfall associated with summertime and springtime blocking events (Mendes et al, 2008) show no increase in precipitation in tropical South America associated with blocking events. Since blocking events are relatively rare in the South American sector (only four summertime events and 21 springtime events over the 41 years covered by Mendes et al, 2008), it is unlikely that blocking causes monsoon onset in the SACZ or that the active phases of the SACZ are associated with blocking as suggested by Silva and Kousky (2001).…”

“…Between day +2 and day +6, the geopotential anomaly pattern of the onset cold front composite over South America resembles a blocking pattern (e.g. Mendes et al, 2008). However, composites of rainfall associated with summertime and springtime blocking events (Mendes et al, 2008) show no increase in precipitation in tropical South America associated with blocking events.…”

Precipitation linked to Atlantic moisture transport: clues to interpret Patagonian palaeoclimate

“…A daily blocking index devised by Tibaldi & Molteni (1990), known as the TM index and updated for the Southern Hemisphere by Mendes et al (2008), was applied to ERA-Interim 500 hPa daily geopotential height (GPH) at 90°and 60°W in order to estimate the blocking action of specific days. Therefore, two 500 hPa GPH meridional gradients GHGS (south) and GHGN (north) were evaluated: GHGS = GPH(75°S) − GPH(65°S) and GHGN = GPH(65°S) − GPH(50°S).…”

Precipitation extremes over La Plata Basin – Review and new results from observations and climate simulations

“…The region undergoes interdecadal, interannual, and intraseasonal variability (Robertson and Mechoso, 1998;Grimm et al, 2000;Nogués-Paegle and Mo, 1997 by teleconnections such as the Pacific South America (PSA) pattern (Cunningham and Cavalcanti, 2006;Zamboni et al, 2012;Cherchi et al, 2014) and blocking episodes (Mendes et al, 2008). The LPB region is also influenced by sea surface temperature (SST) forcing in both the Pacific and Atlantic Oceans (Diaz et al, 1998;Doyle and Barros, 2002;Berri and Bertossa, 2004) and by moisture flux from tropical South America (Rodrigues and Cavalcanti, 2006;Zamboni et al, 2010).…”