Extended Abstract (168K)2.10
The Hadley and Walker circulations associated with the ENSO episodes during 1970s, 1980s and 1990s: Impacts on the South American seasonal rainfall
Tércio Ambrizzi, University of São Paulo, São Paulo, Brazil; and E. B. Souza
Most of the annual total rainfall observed over the South America usually occurs during the austral summer (December to February - DJF) and autumn (March to May - MAM) months. The large and synoptic meteorological systems that modulate the rainfall in this period are related to the South Atlantic Convergence Zone - SACZ, Bolivian High and the upper tropospheric cyclonic vortices. During MAM, the Inter Tropical Convergence Zone - ITCZ also plays an important role in modulating the rainfall. It is well known nowadays that ENSO (El Niño/Southern Oscillation) deflagrates changes in the general circulation of the atmosphere, resulting in climatic impacts in several continental areas located in the tropics and extratropics. These changes are basically related to the weakness, intensification and/or displacements of the large-scale atmospheric circulation in the meridional and zonal planes, mainly those linked to the Hadley and Walker circulations. The main goal of this study is to analyze the changes in the Hadley and Walker cells and their respective impacts on the South American rainfall during the ENSO episodes observed in the decades of 1970s, 1980s and 1990s. Cross-sections analyses of the atmospheric circulation in altitude, averaged in the zonal and meridional planes will be investigated.
In comparison to the other decades, the 90s can be considered as having a "typical" atmospheric circulation and precipitation anomalies pattern. A large upward vertical motion going from lower to the upper levels is observed on the equatorial region around 150oW and a maximum of the descending motion is seen around 50oW during DJF (Figure not shown). In the next season (MAM), ascend motion seems spread over the equator and the subsidence is weaker over the Northeast Brazil. As a consequence of this anomalous Walker circulation, rainfall deficits are observed in the north of South America. The Hadley circulation anomalies show weak ascending motion between 10oN and Equator and around 35oS during DJF. The overall downward motion pattern has decreased during the autumn season. Positive precipitation anomalies are observed over the subtropical South America, particularly in the South of Brazil. This general pattern is in agreement with many previous studies.
The above described patterns vary significantly from one decade to another. In particular the ENSO composites of the 1980s have shown larger amplitudes in terms of Walker and Hadley circulation as well as rainfall deficits over the South America. On the other hand, the 1970s has presented a very different picture, particularly in the precipitation patterns, with reverse signs in the Northeast and South- Southeast Brazil, where positive anomalies were observed in the first and negative in the last regions. It seems that the Atlantic ocean has played a very import role during this decade.
Acknowledgements: The authors would like to acknowledge the financial support from CAPES, CNPq (Proc. No. 301111/93-6) and FAPESP. T.A. has also been partially supported by IAI - CRN-055.
Session 2, Tropical-Extratropical Interactions and Teleconnections over the Southern Hemisphere I
Monday, 24 March 2003, 10:30 AM-3:00 PM
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