Assessing the role of warm-season mesoscale convective complexes in subtropical South American precipitation variability
Durkee, Joshua David
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An investigation of the climatology and rainfall from warm-season (October-May) mesoscale convective complexes (MCCs) for subtropical South America (SSA) revealed the unique nature of these systems. Characteristic changes in MCCs in relation to the South Atlantic Convergence Zone (SACZ) were also explored. For the austral warm seasons of 1998-2007, 330 MCCs were documented across SSA. An average of 37 MCCs occurred each season and reached a maximum cloud-shield size of 256,500 km, and lasted 14 hours. The highest frequency and concentration of MCCs was centered east of the Andes Mountains between 20°S and 30°S over Paraguay, northern Argentina, and southern Brazil. Compared to the United States, MCCs in SSA are significantly larger with longer durations. An analysis of MCC rainfall was conducted with the use of the Tropical Rainfall Measuring Mission satellite 3B42 (v.6) three-hourly blended precipitation data. The average MCC produces 15.7 mm of rainfall across 381,000 km, with a volume of 7.0 km. MCCs accounted for 15-21% of the total rainfall across portions of northern Argentina and Paraguay during 1998-2007. MCCs accounted for 20-30% of the total rainfall between November-February, and 30-50% in December, primarily across northern Argentina and Paraguay. MCCs also produced 25-66% of the total rainfall in portions of west-central Argentina. Results further showed that for October-March, the SACZ occurred during 23% of the warm season days, during which 75 MCCs formed, versus 216 on non-SACZ days. Most of the interannual variation (89%) in the number of MCCs during periods of SACZ was simply due to changes in the frequency of SACZ. A composite analysis of several lower and middle tropospheric data fields show the large-scale atmospheric patterns favorable for MCCs in preferred areas of the study region during SACZ and non-SACZ periods. This study is unique in showing a link in a preferred region of MCC activity to forms of low-level circulation other than advections of heat and moisture from Amazonia.