92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Tuesday, 24 January 2012: 9:00 AM
Observed Changes in Intraseasonal Variability of Summer Convection Over South America
Room 354 (New Orleans Convention Center )
Fernando Hirata, Georgia Institute of Technology, Atlanta, GA; and P. J. Webster, H. M. Kim, and C. Hoyos

Intraseasonal variations of summer convection in the 10-90 day band in the South Atlantic Convergence Zone (SACZ) represent almost 50% of the total variance. During periods of a convectively active SACZ, the combination of heavy rainfall and high infrastructural vulnerability poses great risk to some of the largest metropolitan areas in the world (e.g. Rio de Janeiro and Sao Paulo). Previous studies have related these intraseasonal variations of convection over the SACZ region to extratropical wave activity and the Madden-Julian Oscillation (MJO). Although the precise mechanisms leading to the SACZ formation and variability are still unclear, this work intends to highlight some ubiquitous aspects of the summertime convective variance over the region and describe how they have changed since the beginning of the satellite era. The intraseasonal signal of convection over the SACZ region can be decomposed in two main spatial patterns defined by EOF analysis. The two modes are uncorrelated but present similar periodicities. Together they explain around 43% of the intraseasonal variance of summer OLR. However, this association between the intraseasonal OLR variance and its representation by the EOF modes is stronger before the 90's. After the mid-1990's, this association weakens concurrently with a decrease in MJO activity. In this later period, OLR variance over the SACZ region peaks during the transition of spring to summer. Correlations between seasonally-averaged OLR intraseasonal variance and SST suggests that OLR variance is more strongly associated with ENSO in the first period (1979 to 1995) and with Atlantic Ocean SSTs in the second period (after 1995). We show that intraseasonal OLR variance over South America is not completely determined by remote forcing. Although both the MJO and ENSO can affect convective activity over tropical South America, we argue that local processes are also important in generating intraseasonal oscillations of convection in the region. The changes in the relationships among OLR variance and MJO activity or SST observed here may favor local processes as a forcing mechanism acting as an energy source for the SACZ active and break convective cycles.

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