ATMOSPHERIC SCIENCE LETTERS Atmos. Sci. Let. 6: 219–223 (2006) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/asl.119 South American monsoon indices Manoel A. Gan,* Vadlamudi B. Rao and Marley C. L. Moscati National Institute for Space Research (INPE), S˜ ao Jos´ e dos Campos — S˜ ao Paulo, Brazil *Correspondence to: Manoel A. Gan, Instituto Nacional de Pesquisas Espaciais — INPE, Centro de Previs˜ ao de Tempo e Estudos Clim´ aticos — CPTEC, S˜ ao Jos´ e dos Campos, Brazil. E-mail: alonso@cptec.inpe.br Received: 3 August 2005 Revised: 18 November 2005 Accepted: 22 November 2005 Abstract Four monsoon indices (the meridional wind shear index (MWSI), the zonal wind shear index (ZWSI), the 850-hPa zonal wind index (850ZWI) and the 850-hPa zonal and meridional wind index (UVI)) based on the characteristics of the wind circulation are used to identify the onset and the demise dates and the intraseasonal variability of the rainy season over the west central Brazil (WCB) region. All the four-index time series have a high correlation with the precipitation series over WCB. The UVI, MWSI and the 850ZWI represent very well the intraseasonal variability (break and active periods) of the precipitation over WCB and the 850ZWI is also useful for identifying the onset dates. Copyright  2006 Royal Meteorological Society Keywords: South American monsoon; wind; index; precipitation 1. Introduction South America is not usually considered to exhibit a monsoon regime (Ramage, 1971). However, avail- ability of good quality data, such as National Cen- ters for Environment Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data, revealed that a large part of subtropical South Amer- ica experiences typical monsoonal circulation (Zhou and Lau, 1998; Gan et al., 2004). In this region, the rainfall season is the austral summer (Decem- ber – January – February) (Rao and Hada, 1990), similar to the Asian summer monsoon season (June–July– August). Recent studies of Gan et al. (2004) and Zhou and Lau (1998) described many similarities between the Asian and South American monsoon regimes. The South American monsoon system is characterized by an upper level anticyclonic circulation (called Boli- vian high), a low-level cyclone (called Chaco low) and a temperature maximum just before the onset of the rainy season. However, the wind reversal in low levels does not occur as in the Asian monsoon. But the upper and low-level zonal wind over west central Brazil (WCB) changes its direction (Gan et al., 2004). Zhou and Lau (1998) note that even the wind rever- sal at low levels can be seen if the annual cycle is removed. Several studies have been made to define the onset and demise dates of monsoon systems, e.g. Fasullo and Webster (2003) for the Indian monsoon and Gan et al. (2004) and Wang and Fu (2002) for the South American monsoon. Kousky (1988), Liebmann and Marengo (2001) and Marengo et al. (2001) used either outgoing long wave radiation (OLR) or precipitation data to define the onset of rainy season over South America. In these studies, different criteria, mostly based on rainfall or OLR, were used to identify the monsoon onset and demise dates over different monsoon regions. However, monsoon indices based on wind changes are important because the skill of the climatic models to predict the wind components is better than to predict the precipitation. Further, a monsoon index based on wind changes is useful in identifying the onset and demise dates and interannual variability of the monsoon activity. Thus, the purpose of the present study is to evaluate the applicability of some monsoon indices based on the wind components to identify the onset and demise dates of the rainy season in the WCB (60 – 50 ◦ W; 20–10 ◦ S). We choose the WCB region because it includes a portion of the summertime rainfall maximum where the mean annual cycle of circulation is strongly related to the South American monsoon system (SAMS) as identified by Gan et al. (2004). Also, this region contains the headwaters of major rivers, such as Araguaia and Paraguay, which flow into the Amazon and La Plata basins, respectively. In some years the lack of rain in the headwaters leads to dramatic situations, such as the one that occurred during the summer of 2000/2001 when power rationing was introduced in the state of S˜ ao Paulo (Rao et al., 2001). To our knowledge, this type of analysis defining indices for the South American monsoon has not been done earlier. 2. Data and methodology The data used in this study are pentad (five day averages) gridded precipitation values for Brazil (for more information see Gan et al., 2004) obtained from the Climate Prediction Center and pentads of the daily averaged fields of wind from the NCEP/NCAR reanalysis (Kalnay et al., 1996). The period used is July 1979 through June 1997. The precipitation time series are the pentad mean values averaged over WCB (10–20 ◦ S, 60–50 ◦ W). Copyright  2006 Royal Meteorological Society