Determination of the onset dates of the rainy season in central Amazon with equatorially antisymmetric outgoing longwave radiation

The present work provides a new methodology to determine onset dates of the rainy season (ONR) in central Amazon (CAM) using the antisymmetric in relation to the equator outgoing longwave radiation (OLR) ((AOLR) over bar) data, for the 1979-2006 period. Spatial averages of the AOLR ((AOLR) over bar) over the CAM for the ONR periods are obtained. These periods correspond to 25 pentads centered on the mean pentad for the ONR. The sign changes from positive to negative of the AOLR for the ONR periods indicate the transition from dry to wet season. Composites of several variables are done for pentads before and after the ONR dates. These composites show physically consistent features. The potential of the (AOLR) over bar time series as an index for monitoring tasks is analyzed. The results here show that the (AOLR) over bar for the ONR period captures the transition from dry to wet conditions in the CAM area during 2006. The advantages of this method are discussed. The new simple method proposed here seems to be efficient in determining the ONR in the CAM.