South American monsoon onset and end date prediction using outgoing longwave radiation and sea surface temperature

The convection of the Intertropical Convergence Zone (ITCZ) propagates to the Southern Hemisphere over South America in austral spring and leaves it in autumn. This fact gives rise to the South American monsoon and a large area of South America is greatly influenced by the rainfall produced by the associated convection. The onset and end dates were calculated for the period 1975-1993 using outgoing longwave radiation (OLR) data following Kousky’s definition (1988). The onset date occurs during September and it is correlated (0.44) with the August sea surface temperature (SST) over the northern Caribbean, between 20ºN and 40ºN and west 290ºE. Therefore, warm (cold) Northern Caribbean SST is associated to a later (earlier) than normal beginning of the South America monsoon. Average onset date for years when warmest (coldest) SST is September 30th (September 16th). The onset date is also significantly correlated (0.64) with the August mean OLR anomaly in the area between 0º-15ºS and 45º-60ºW, indicating that when there is more (less) convection than normal in this area, the monsoon begins before (after) than normal. When August data of northern Caribbean SST and of this mean OLR anomaly are used as predictors for the onset date in a multiple linear regression model, they explain the 53% of the total variance. The same methodology was applied to the end monsoon date, which occurs during May. This date was significantly correlated (0.58) with April SST in Southern Caribbean between 10º and 20ºN, west of 290ºE. This result indicates that when the SST is warmer (colder) than normal, the monsoon ends before (after) than normal. Average end dates for year’s warmer (colder) than normal is May 4th (May 14th). The correlation between the end date and the April mean OLR anomaly, in the same area using with the onset, is significant although not very high (0.25). These two parameters used in a multiple linear regression model explain the 57% of the total variance of the end date. A similar model based on the March data of the Southern Caribbean SST and mean OLR anomaly explains the 44% of the variance of the onset date. These results seems to indicate that the beginning and the end of the summer tropical convection over South America depend on the heat sources both over the continent (latent heat release) and over the tropical sea in the ITCZ region, north of the continent.