The relationship between tropical convection in South America and the end of the dry period in subtropical Argentina

Abstract

The connection between the tropical convection over South America and rainfall in subtropical Argentina is studied using outgoing longwave radiation (OLR) measurements obtained from the scanning radiometer aboard the polar orbiting NOM operational satellites and rainfall measurements of 32 selected Argentine stations for the period 1975–1991. The mean annual rainfall cycle in subtropical Argentina can be described by the composition of the first three eigenvectors. The first eigenvector explains 59% of the variance having a strong influence in north-eastern Argentina and little variability during the year. The second, explaining 19% of the variance, represents the summer rainfall with maximum variability in the northwest part of the country. The third eigenvector explains 4% of the variance and is related to the semiannual cycle observed in the southeast of subtropical Argentina. The tropical convection may be related to rainfall in subtropical Argentina. It influences the vertical movements by compensatory subsidence and the higher humidity at low levels accompanying the convection, which can be advected towards Argentina. Also, the position of the mean centres of tropical convection is related to the latitude of the subtropical jet and of the tropospheric circulation. As a consequence, it influences the possible advection of humidity from the Atlantic Ocean. Different parameters are used to detect the influence of the onset of the summer monsoon on the end of the dry winter over almost all subtropical Argentina. Significant correlation is evident when rainfall in September in subtropical Argentina is correlated with the onset monsoon date anomaly. When the convection is advanced (delayed) from the climatic value, there are positive (negative) rainfall anomalies in the south-eastern region of subtropical Argentina. © 1998 Royal Meteorological Society