The moisture budget over the Atlantic Ocean and South America is evaluated for two contrasting 5-year periods, 1985-1989 (wet) and 1990-1994 (dry), of rainfall over Northeast Brazil (NEB) using NCEP/NCAR reanalysis data. The linear correlation coefficients between the 5-year averages of rainfall over NEB and sea surface temperature (SST) anomalies over the Atlantic Ocean show a dipole configuration over the Tropical Atlantic. It is found that evaporation is higher over the Tropical North Atlantic, and lower over the Tropical South Atlantic during the 5-year period of higher rainfall over NEB. Higher evaporation causes lower SST and vice versa. This seems to be the main mechanism which generates dipole configuration in the Atlantic SST. During the rainy season (February through May) in NEB moisture is transported towards NEB and converges, providing the necessary moisture source for the rainfall. This low level convergence is associated with the rising branch of a local meridional circulation cell, which in turn is generated by the SST dipole. This series of interlinked mechanisms seems to fit in a physical picture responsible for the rainfall decadal variation over NEB.
Cold cloud index (CCI) data derived from Meteosat infrared imagery are used to detect periodicities in convective activity in South America. The generally used Fourier transform (FT) cannot provide time-localized information but gives information on the average periodicity of oscillations over the entire time domain. As many events in the atmosphere are intermittent, wavelet transform (WT) is used to identify periodic events in CCI data. First, the Morlet WT is applied to different combinations of time series data of known periodicities to demonstrate the advantage of WT over FT. Later it is applied to CCI data over four 9 square areas between the latitudes 4.5N and 31.5S, and longitudes 54-45W. Near the equator periodic convective activities are observed to be more prominent in the boreal summer than in the austral summer. Between the latitudes 4.5 and 22.5S, 1-, 2-3-, approximately 5-, and 8-10-day oscillations are seen in the austral summer and seldom is any convective activity seen in the winter. In January semidiurnal variation of cloudiness is also observed for a few days. Farther south in the extratropics, approximately 10-and approximately 20-day periodic events, which refer to the baroclinic waves, are seen more prominently in the austral autumn and winter, and 1-and approximately 5-day oscillations are seen in the summer, perhaps due to convective cloudiness.
A diagnosis of rainfall over South America (SA) during the 1997/98 El Niño year is made examining the roles of water vapor transport and stationary waves. It is found that the low-level jet (LLJ) on the eastern side of the central Andes is stronger during the El Niño event and transports more moisture. This seems to be the source for higher rainfall over southeast SA noted during the El Niño years. A calculation of three-dimensional stationary wave activity (F s) for 1997 and 1998 showed that in the summer of 1996/97 stationary waves propagate poleward and eastward from midlatitudes into the higher latitudes to the west of south SA and then propagate equatorward to the east of SA. During the autumn of 1997, the vertical component of F s is consistent with a blocking high over the southeast Pacific. To the east of this high cold air advection from Antarctica occurs, and to the west warm air advection occurs. This is consistent with negative and positive centers seen in the vertical component of F s to the east and west of south SA. The rainfall anomalies during a particular season seem to be due to multiple causes and this complicates a direct connection between them. Although over the southeast of SA, the higher rainfall during 1997 and 1998 can be attributed to some specific causes such as higher water vapor transport by LLJ in summer of 1997/98, in other parts such an association was not possible.
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