chapter - Atmospheric and Oceanic Science

Interannual low frequency variability: Background
Others studies have shown that the tropical Atlantic Ocean also plays an
important role in the interannual variability of the rainy season of the Amazon and
Northeast region of Brazil (Hastenrath and Heller 1977; Moura and Shukla 1981;
Servain 1991; Pulwarty, 1994; Nobre and Shukla 1996; Uvo et al. 1998; Souza et
al. 2000 and others). However, for the Uruguay and southern Brazil, Diaz et al.
(1998) show that both Pacific and Atlantic SST anomalies influence the rainfall
regime in these regions during austral Spring and Summer, although during the fall-
Winter period (April-July) only the SST anomalies in the southwestern Atlantic
seem to have an influence on rainfall anomalies in this region.
Much of the precipitation during the rainfall season in La Plata basin is produced
by large mesoscale precipitation systems. Mesoscale Convective Processes
(MCCs; Maddox 1980) are a subset of mesoscale precipitation systems characterized
by their large size and longevity. The term MCC is an infrared satellite imagebased
definition of cloudiness that was named by Maddox (1980) to classify the
mesoscale systems that produce great rainfall amount over the North American
Great Plains during spring and summer. Most MCCs occur over land regions situated
on the lee side of major topographic features and downstream from a low level
jet that brings a continuous supply of warm-moist tropical air to feed the convection
(Laing and Fritsch 1997; Velasco and Fritsch 1987). In that sense, La Plata
basin is a favourable region for the occurrence of MCCs because it is located on the
lee side of the Andes Mountains and downstream of the SALLJ, which supplies
warm and moist air from the Amazon basin to feed the convection (Marengo et al,
2004). Recently, Nieto Ferreira et al (2003) have presented an observational study
of the January-March 1998-99 differences in precipitation, atmospheric circulation,
and occurrence of large, long-lived convective cloud systems in South America.
They chose this period because of the strongly contrasting Southern Oscillation
conditions, where one of the strongest El Niño episodes on record was under way
in the Pacific Ocean during JFM 1998 and a strong La Niña conditions prevailed in
the Pacific Ocean during JFM 1999. They found that the SALLJ was about twice
as strong during the warm ENSO phase than during the cold one. The stronger
SALLJ was accompanied by large, long-lived convective cloud systems and nearly
twice as much rainfall in subtropical South America (parts of southern Brazil,
Uruguay, and Argentina).
3.2. Variability over the La Plata basin
Studies of precipitation variability have been hampered by the lack of an
extensive observational network. Nevertheless, there are some results, mainly in
connection with the El Niño Southern Oscillation (ENSO) that show that it has a
considerable signal in the interannual variability of the precipitation over the La
Plata basin (Aceituno 1988; Ropelewski and Halpert 1987, 1989; Kiladis and Diaz
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