El Niño/Southern Oscillation (ENSO)-related patterns of monthly reanalyzed upper-level circulation data and monthly rainfall time series over South America are revised for the 1948-1999 period considering the phases of the Pacific interDecadal Oscillation (PDO). The El Niño (EN) related composites show differences relative to the PDO phases as well as seasonal differences. EN signals in the South American rainfall are more conspicuous for the warm PDO (WPDO) regime, when seasonal differences are more pronounced, than for the cold PDO (CPDO). Differences for the 200-hPa stream function composites seem to determine the precipitation composite differences. In fact, the negative precipitation anomalies over northeast Brazil and the excessive rainfall to the south are explained respectively by a cyclonic center over eastern and northeastern Brazil and a weak anticyclonic center over southeastern South America for the WPDO regime. Positive precipitation anomalies over southeastern South America are located in a southern position for the CPDO regime when compared to those for the WPDO regime. These anomalies might be related to a strengthened upper-level subtropical jet stream associated with strong cyclonic circulation extending over southern South America for the CPDO regime. With regard to the linear and nonlinear parts of the precipitation anomaly patterns related to the ENSO, the nonlinear component is considerably smaller than the linear component, in particular, over northern and southeastern South America. This suggests that the linear approach of the South American precipitation response to the ENSO seems to be appropriated.

Abstract:The anomaly patterns of rainfall in South America for El Niño/Southern Oscillation (ENSO) extreme conditions stratified according to the high, low, and normal Pacific (inter-) Decadal Oscillation (PDO) phases (HPDO, LPDO and NPDO) are examined for the three bi-months of the season from November to April. El Niño (EN) and La Niña (LN) composites as well as the linear (EN − LN) and nonlinear (EN + LN) components of the precipitation anomaly patterns relative to ENSO show substantial differences among the three PDO phases. The differences in the strength of ENSO teleconnections for the South American rainfall might be related to the PDO, which creates a background for these teleconnections acting constructively (destructively) when ENSO and PDO are in the same (opposite) phase. An interesting aspect is the occurrence of robust structures of the nonlinear component, which are due to the same sign rainfall anomalies for EN and LN composites. This is particularly conspicuous for the HPDO over eastern Brazil in the South Atlantic Convergence Zone (SACZ) region during Nov/Dec and Jan/Feb, for the HPDO over northern/northwestern South America during Mar/Apr, and for the NPDO over northeastern Brazil during Mar/Apr. The results presented here might have relevant implications for climate monitoring purposes.

ABSTRACT:The impacts of El Niño (EN) on the rainfall over South America are investigated considering three EN types differing in the locations of the maximum sea surface temperature (SST) anomalies in the equatorial Pacific: the Central-Pacific (CP), Eastern-Pacific (EP) and the Mixed (MIX) types. The largest positive (SST) anomalies for the EP and CP types occur, respectively, in the eastern and central sectors of the tropical Pacific during all seasons. The SST anomaly pattern for the MIX-EN resembles that of the EP-EN during its onset stage, and of the CP-EN during its mature and demise stages. The different SST anomaly patterns affect the large-scale (Walker circulation and the tropospheric Rossby-wave trains) and local (South American low-level jet -SALLJ) atmospheric circulation patterns in different ways and lead to distinct precipitation anomaly patterns over South America. Variations in the position and longitudinal extension of the downward motions of the EN-related eastward-displaced Walker circulation explain the differences in the dryness over equatorial South America. For the CP-EN, a double Walker cell defines a more zonal configuration of the equatorial dryness over South America during the first three analysed seasons. This feature is not noted for the other two EN types. The Rossby-wave train pattern path depends on the EN types and seasons. In consequence, the associated local atmospheric circulation patterns depend also on the season and EN types. In all seasons, an intense SALLJ for the EP EN contrasts with weak or inexistence SALLJ for the other two EN types. Thus, a wetter condition over southeastern South America, southern and eastern Brazil occurs for the EP EN in relation to the other EN types. The results shown here, in particular considering the MIX EN type, have not been discussed before and might be useful mainly for climate monitoring purposes.

[1] In this paper, sea surface temperature (SST) and sea level pressure variability modes associated with climate extremes (droughts and floods) over northeastern Brazil (NEB) stratified according to the El Niño-Southern Oscillation (ENSO) phases (El Niño, La Niña, and neutral) are reexamined. The analyses indicate that only 36% of the time an ENSO-based forecast for the NEB climate would be right. This relatively low percentage is mostly because the interannual variations of the NEB climate are more closely tied to the tropical South Atlantic SST variability modes than to the tropical Pacific variability mode. An interesting aspect revealed in the present analysis is that hints of the FebruaryApril SST anomaly patterns in the tropical Atlantic for dry and wet cases which are not directly related to the ENSO can be found months prior to the NEB rainy season. Since these hints are particularly strong in the tropical South Atlantic, the SST variations in this sector during months prior to the rainy season should be carefully monitored in the diagnostic activities.Citation: Kayano, M. T., and R. V. Andreoli (2006), Relationships between rainfall anomalies over northeastern Brazil and the El Niño -Southern Oscillation,