344
B. Kjerfve eta!.
substantial, especially in the case of Rio San Juan (Restrepo and Kjerfve
2000).
2 El Nino-La Nina Cycle
All rivers in South America display a strong seasonal signal of water
discharge and sediment load, typically varying by a factor of 5-10
between the low and the high monthly discharge. Additionally, interannual variability of water discharge and sediment load is associated with
the El Nino-Southern Oscillation (ENSO) or El Nino-La Nina cycle with
a factor of 2-4 between low and high annual discharge (Richey et al. 1986,
1989; Depetris et al. 1996; Vorosmarty et al. 1996; Restrepo and Kjerfve
2000). This cycle can be quantified by the Southern Oscillation Index
(SOl), which is defined as the difference in atmospheric sea level pressure
between Tahiti and Darwin (Glantz 1997). The cold La Nina phase of the
SO I is characterized by a positive SOl index (ca. + 5 hPa), whereas the
warm El Nino phase is characterized by a negative SOl index (ca. -5hPa).
The El Nino-La Nina cycle gives rise to a significant variability in regional rainfall, river discharge, and sediment load. This hydrological variability is probably the most important forcing function for the coastal
marine ecosystems since the temperature signal is relatively small, at least
for the subtropical and tropical portions of the continent.
However, the northern and southern portions of the South American
continent, can be divided by the subtropical jetstream, stretching from
approximately Quito, Ecuador, to Sao Paulo, Brazil, and have interannual
hydrological regimes that are opposite in phase. El Nino brings about
heavy rainfall south of the jetstream, and rivers have high flow and suspended loads during the southern-hemisphere late summer, when extensive flooding impacts river basins in southern Brazil and the delta of the
Parana River in Argentina (Probst and Tardy 1989; Mechoso and Perez-Iribarren 1992). At the same time, river basins north of the jetstream suffer
from drought conditions and low river discharge. In contrast, during the La
Nina phase southeast trade winds are well developed and the Inter Tropical Convergence Zone (ITCZ) remains north of its typical position in the
eastern Pacific. As a result, drier than normal conditions prevail in the southern portion of the South American continent but intense rainfall occurs
in the northern parts and in portions of Central America (Ropelewski and
Halpert 1987). Rivers in Colombia and Venezuela, in particular, experience
catastrophic floods during La Nina seasons, often having drastic social and
economic impacts.
B. Kjerfve eta!.
substantial, especially in the case of Rio San Juan (Restrepo and Kjerfve
2000).
2 El Nino-La Nina Cycle
All rivers in South America display a strong seasonal signal of water
discharge and sediment load, typically varying by a factor of 5-10
between the low and the high monthly discharge. Additionally, interannual variability of water discharge and sediment load is associated with
the El Nino-Southern Oscillation (ENSO) or El Nino-La Nina cycle with
a factor of 2-4 between low and high annual discharge (Richey et al. 1986,
1989; Depetris et al. 1996; Vorosmarty et al. 1996; Restrepo and Kjerfve
2000). This cycle can be quantified by the Southern Oscillation Index
(SOl), which is defined as the difference in atmospheric sea level pressure
between Tahiti and Darwin (Glantz 1997). The cold La Nina phase of the
SO I is characterized by a positive SOl index (ca. + 5 hPa), whereas the
warm El Nino phase is characterized by a negative SOl index (ca. -5hPa).
The El Nino-La Nina cycle gives rise to a significant variability in regional rainfall, river discharge, and sediment load. This hydrological variability is probably the most important forcing function for the coastal
marine ecosystems since the temperature signal is relatively small, at least
for the subtropical and tropical portions of the continent.
However, the northern and southern portions of the South American
continent, can be divided by the subtropical jetstream, stretching from
approximately Quito, Ecuador, to Sao Paulo, Brazil, and have interannual
hydrological regimes that are opposite in phase. El Nino brings about
heavy rainfall south of the jetstream, and rivers have high flow and suspended loads during the southern-hemisphere late summer, when extensive flooding impacts river basins in southern Brazil and the delta of the
Parana River in Argentina (Probst and Tardy 1989; Mechoso and Perez-Iribarren 1992). At the same time, river basins north of the jetstream suffer
from drought conditions and low river discharge. In contrast, during the La
Nina phase southeast trade winds are well developed and the Inter Tropical Convergence Zone (ITCZ) remains north of its typical position in the
eastern Pacific. As a result, drier than normal conditions prevail in the southern portion of the South American continent but intense rainfall occurs
in the northern parts and in portions of Central America (Ropelewski and
Halpert 1987). Rivers in Colombia and Venezuela, in particular, experience
catastrophic floods during La Nina seasons, often having drastic social and
economic impacts.