On interannual timescales, low-frequency large-scaleocean–atmosphere phenomena are highly coupled to thehydroclimatology of the region. This region is favored fordevelopment of tropical mesoscale convective complexes (Velasco and Fritsch 1987). The confluence of the twowinds, combined with the effects of surface warmingand orographic lifting, produces a highly unstable atmospheric profile causing strong convection and heavyprecipitation along the Pacific coast and western flanksof the Cordillera Occidental. The origin of this highly wet region over northwestern South America lies in the low-level westerlyflow from the Pacific Ocean over inland Colombia.These winds are colder and moister than the dominanteasterly trades from the Atlantic and the Caribbean( López and Howell 1967). The Atrato River in Colombia drains 35702km 2 of the wettest areas of the planet, producing a meanannual discharge of 4557 m 3 s −1 and an equivalent runoff of 127.6 L s −1 km −2, 5–6 times larger than the Amazon. The excessof precipitation over evapotranspiration in the region issuch that the combined runoffs of the Amazon, Orinoco,and Magdalena rivers account for 18.3% of the totalinflow to the world oceans ( Baumgartner and Reichel1975, 95). Theregion is a major center of convective activity, mostlydeveloped within large cumulonimbus clouds, fromwhich latent heat is continuously released into the atmosphere thus influencing the Hadley cells and overallglobal circulation ( Riehl and Malkus 1958). Large quantities of precipitation, evapotranspiration, soil moisture, and runoff are present in tropicalSouth America, as compared with world averages. The main controls of the rain space distribution are the presence ofthe Andes mountains and the eastern Pacific, and western Atlantic Oceans, the atmospheric circulation overthe Amazon basin, and vegetation and soil moisturecontrasts. The annual distribution of rainfall over tropical SouthAmerica is primarily influenced by the position of theintertropical convergence zone (ITCZ). The proposed mechanisms would constitute the “land–atmosphere” bridge connecting Pacific and Atlantic SST anomalies. To begin with, the occurrence of both phases of ENSO affects all those fields. A hypothesis is formulated to explain these feedback mechanisms throughperturbations in precipitation, soil moisture, and evapotranspiration over the continent. Evidence is also presented to show that processes arisingfrom land–atmosphere interactions in tropical South America affect sea surface temperatures in the Caribbeanand the north tropical Atlantic. Also, the impactsof La Niña are more pronounced than those of El Niño. Observations show that ENSO’s effect on river dischargesoccurs progressively later for rivers toward the east in Colombia and northern South America. Such dependence is illustrated in the hydroclimatology of Colombia throughseveral empirical analyses: correlation, empirical orthogonal functions, principal component, and spectral analysis, and discussion of the major physical mechanisms. In particular, El Niño–SouthernOscillation (ENSO) affects climatic and hydrologic conditions on timescales ranging from seasons to decades.With some regional differences in timing and amplitude, tropical South America exhibits negative rainfall andstreamflow anomalies in association with the low–warm phase of the Southern Oscillation (El Niño), and positiveanomalies with the high–cold phase. The hydroclimatology of tropical South America is strongly coupled to low-frequency large-scale oceanicand atmospheric phenomena occurring over the Pacific and the Atlantic Oceans.
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