Feedbacks between hydrological processes in tropical South America and large-scale ocean-atmospheric phenomena

摘要

The hydroclimatology of tropical South America is strongly coupled to low-frequency large-scale oceanicudand atmospheric phenomena occurring over the Pacific and the Atlantic Oceans. In particular, El Nin˜o–SouthernudOscillation (ENSO) affects climatic and hydrologic conditions on timescales ranging from seasons to decades.udWith some regional differences in timing and amplitude, tropical South America exhibits negative rainfall andudstreamflow anomalies in association with the low–warm phase of the Southern Oscillation (El Nin˜o), and positiveudanomalies with the high–cold phase. Such dependence is illustrated in the hydroclimatology of Colombia throughudseveral empirical analyses: correlation, empirical orthogonal functions, principal component, and spectral analysis,udand discussion of the major physical mechanisms. Observations show that ENSO’s effect on river dischargesudoccurs progressively later for rivers toward the east in Colombia and northern South America. Also, the impactsudof La Nin˜a are more pronounced than those of El Nin˜o. Evidence is also presented to show that processes arisingudfrom land–atmosphere interactions in tropical South America affect sea surface temperatures in the Caribbeanudand the north tropical Atlantic. A hypothesis is formulated to explain these feedback mechanisms throughudperturbations in precipitation, soil moisture, and evapotranspiration over the continent. To begin with, the occurrenceudof both phases of ENSO affects all those fields. The proposed mechanisms would constitute the ‘‘land–udatmosphere’’ bridge connecting Pacific and Atlantic SST anomalies.