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Diagnostics and Prediction of Anomalous River Discharge in Northern South America

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  • 1 Department of Meteorology, University of Wisconsin, Madison, Wisconsin
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Abstract

The annual cycles of rainfall and river discharge in northern South America are dominated by the seasonal latitude migration of the intertropical convergence zone. The catchments in the west (Madden. Magdalena and Orinoco) have their high stands between August and December, while those of the Guyanas (Cuyuni, Essequibo, Suriname, Maroni and Oyapock) peak between May and July. Anomalously abundant discharge is in almost all catchments (except Orinoco) associated with the high SO phase (defined by anomalously high/low pressure at Tahiti/Darwin), weakened Caribbean tradewinds. and accelerated cross-equatorial southerly flow over the eastern Pacific.

In a series of experiments a sharp distinction was kept between a “dependent” dataset (1940–70, or the part available in the river series) used as training period and an “independent” portion of the record (1971–87) reserved for prediction. Stepwise multiple regression models for bimonthly “seasons” used as input river discharge as regressand, and as regressors index series of Tahiti minus Darwin pressure difference, equatorial Pacific sea surface temperature PWT, zonal wind component over the Caribbean and meridional wind component over the eastern equatorial Pacific, all two seasons earlier. The resulting equations were then used to predict the discharge anomalies in the independent dataset 1971–87. There is considerable predictive skill for various rivers/seasons, with the overall best predictability for the low discharge time of year. In particular, for Magdalena 55% and for Essequibo 74% of the interannual variance of January-February discharge during 1971–87 is predictable by this method, in which for Essequibo PWT serves as sole predictor.

Abstract

The annual cycles of rainfall and river discharge in northern South America are dominated by the seasonal latitude migration of the intertropical convergence zone. The catchments in the west (Madden. Magdalena and Orinoco) have their high stands between August and December, while those of the Guyanas (Cuyuni, Essequibo, Suriname, Maroni and Oyapock) peak between May and July. Anomalously abundant discharge is in almost all catchments (except Orinoco) associated with the high SO phase (defined by anomalously high/low pressure at Tahiti/Darwin), weakened Caribbean tradewinds. and accelerated cross-equatorial southerly flow over the eastern Pacific.

In a series of experiments a sharp distinction was kept between a “dependent” dataset (1940–70, or the part available in the river series) used as training period and an “independent” portion of the record (1971–87) reserved for prediction. Stepwise multiple regression models for bimonthly “seasons” used as input river discharge as regressand, and as regressors index series of Tahiti minus Darwin pressure difference, equatorial Pacific sea surface temperature PWT, zonal wind component over the Caribbean and meridional wind component over the eastern equatorial Pacific, all two seasons earlier. The resulting equations were then used to predict the discharge anomalies in the independent dataset 1971–87. There is considerable predictive skill for various rivers/seasons, with the overall best predictability for the low discharge time of year. In particular, for Magdalena 55% and for Essequibo 74% of the interannual variance of January-February discharge during 1971–87 is predictable by this method, in which for Essequibo PWT serves as sole predictor.

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