Potential Predictability of Two-Year Droughts in the Missouri River Basin

Potential predictability of two-year droughts indicated by low runoff in consecutive April-September in the Upper (UMRB) and Lower (LMRB) Missouri River Basin are examined with observed estimates and climate models. The majority of annual runoff is generated in April-September, which is also the main precipitation and evapotranspiration season. Physical features related to low runoff of less than -0.5 standardized departures in both UMRB and LMRB include a wave train across the Pacific-North American region that can be generated internally by the atmosphere or forced by the La Niña phase of the El Niño-Southern Oscillation, a dry land surface state indicated by 1-m soil moisture and La Niña during the prior March. The latter two features contribute to low runoff in two consecutive April-September. A dry land surface in March leads to statistically significant increases in the frequency of low runoff in one (50-60%) and two (40-50%) subsequent April-September. A dry land surface in March decreases subsequent April-September runoff efficiency, as more precipitation is required to wet the land surface and less available for runoff. La Niña in March significantly increases the probability of low runoff in the following April-September (40-50%) by inhibiting precipitation and runoff by dynamical-induced atmospheric circulation anomalies. However, La Nina itself in March does not significantly increase the probability of low runoff in April-September of the following year. A wet antecedent land surface provides a buffer to avert low runoff, as simultaneous high 1-m soil moisture and La Nina do not lead to increases in the probability of drought in either of the two following April-September.