ENSO SIGNALS WITHIN STREAMFLOW REGIMES

Bradley Todd Luff

doi:10.7273/000005439

The El Niño Southern Oscillation (ENSO) is known to cause episodic changes in seasonal temperature and precipitation patterns across a large portion of the globe. ENSO climatological patterns are well described, but the associated changes in stream discharge are not. I selected 1,776 USGS stream gages across the contiguous United States and correlated four discharge metrics to the Oceanographic Niño Index (ONI). I calculated the 7-day minimum discharge, the 15-day maximum discharge, the mean annual discharge, and the total annual discharge for every water year at each gage. The Watershed Boundaries Dataset (WBD) was used to select regions within the contiguous US that may have significant ENSO responses according to the existing literature and the correlation between the four discharge metrics and ONI. This resulted in selecting the Pacific Northwest (PNW), Southwest (SW), Gulf-Mexican (GM), and Northeast (NE) regions. Within each region I used a Kruskal-Wallis test and then conducted a Dunn post hoc test to determine the between group significance for each metric. I found that the stream discharge response to ENSO is not consistent across the contiguous US. The PNW region experiences increases in discharge for both El Niño and La Niña conditions, but La Niña conditions have a larger magnitude effect on discharge. Increases in discharge during El Niño conditions are likely due to an increased rate of melting of permanent snowpack while increases in discharge during La Niña are likely a result of increase in precipitation. Both ENSO conditions result in increases in discharge through different processes that are not significantly different from each other despite being a significant increase over Neutral conditions. The GM region experiences decreases in discharge for both ENSO conditions which is likely a result of different hydrological processes again. The NE also experiences decreases in discharge during ENSO conditions, but El Niño conditions are more significant than La Niña conditions. The SW is the only region that has a linear streamflow response to ENSO. I propose that ENSO conditions are separate climate anomalies that are related and should be examined individually. All methods are repeatable using my R package ENSOsignal.

ENSO SIGNALS WITHIN STREAMFLOW REGIMES

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