Impact of two different types of El Nino events on runoff over the conterminous United States

The responses of river runoff to shifts of largescale climatic patterns are of increasing concerns to water resource planners and managers for long-term climate change adaptation. El Nino, as one of the most dominant modes of climate variability, is closely linked to hydrologic extremes such as floods and droughts that cause great loss of lives and properties. However, the different impacts of the two types of El Nino, i.e., central Pacific (CP-) and eastern Pacific (EP-) El Nino, on runoff across the conterminous US (CONUS) are not well understood. This study characterizes the impacts of the CP- and EP-El Nino on seasonal and annual runoff using observed streamflow data from 658 reference gaging stations and the NCAR-CCSM4 model. We found that surface runoff responds similarly to the two types of El Nino events in southeastern, central, southern, and western coastal regions, but differently in northeast (NE), Pacific northwest (PNW) and west north central (WNC) climatic zones. Specifically, EP-El Nino events tend to bring above-average runoff in NE, WNC, and PNW throughout the year while CP-El Nino events cause below-than normal runoff in the three regions. Similar findings were also found by analyzing NCAR-CCSM4 model outputs that captured both the CP- and EP-El Nino events, representing the best data set among CMIP5 models. The CCSM4 model simulates lower runoff values during CP-El Nino years than those in EP-El Nino over all of the three climatic regions (NE, PNW, and WNC) during 1950-1999. In the future (2050-2099), for both types of El Nino years, runoff is projected to increase over the NE and PNW regions, mainly due to increased precipitation (P). In contrast, the increase of future evapotranspiration (ET) exceeds that of future P, leading to a projected decrease in runoff over the WNC region. In addition, model analysis indicates that all of the three regions (NE, PNW, and WNC) are projected to have lower runoff in CP-El Nino years than in EP-El Nino years. Our study suggests that the US water resources may be distributed more unevenly in space and time with more frequent and intense flood and drought events. The findings from this study have important implications to water resource management at regional scales. Information generated from this study may help water resource planners to anticipate the influence of two different types of El Nino events on droughts and floods across the CONUS.