Impact of two different types of El Niño events on runoff over the conterminous United States
The responses of river runo to shifts of large-scale climatic patterns are of increasing concerns to water resource planners and managers for long-term climate change adaptation. El Niño is one of the most dominant modes of climate variability that 5 is closely linked to hydrologic extremes such as floods and droughts that cause great loss of lives and properties. However, the dierent impacts of the two types of El Niño-Central Pacific (CP) and Eastern Pacific (EP)-El Niño on runo across the conterminous US (CONUS) are not well understood. This study characterizes the impacts of the CP- and EP-El Niño on seasonal and annual runo using observed 10 historical streamflow data from 658 reference gaging stations and NCAR-CCSM4 model. We found that surface runo responds similarly to the two types of El Niño events in Southeast, Central, South and Western coastal regions, but dierently in Northeast (NE), Pacific Northwest (PNW) and West North Central (WNC) climatic zones. Specifically, EP-El Niño events tend to bring above-average runo in NE, WNC, 15 and PNWthroughout the year while CP-El Niño events cause below-than normal runo in the three regions. Similar findings were also found by analyzing NCAR-CCSM4 model outputs that captured both the CP- and EP-El Niño events representing the best datasets among selected CMIP5 models. The CCSM4 model simulates lower runo values during CP-El Niño years than those in EP-El Niño in all of the three 20 climatic regions (NE, PNW and WNC) during 1950–1999. In the future (2050–2099), for both types of El Niño years, runo is projected to increase over the NE and PNW regions, mainly due to increased precipitation (P ). In contrast, the increase of future evapotranspiration (ET) is higher than that of future P , leading to a projected decrease in runo over the WNC region. In addition, model analysis indicates that all of the 25 three regions (NE, PNW and WNC) are projected to have lower runo values during CP-El Niño years than EP-El Niño. Our study suggests that US water resources may be distributed more unevenly in space and time with more frequent and intense floodresource management at the regional scale. Information generated from this study is useful for water resource planners to anticipate the influence of two dierent types of El Niño events on droughts and floods across the CONUS. and drought events. The findings from this study have important implications to water
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