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Abstract
The Colorado River is one of the most important rivers in the southwestern U.S., with ~ 90% of the total flow originating from the Upper Colorado River Basin (UCRB). The UCRB April–July streamflow is well-correlated to the UCRB spring precipitation. It is known that the UCRB precipitation is linked to an El Niño-like sea surface temperature (SST) pattern, but the causal effect of the tropical Pacific SST on the UCRB spring precipitation is still uncertain. Here, we apply a Granger causality approach to understand the causal effect of the tropical Pacific averaged SST in previous three seasons (winter, fall, and summer) on the UCRB averaged precipitation in spring in observations and two climate models. In observations, only the winter SST has Granger causal effect (with p-value ~ 0.05) on spring precipitation, while historical simulations of the two climate models overestimate the causal effect for winter and fall (with p-value < 0.01 and < 0.05, respectively) due to model biases. Moreover, future projections of the two climate models show divergent causal effects, especially for the scenario with high anthropogenic emissions. The divergent projections indicate that (1) there are large uncertainties in model projections of the causal effect of the tropical Pacific SST on UCRB spring precipitation and (2) it is uncertain whether climate models can reliably capture changes in such causality. These uncertainties may result in large uncertainties in seasonal forecasts of the UCRB hydroclimate under global climate change.
Highlights
The Colorado River is one of the most important rivers in the southwestern U.S It provides water for nearly 40 million people, irrigates ~ 20,000 km2 of land, and generates more than 4,200 megawatts of hydroelectric power per year (U.S Bureau of Reclamation 2012)
The CESM is unable to capture the large variance over the western boundary of the Upper Colorado River Basin (UCRB), while the GFDL-ESM4 overestimates the variance over lower latitudes
This study investigates the causal effect of the tropical Pacific sea surface temperature (SST) in summer, fall, and winter on the UCRB precipitation in following spring using observations and climate model simulations
Summary
The Colorado River is one of the most important rivers in the southwestern U.S It provides water for nearly 40 million people, irrigates ~ 20,000 km of land, and generates more than 4,200 megawatts of hydroelectric power per year (U.S Bureau of Reclamation 2012). The Upper Colorado River Basin (UCRB), defined as the catchment area upstream of the U.S Geological Survey stream gauge at Lees Ferry, Arizona, provides ~ 90% of the streamflow for the Colorado River (Jacobs 2011). A significant negative trend of April–July streamflow was observed over the UCRB (e.g., Xiao et al 2018). The climate change impacts on the UCRB streamflow have been widely investigated (e.g., Barnett et al 2005; Alley et al 2007; Harding et al 2012; Ficklin et al 2013; Vano et al 2014; Vano and Lettenmaier 2014; Ayers et al 2016; Udall and Overpeck 2017; Solander et al 2018). Solander et al (2018) found that more severe changes in streamflow for Colorado River Basin
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