The US Southwest is among the most suitable regions for the employment of concentrated solar power (CSP). The high fluctuations of direct normal irradiance (DNI) lead to significant variabilities in CSP plants power output El Nino Southern Oscillation (ENSO) has been proven as a large-scale climate phenomenon that influences the climatic behaviors and meteorological variables in the US southwest In this study, the impacts of ENSO on DNI and CSP plants electricity production are investigated in four US southwest States of Arizona, California, Nevada and New Mexico, using 50 years (1961-2010) collected DNI data. The results demonstrate that responses of DNI to ENSO are both location and seasonal dependent due to the specific climate and DNI features of each site. Furthermore, the conducted analysis shows that each ENSO type and intensity has distinct impacts on DNI. The changes in the variability, distribution and magnitude of DNI during ENSO events can be due to changes in the atmospheric contents, cloud amounts and precipitation level caused by ENSO events. These changes lead to magnitude and continuity variations of CSP plants power output. Such variations necessitate optimizing the thermal energy storage utilization schedule and back-up energy source requirements for CSP power plants.