No access causality of the drought in the Southwestern United States based on observations

Slow feature analysis is used to extract driving forces from the monthly mean anomaly time series of the precipitation in the southwestern United States (1895-2015). Four major spectral scales pass the 95% confidence test after wavelet analysis of the derived driving forces. Further harmonic analysis indicates that only two fundamental frequencies are dominant in the spectral domain. The frequencies represent the influence of the Pacific decadal oscillation (PDO) and solar activity on the precipitation from the southwestern United States. In addition, solar activity has exerted a greater effect than the PDO on the precipitation in the southwestern United States over the past 120 years. By comparing the trend of droughts with the two fundamental frequencies, it is found that both the droughts in the 1900s and in the twenty-first century were affected by the PDO and solar activity, whereas the droughts from the 1950s to the 1970s were mainly affected by solar activity.