“Intrinsic” correlation and its temporal evolution between winter-time PNA/EPW and winter drought in the west United States

Effects of natural variability, such as El Nino-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), etc., have been the focus of several recent studies on the change of drought patterns. But the influence of atmospheric patterns was rarely discussed. In this study, relations between winter-time Pacific-Northern America pattern (PNA)/East Pacific wave-train (EPW) and winter-time drought in the west United States over the period of 1951-2010 are analyzed. Considering traditional Pearson's Correlation Coefficient can be influenced by non-stationarity and nonlinearity, a recently proposed new method, Detrended Partial-Cross-Correlation Analysis (DPCCA) is applied. With DPCCA, we analyzed the “intrinsic” correlations between PNA/EPW and the winter drought with possible effects of ENSO and PDO removed. We found, i) significant negative correlations between PNA/EPW and drought on time scales of 5-6 years after removing the effects of ENSO, ii) while significantly stronger negative correlations between PNA/EPW and drought on time scales of 15-25 years after the influence of PDO is removed. By further studying the temporal evolutions of the “intrinsic” correlations, we found on time scales of 5-6 years, after removing the effects of ENSO, the “intrinsic” correlations between PNA/EPW and drought can vary severely with time, but for most time, the correlations are negative. While on interdecadal (15-25 years) time scales, after removing the effects of PDO, unlike the relations between PNA and drought, the “intrinsic” correlations between EPW and drought takes nearly homogeneous-sign (negative) over the whole period, which indicates a better model can be designed by using EPW.