Regional trends of daily precipitation indices in northwest Mexico and southwest United States

A regionalization based on a rotated principal component analysis (PCA) was used to produce six precipitation regions in northwest Mexico and the southwest United States. Monthly precipitation data from 184 meteorological stations for the 1960-1997 period were used in the PCA. The aim of this study was to estimate annual and seasonal trends of 10 daily precipitation indices in the six regions, including four indices related with extreme precipitation. The annual indices show a larger number of statistically significant trends than the seasonal indices, especially in Arizona-New Mexico and in the monsoon region in northwest Mexico (MON). Significant positive trends common to these two contiguous regions are extreme precipitation exceeding the 95th (R95p) and 99th (R99p) percentiles. The analysis of summer (June-October) daily precipitation indices also reveals the occurrence of significant positive trends in R95p in MON, mainly due to tropical cyclone activity. With the exception of the trends in MON, the most important contribution to the annual trends comes from the winter indices. Four of the six regions in the study area show significant positive trends in extreme winter precipitation (R10mm, R95p or R99p) during the study period. The variability of the annual indices that show statistically significant trends in extreme precipitation are partially linked to natural variations resulting from the combined effects of El Nino/Southern Oscillation and the Pacific decadal oscillation (PDO) and, in most cases, the trends are explained by the PDO.