Pacific and Atlantic Ocean influence on the spatiotemporal variability of heavy precipitation in the western United States

In this study, we test our hypothesis that no single index such as El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), Atlantic Multi-decadal Oscillation (AMO) or North Atlantic Oscillation (NAO) derived from the Pacific and Atlantic Oceans can explain the multi-scale temporal variability and spatial distribution of heavy precipitation in the western United States. Instead, it may be possible to utilize a characterization of their integrated effect or some other unidentified factors which reflects the combined physical oceanic–atmospheric processes that occur. For this purpose, Empirical orthogonal function (EOF) analysis is performed on summer (April–September) and winter (October–March) heavy precipitation expressed as total precipitation when daily precipitation is larger than 95th percentile (R95) to indentify the leading modes of variability during the period 1948–2009. The correlation between the principle components (PCs) of each EOF mode with Sea Surface Temperature (SST) anomalies is evaluated. The analysis has shown that the leading modes of R95 variability and the connections between local R95 and SST over western United States are seasonally dependent. The first EOF mode of summer R95 is associated with AMO. The first two EOF modes of winter R95 are related to an integrated effects of ENSO, PDO, and NAO which explain nearly half (49%) of the spatial and temporal variance in R95 in this region. Additionally, the coupled effects of these three oceanic–atmospheric oscillations on winter R95 are evaluated by investigating the ENSO-R95 responses modulated by a combination of different PDO and NAO phases. Based on our analyses and predicted future states of these oceanic–atmospheric oscillations, we suggest possible heavy precipitation scenarios for upcoming decades which may be useful to forecasters and water managers.