ENSO, Pacific decadal variability, and U.S. summertime precipitation, drought, and stream flow

Abstract

The relationship between the three primary modes of Pacific sea surface temperature (SST) variability-the El Niño-Southern Oscillation (ENSO), the Pacific decadal oscillation, and the North Pacific mode-and U.S. warm season hydroclimate is examined. In addition to precipitation, drought and stream flow data are analyzed to provide a comprehensive picture of the lower-frequency components of hydrologic variability. ENSO and the two decadal modes are extracted from a single unfiltered analysis, allowing a direct inter-comparison of the modal structures and continental linkages. Both decadal modes have signals in the North Pacific, but the North Pacific mode captures most of the local variability. A summertime U.S. hydroclimatic signal is associated with all three SST modes, with the linkages of the two decadal modes comparable in strength to that of ENSO. The three SST variability modes also appear to play a significant role in long-term U.S. drought events. In particular, the northeastern drought of the 1960s is shown to be closely linked to the North Pacific mode. Concurrent with the drought were large positive SST anomalies in the North Pacific, quite similar in structure to the North Pacific mode, and an example of a physical realization of the mode. Correspondingly, the 1962-66 drought pattern had considerable similarity to the drought regression associated with the North Pacific mode. Analysis of upper-level stationary wave activity during the drought period shows a flux emanating from the North Pacific and propagating over the United States. The near-equivalent-barotropic circulation anomalies originating in the North Pacific culminate in a cyclonic circulation over the East Coast that, at low levels, opposes the climatological inflow of moisture in an arc over the continent from the Gulf Coast to the Northeast, consistent with the observed drought.