ENSO, Pacific decadal variability, and US summertime precipitation, drought, and stream flow

The relationship between the three primary modes of Pacific sea surface tem perature (SST) variability-the El Nino-Southern Oscillation (ENSO), the Pac ific decadal oscillation, and the North Pacific mode-and U.S. warm season h ydroclimate is examined. In addition to precipitation, drought and stream f low data are analyzed to provide a comprehensive picture of the lower-frequ ency components of hydrologic variability. ENSO and the two decadal modes are extracted from a single unfiltered analy sis, allowing a direct intercomparison of the modal structures and continen tal 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 li nkages 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 l ong-term U.S. drought events. In particular, the northeastern drought of th e 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, q uite similar in structure to the North Pacific mode, and an example of a ph ysical realization of the mode. Correspondingly, the 1962-66 drought patter n had considerable similarity to the drought regression associated with the North Pacific mode. Analysis of upper-level stationary wave activity durin g the drought period shows a flux emanating from the North Pacific and prop agating over the United States. The near-equivalent-barotropic circulation anomalies originating in the North Pacific culminate in a cyclonic circulat ion over the East Coast that, at low levels, opposes the climatological inf low of moisture in an arc over the continent from the Gulf Coast to the Nor theast, consistent with the observed drought.