U.S. streamflow patterns in relation to the El Niño/Southern Oscillation

Abstract

The relationship between the E1Nifio/Southern Oscillation (ENSO) and unimpaired streamflow over the contiguous United States is studied. The extreme phases of the Southern Oscillation have been linked to fairly persistent classes of atmospheric anomalies over the low and middle latitudes at regional and global scales. Of particular interest in this investigation is the identification of regions of land that appear to have strong and consistent ENSO-related streamflow signals. The first harmonic extracted from a 24-month ENSO composite at each station is assumed to be the ENSO-related signal appearing in streamflow anomalies. These regions were identified by the similarity in phase of the harmonic vectors. The vectorial display of these harmonics over a map of the United States provides the areal extents of ENSO influence on streamflow. Coherent and significant streamflow responses to hypothesized ENSO forcing are found in four regions of the United States: the Gulf of Mexico, the Northeast, the North Central, and the Pacific Northwest. Once an ENSO event sets in, a long-range forecasting utility may be available for these regions. The results of this analysis, which are consistent with previous studies on precipitation and temperature, demonstrate the mid-latitude hydrologic response to the tropical ENSO phenomena. most extensive region in the world of water warmer than 28øC is located in the western tropical Pacific-Indonesia region. This large warm water pool is associated with heavy rainfall and is considered a major source of atmospheric heating, which drives large-scale convective circulation patterns [Rasmusson, 1987]. Normally, the trade winds (easterlies) are driven from the eastern to the western Pacific by the differences in atmospheric pressures. These winds tend to move water westward along the equator, maintaining a buildup of warm surface water and creating, in turn, a higher sea level elevation in the western Pacific. When the Southern Paper number 93WR00744. .0043.1397/93/93WR-00744505.00 and the trade winds usually weaken (or become westerlies as the case in 1982-1983). The warm water pool extends eastward, piling up off the coast of Peru and southern Ecuador. This is the sign of the development of an E1 Nifio event [Nicholls, 1987; Vogel, 1989]. E1 Nifio events have been observed and recorded since 1726 [Cane, 1983]. They occur approximately once every 4 years; however, the time interval between successive events varies from 2 to 10 years. No two events are completely alike: They evolve according to a consistent pattern, but differ in timing, intensity, extent, and duration. Quinn et al. [1978] introduce four E1 Nifio categories: strong, moderate, weak, and very weak. However, there are noticeable differences within each category. The strongest El Nifio episode in this century, in 1982-1983, had a unique pattern in terms of the sequence of the waxming and the time of onset [Cane, 1983]. The El Nifio/Southern Oscillation (ENSO) is a warm event in the tropical Pacific Ocean and is considered a sig...

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