Comparison of a proxy for wet and dry conditions for the Great Plains based on tree-ring studies (Meko, 1992) and the flow record of the Mississippi River at Vicksburg is shown in figure 13. Inspection of
indicates that the tree-ring-derived proxy and the discharge record are similar. With minor exceptions, the two data sets covary between 1865 and 1965. The correlation between the records is very high (
= +0.82) between 1890 and 1930. Between 1840 and 1865, the correspondence between the tree-ring proxy and the discharge record degrades, largely due to a strong trend in the tree-ring proxy from wet conditions (positive values) to dry conditions (negative values) that is not present in the flow data. Between 1820 and 1840, the discharge record appears to lead the tree-ring proxy by a few years, creating an offset in the two time series. We cannot explain the offset for 1820-40 but suspect that errors in counting rings to develop the tree-ring chronology are the most likely causes. Overall, our data indicate that the discharge record of the Mississippi River at Vicksburg matches major features of the climate record in the Great Plains region derived and compiled from historical and tree-ring studies.
Assessing natural discharge variations for the Rio Grande is complicated. Construction of reservoirs, removal of water for irrigation, and rerouting and reshaping of the river channel have greatly reduced flow and altered the natural pattern of flow variation (Mueller, 1975). Thus, discharge near the mouth of the Rio Grande is highly perturbed by human activities. Another complication is that Rio Grande flow is not always continuous. Flow is greatly reduced or even eliminated in dry years downstream of El Paso, Tex. In these dry years, the flow then increases or resumes in the Rio Grande near Presidio due to the intersection of the Rio Conchos with the Rio Grande. The Rio Conchos contributes up to 75 percent of flow in the lower Rio Grande downstream of Presidio (Moring and Setser, 2000). However, in modern times, human activities have reduced the flow in the lower Rio Grande below the junction with the Rio Conchos by about 87 percent (Mueller, 1975).
In order to study discharge records showing the fewest effects of human activities, we used long-term gaging records from the upper part of the Rio Grande upstream from Albuquerque, N. Mex., and from just downstream of the intersection of the Rio Conchos with the Rio Grande to represent historical flow. Inspection of available records suggests that the record from Embudo, N. Mex. (fig. 14
), is the best available record for the upper Rio Grande. The Embudo station is upstream of major dams and has a continuous record from 1912 to 1999 and a separate series from 1890 to 1903. For the lower Rio Grande downstream of the intersection of the Rio Conchos and the Rio Grande, we selected the record from Presidio because it was the longest, albeit discontinuous, record (fig. 14
Thus, like those for the Mississippi River, Rio Grande discharge records contain decadal-scale features that correspond to known droughts and extreme high-flow years that match known major floods. Differences between discharge records for the Mississippi River at Vicksburg and the Rio Grande reflect the regional nature of droughts and wet events. For example, the Dust Bowl interval of the 1930's is not well represented in the Rio Grande discharge records because the 1930's drought was most severe and persistent in the central and northern Great Plains; see Woodhouse and Overpeck (1998) for a summary of regional impacts of major historical and prehistorical drought events.
EL NIÑO/ SOUTHERN OSCILLATION (ENSO)
Interannual climate variability in the Southwestern United States and many areas of the Gulf Coast and Great Plains is highly influenced by strong El Niño/Southern Oscillation events (Stahle and Cleaveland, 1993), which are briefly described below. Therefore, we studied flow records for the Mississippi River and Rio Grande to test for an ENSO influence in the flow records.
Conditions during ENSO Events
El Niño is the name given to the quasi-periodic occurrence of anomalously warm surface waters off the coast of South America and in the eastern equatorial Pacific. The surface water warming usually begins in December, thus the association with Christmas and reference to "the child." The warming of eastern equatorial surface waters corresponds to a quasi-periodic change in the difference in sea-level pressure between the eastern and western tropical Pacific, which is the Southern Oscillation. These coupled changes in sea-surface temperature and sea-level pressure are referred to as ENSO. ENSO influences atmospheric circulation outside of the tropical Pacific region, which, in turn, causes regional climate variations linked to ENSO through the atmospheric "teleconnections" (Diaz and Kiladis, 1992).
In the "normal" condition, sea-level pressure in the eastern equatorial Pacific is high relative to sea-level pressure in the western equatorial Pacific. Surface winds blow from the eastern equatorial Pacific to the west. The winds move surface water west and toward the poles (due to the Coriolis effect), resulting in upwelling of cool, subsurface nutrient-rich water. The persistent upwelling results in cool sea-surface temperatures along the coast of South America and in the eastern equatorial Pacific. Warm surface waters are moved to the west and pile up in the western Pacific (see fig. 15
During an El Niño, or the warm extreme of ENSO, sea-level pressure in the eastern equatorial Pacific is reduced, and surface winds decline in intensity or even reverse direction and blow to the east. Upwelling of cool waters along the South American Coast declines or stops, and the warm surface waters of the western Pacific expand eastward toward the Americas.
A La Niña event is the opposite of an El Niño event and represents an extreme case of the normal situation. During La Niña, sea-surface pressure in the eastern equatorial Pacific increases, wind intensity increases, and surface waters become cooler than normal due to increased upwelling. More extensive discussions of ENSO are provided by Diaz and Kiladis (1992) and Zebiak (1999).
Establishing a record of significant individual El Niño and La Niña events is complicated because different indicators and boundary conditions have been used to define specific events. Some workers use an index based on the sea-surface-pressure difference between Darwin, Australia, and Tahiti, which is the Southern Oscillation Index or SOI. Other workers use sea-surface-temperature anomalies based on particular areas of the tropical Pacific Ocean, and the National Oceanic and Atmospheric Administration (NOAA) has developed a composite index that combines several ENSO indicators. In addition, records of the presumed proxies for El Niño and La Niña in areas within and outside of the equatorial Pacific are often used to identify ENSO events to augment or extend instrumental records (for example, see Quinn, 1992).
ENSO Events and Discharge Records
We compiled records of El Niño and La Niña events from several sources in order to develop a composite record for our study (fig. 4
). Records from 1950 to the present are well constrained and show a great deal of agreement. Records before 1950 are based on sparse and less reliable data.
Flow records for the Ohio River at Metropolis, the Mississippi River at St. Louis and at Vicksburg, and the Rio Grande at Embudo and at Presidio are shown in figures 16
with major ENSO events from figure 4
. We first used the Mann-Whitney U-test (Mann and Whitney, 1947) to determine if ENSO extremes (El Niño versus La Niña) were associated with significantly increased or decreased river flow. For each flow record, average daily flow values were obtained for each of the years designated as either El Niño or La Niña. A Mann-Whitney U test was performed on each data set using
The results of the analysis show that there is no statistically significant difference (either positive or negative) in flow rate due to El Niño or La Niña in the St. Louis, Vicksburg, or Metropolis data sets (table 2
). An interesting point is that the effects of El Niño on the Mississippi River flow at St. Louis and on Ohio River flow at Metropolis, while not statistically significant, are opposite. In contrast to the Mississippi River results, our analysis shows that El Niño years have statistically higher average daily flow rates than do La Niña years in the Rio Grande data sets (table 2
To further explore the relation between ENSO events and flow in the Rio Grande, we did simple time-series experiments with the flow data between 1915 and 1997 for the Embudo record and between 1930 and 1997 for the Presidio record. We performed a cross-spectral analysis on flow data and the SOI. To make the SOI comparable to our data, we converted the monthly SOI index (see http://daac.gsfc.nasa.gov/COMPAIGN_DOCS/FTP_SITE/INT_DIS/readmes/soi.html
) to an annual mean (fig. 18
). The power spectra for SOI (figs. 19
) show peaks at frequencies of approximately 0.150, 0.175 (Embudo only), and 0.237, which translate to cycles having periods of 4.5 to 6.5 years. Note that the difference in the SOI power spectra for comparison with Embudo and Presidio flow data is due to the different length of the records. In other words, the 0.175 frequency is not well represented in the shorter SOI data set.
Analysis of the Embudo flow record results in cycles having periods similar to the SOI. The variation in the SOI and the Embudo flow records is coherent at periods of approximately 4.5, 5.4, and 6.5 years (fig. 19
). The analyses suggest a clear relation between ENSO events and flow in the upper part of the Rio Grande. Analyses of the Presidio flow record show the presence of the 4.5-year cycle and demonstrate that variation in the SOI and flow are coherent at the 4.5-year period. The 6.5-year period observed in the 1930-95 SOI record is not present in the Presidio flow record. Our analyses indicate that flow in the Rio Grande downstream from the Rio Conchos is related to ENSO events but less strongly than flow variations in the upper part of the basin as monitored by the Embudo record.
The link between Rio Grande discharge and ENSO events is consistent with previous studies based on instrumental records (Diaz and Kiladis, 1992) and tree-ring records (Stahle and Cleaveland, 1993), which demonstrate correlation between ENSO and climate in the Southwestern United States. Tree-ring data (D'Arrigo and Jacoby, 1992) demonstrate that the El Niño results in increased winter precipitation in northern New Mexico, which would have a direct effect on the Embudo record due to melting of increased winter snow pack. It seems likely that the ENSO signal in the lower Rio Grande is complicated by variations in the summer monsoon. Most of the flow in the lower Rio Grande derives from the Rio Conchos, and the Rio Conchos is influenced by summer monsoon rains (Moring and Setser, 2000). However, the Southwest summer monsoon also shows some correlation with ENSO (Harrington and others, 1992).
Another complication in the ENSO signal is due to decadal-scale variations in sea-surface temperature of the North Pacific Ocean known as the Pacific Decadal Oscillation (PDO). Historical trends indicate that when the PDO is positive (indicating that central North Pacific sea-surface temperatures are cooler than normal), ENSO influence on the Western United States diminishes (McCabe and Dettinger, 1999). Despite these complications, our data indicate that variation in flow of the Rio Grande is related to ENSO and that the correlation is strongest in the upper part of the drainage basin.
In contrast to correlations with individual ENSO events or short-term variability, a long-term reconstruction of Mississippi River flow may provide important information on changes in the long-term variation of ENSO and changes in mean climate state. For example, instrumental and coral-based studies show that the tropical Pacific climate shifted to warmer and wetter conditions in 1976. This shift coincided with a change in variability of ENSO from predominantly interannual to a period of about 4 years (Urban and others, 2000). The flow records from the Mississippi River and the Rio Grande show a shift to wetter conditions in the late 1970's. The change is most striking in the Vicksburg record, which shows that the last 25 years has the highest overall flow of any 25-year period since 1815. A smoothed Vicksburg flow record (fig. 21
) shows the change to wetter conditions in the 1970's.